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The MOO Programming Language

MOO stands for ``MUD, Object Oriented.'' MUD, in turn, has been said to stand for many different things, but I tend to think of it as ``Multi-User Dungeon'' in the spirit of those ancient precursors to MUDs, Adventure and Zork.

MOO, the programming language, is a relatively small and simple object-oriented language designed to be easy to learn for most non-programmers; most complex systems still require some significant programming ability to accomplish, however.

Having given you enough context to allow you to understand exactly what MOO code is doing, I now explain what MOO code looks like and what it means. I begin with the syntax and semantics of expressions, those pieces of code that have values. After that, I cover statements, the next level of structure up from expressions. Next, I discuss the concept of a task, the kind of running process initiated by players entering commands, among other causes. Finally, I list all of the built-in functions available to MOO code and describe what they do.

First, though, let me mention comments. You can include bits of text in your MOO program that are ignored by the server. The idea is to allow you to put in notes to yourself and others about what the code is doing. To do this, begin the text of the comment with the two characters /* and end it with the two characters */; this is just like comments in the C programming language. Note that the server will completely ignore that text; it will not be saved in the database. Thus, such comments are only useful in files of code that you maintain outside the database.

To include a more persistent comment in your code, try using a character string literal as a statement. For example, the sentence about peanut butter in the following code is essentially ignored during execution but will be maintained in the database:

for x in (players())
  "Grendel eats peanut butter!";
  player:tell(x.name, " (", x, ")");
endfor

Expressions

Expressions are those pieces of MOO code that generate values; for example, the MOO code

3 + 4
is an expression that generates (or ``has'' or ``returns'') the value 7. There are many kinds of expressions in MOO, all of them discussed below.

Errors

Most kinds of expressions can be used improperly in some way. For example, the expression

3 / 0
is improper because it tries to divide by zero. In such cases, MOO returns an error value (E_DIV for the example). Usually, though, MOO-code doesn't actually see that error value because the d (for ``debug'') flag is usually set on the verb. This flag causes the server to print a message to the current player and abort the current command whenever an error value is generated. For simplicity in describing the language below, though, I just say that the error value is returned by the expression.

Literals

The simplest kind of expression is a literal MOO value, just as described in the section on values at the beginning of this document. For example, the following are all expressions:

17
#893
"This is a character string."
E_TYPE
{"This", "is", "a", "list", "of", "words"}

In the case of lists, like the last example above, note that the list expression contains other expressions, several character strings in this case. In general, those expressions can be of any kind at all, not necessarily literal values. For example,

{3 + 4, 3 - 4, 3 * 4}
is an expression whose value is the list {7, -1, 12}.

Variables

As discussed earlier, it is possible to store values in properties on objects; the properties will keep those values forever, or until another value is explicitly put there. Quite often, though, it is useful to have a place to put a value for just a little while. MOO provides local variables for this purpose.

Variables are named places to hold values; you can get and set the value in a given variable as many times as you like. Variables are temporary, though; they only last while a particular verb is running; after it finishes, all of the variables given values there cease to exist and the values are forgotten.

Variables are also ``local'' to a particular verb; every verb has its own set of them. Thus, the variables set in one verb are not visible to the code of other verbs.

The name for a variable is made up entirely of letters, digits, and the underscore character (_) and does not begin with a digit. The following are all valid variable names:

foo
_foo
this2that
M68000
two_words
This_is_a_very_long_multiword_variable_name

Note that, along with almost everything else in MOO, the case of the letters in variable names is insignificant. For example, these are all names for the same variable:

fubar
Fubar
FUBAR
fUbAr

A variable name is itself an expression; it's value is the value of the named variable. When a verb begins, almost no variables have values yet; if you try to use the value of a variable that doesn't have one, the error value E_VARNF is returned (MOO is unlike many other programming languages in which one must `declare' each variable before using it; MOO has no such declarations). The following variables always have values, though:

NUM         OBJ          STR
LIST        ERR
player      this         caller
verb        args         argstr
dobj        dobjstr      prepstr
iobj        iobjstr

The values of some of these variables always start out the same:

NUM
a number, the type code for numbers (see the description of the function typeof(), below)
LIST
a number, the type code for lists
STR
a number, the type code for strings
OBJ
a number, the type code for objects
ERR
a number, the type code for error values

For others, the general meaning of the value is consistent, though the value itself is different for different situations:

player
an object, the player who typed the command that started the task that involved running this piece of code.
this
an object, the object on which the currently-running verb was found.
caller
an object, the object on which the verb that called the currently-running verb was found. For the first verb called for a given command, caller has the same value as player.
verb
a string, the name by which the currently-running verb was identified.
args
a list, the arguments given to this verb. For the first verb called for a given command, this is a list of strings, the words on the command line.

The rest of the so-called ``built-in'' variables are only really meaningful for the first verb called for a given command. Their semantics is given in the discussion of command parsing, above.

To change what value is stored in a variable, use an assignment expression:

variable = expression

For example, to change the variable named x to have the value 17, you would write x = 17 as an expression. An assignment expression does two things:


Thus, the expression

13 + (x = 17)

changes the value of x to be 17 and returns 30.

Arithmetic

All of the usual simple operations on numbers are available to MOO programs:

+    -    *    /    %

These are, in order, addition, subtraction, multiplication, division, and remainder. In the following table, the expressions on the left have the corresponding values on the right:

5 + 2     @result{}  7
5 - 2     @result{}  3
5 * 2     @result{}  10
5 / 2     @result{}  2
5 % 2     @result{}  1
5 % -2    @result{}  1
-5 % 2    @result{}  -1
-5 % -2   @result{}  -1
-(5 + 2)  @result{}  -7

Note that division in MOO throws away the remainder and that the result of the remainder operator (%) has the same sign as the left-hand operand. Also, note that - can be used without a left-hand operand to negate a numeric expression.

The + operator can also be used to append two strings. The expression

"foo" + "bar"

has the value

"foobar"

Unless both operands to an arithmetic operator are numbers (or, for +, both strings), the error value E_TYPE is returned. If the right-hand operand for the division or remainder operators (/ or %) is zero, the error value E_DIV is returned.

Comparisons

Any two values can be compared for equality using == and !=. The first of these returns 1 if the two values are equal and 0 otherwise; the second does the reverse:

3 == 4                              @result{}  0
3 != 4                              @result{}  1
"foo" == "Foo"                      @result{}  1
#34 != #34                          @result{}  0
{1, #34, "foo"} == {1, #34, "FoO"}  @result{}  1
E_DIV == E_TYPE                     @result{}  0
3 != "foo"                          @result{}  1

Note that comparison of strings is case-insensitive; that is, it does not distinguish between the upper- and lower-case version of letters. To perform a case-sensitive comparison, use the strcmp function described later.

Warning: It is easy (and very annoying) to confuse the equality-testing operator (==) with the assignment operator (=), leading to nasty, hard-to-find bugs. Don't do this.

Numbers, object numbers, strings, and error values can also be compared for ordering purposes using the following operators:

<       <=      >=      >

meaning ``less than,'' ``less than or equal,'' ``greater than or equal,'' and ``greater than,'' respectively. As with the equality operators, these return 1 when their operands are in the appropriate relation and 0 otherwise:

3 < 4           @result{}  1
#34 >= #32      @result{}  1
"foo" <= "Boo"  @result{}  0
E_DIV > E_TYPE  @result{}  1

Note that, as with the equality operators, strings are compared case-insensitively. Also note that the error values are ordered as given in the table in the section on values. If the operands to these four comparison operators are of different types, or if they are lists, then E_TYPE is returned.

Conditionals

There is a notion in MOO of true and false values; every value is one or the other. The true values are as follows:


All other values are false:


There are four kinds of expressions and two kinds of statements that depend upon this classification of MOO values. In describing them, I sometimes refer to the truth value of a MOO value; this is just true or false, the category into which that MOO value is classified.

The conditional expression in MOO has the following form:

expression-1 ? expression-2 | expression-3

First, expression-1 is evaluated. If it returns a true value, then expression-2 is evaluated and whatever it returns is returned as the value of the conditional expression as a whole. If expression-1 returns a false value, then expression-3 is evaluated instead and its value is used as that of the conditional expression.

1 ? 2 | 3           @result{}  2
0 ? 2 | 3           @result{}  3
"foo" ? 17 | {#34}  @result{}  17

Note that only one of expression-2 and expression-3 is evaluated, never both.

To negate the truth value of a MOO value, use the ! operator:

! expression

If the value of expression is true, ! returns 0; otherwise, it returns 1:

! "foo"     @result{}  0
! (3 >= 4)  @result{}  1

The negation operator is usually read as ``not.''

It is frequently useful to test more than one condition to see if some or all of them are true. MOO provides two operators for this:

expression-1 && expression-2
expression-1 || expression-2

These operators are usually read as ``and'' and ``or,'' respectively.

The && operator first evaluates expression-1. If it returns a true value, then expression-2 is evaluated and its value becomes the value of the && expression as a whole; otherwise, the value of expression-1 is used as the value of the && expression. Note that expression-2 is only evaluated if expression-1 returns a true value. The && expression is equivalent to the conditional expression

expression-1 ? expression-2 | expression-1

except that expression-1 is only evaluated once.

The || operator works similarly, except that expression-2 is evaluated only if expression-1 returns a false value. It is equivalent to the conditional expression

expression-1 ? expression-1 | expression-2

except that, as with &&, expression-1 is only evaluated once.

These two operators behave very much like ``and'' and ``or'' in English:

1 && 1                  @result{}  1
0 && 1                  @result{}  0
0 && 0                  @result{}  0
1 || 1                  @result{}  1
0 || 1                  @result{}  1
0 || 0                  @result{}  0
17 <= 23  &&  23 <= 27  @result{}  1

Indexing into Lists and Strings

Both strings and lists can be seen as ordered sequences of MOO values. In the case of strings, each is a sequence of single-character strings; that is, one can view the string "bar" as a sequence of the strings "b", "a", and "r". MOO allows you to refer to the elements of lists and strings by number, by the index of that element in the list or string. The first element in a list or string has index 1, the second has index 2, and so on.

Extracting an Element from a List or String

The indexing expression in MOO extracts a specified element from a list or string:

expression-1[expression-2]

First, expression-1 is evaluated; it must return a list or a string (the sequence). Then, expression-2 is evaluated and must return a number (the index). If either of the expressions returns some other type of value, E_TYPE is returned. The index must be between 1 and the length of the sequence, inclusive; if it is not, then E_RANGE is returned. The value of the indexing expression is the index'th element in the sequence.

"fob"[2]            @result{}  "o"
"fob"[1]            @result{}  "f"
{#12, #23, #34}[3]  @result{}  #34

Note that there are no legal indices for the empty string or list, since there are no numbers between 1 and 0 (the length of the empty string or list).

Replacing an Element of a List or String

It often happens that one wants to change just one particular slot of a list or string, which is stored in a variable or a property. This can be done conveniently using an indexed assignment having one of the following forms:

variable[index-expr] = result-expr
object-expr.name[index-expr] = result-expr
object-expr.(name-expr)[index-expr] = result-expr
$name[index-expr] = result-expr

The first form writes into a variable, and the last three forms write into a property. The usual errors (E_TYPE, E_INVIND, E_PROPNF and E_PERM for lack of read/write permission on the property) may be returned, just as in reading and writing any object property; see the discussion of object property expressions below for details. Correspondingly, if variable does not yet have a value (i.e., it has never been assigned to), E_VARNF will be returned.

If index-expr is not a number, or if the value of variable or the property is not a list or string, E_TYPE is returned. If result-expr is a string, but not of length 1, E_INVARG is returned. Now suppose index-expr evaluates to a number k. If k is outside the range of the list or string (i.e. smaller than 1 or greater than the length of the list or string), E_RANGE is returned. Otherwise, the actual assignment takes place. For lists, the variable or the property is assigned a new list that is identical to the original one except at the k-th position, where the new list contains the result of result-expr instead. For strings, the variable or the property is assigned a new string that is identical to the original one, except the k-th character is changed to be result-expr.

The assignment expression itself returns the value of result-expr. For the following examples, assume that l initially contains the list {1, 2, 3} and that s initially contains the string "foobar":

l[5] = 3          @error{}   E_RANGE
l["first"] = 4    @error{}   E_TYPE
s[3] = "baz"      @error{}   E_INVARG
l[2] = l[2] + 3   @result{}   5
l                 @result{}   {1, 5, 3}
l[2] = "foo"      @result{}   "foo"
l                 @result{}   {1, "foo", 3}
s[2] = "u"        @result{}   "u"
s                 @result{}   "fuobar"

Note that, after an indexed assignment, the variable or property contains a new list or string, a copy of the original list in all but the k-th place, where it contains a new value. In programming-language jargon, the original list is not mutated, and there is no aliasing. (Indeed, no MOO value is mutable and no aliasing ever occurs.)

In the list case, indexed assignment can be nested to many levels, to work on nested lists. Assume that l initially contains the list

{{1, 2, 3}, {4, 5, 6}, "foo"}

in the following examples:

l[7] = 4             @error{}   E_RANGE
l[1][8] = 35         @error{}   E_RANGE
l[3][2] = 7          @error{}   E_TYPE
l[1][1][1] = 3       @error{}   E_TYPE
l[2][2] = -l[2][2]   @result{}   -5
l                    @result{}   {{1, 2, 3}, {4, -5, 6}, "foo"}
l[2] = "bar"         @result{}   "bar"
l                    @result{}   {{1, 2, 3}, "bar", "foo"}

The first two examples return E_RANGE because 7 is out of the range of l and 8 is out of the range of l[1]. The next two examples return E_TYPE because l[3] and l[1][1] are not lists.

Extracting a Subsequence of a List or String

The range expression extracts a specified subsequence from a list or string:

expression-1[expression-2..expression-3]

The three expressions are evaluated in order. Expression-1 must return a list or string (the sequence) and the other two expressions must return numbers (the low and high indices, respectively); otherwise, E_TYPE is returned. If the low index is greater than the high index, then the empty string or list is returned, depending on whether the sequence is a string or a list. Otherwise, both indices must be between 1 and the length of the sequence; E_RANGE is returned if they are not. A new list or string is returned that contains just the elements of the sequence with indices between the low and high bounds.

"foobar"[2..6]                   @result{}  "oobar"
"foobar"[3..3]                   @result{}  "o"
"foobar"[17..12]                 @result{}  ""
{"one", "two", "three"}[1..2]    @result{}  {"one", "two"}
{"one", "two", "three"}[3..3]    @result{}  {"three"}
{"one", "two", "three"}[17..12]  @result{}  {}

Replacing a Subsequence of a List or String

The range assigment replaces a specified subsequence of a list or string with a supplied subsequence. The allowed forms are:

variable[start-index-expr..end-index-expr] = result-expr
object-expr.name[start-index-expr..end-index-expr] = result-expr
object-expr.(name-expr)[start-index-expr..end-index-expr] = result-expr
$name[start-index-expr..end-index-expr] = result-expr

As with indexed assigments, the first form writes into a variable, and the last three forms write into a property. The same errors (E_TYPE, E_INVIND, E_PROPNF and E_PERM for lack of read/write permission on the property) may be returned. If variable does not yet have a value (i.e., it has never been assigned to), E_VARNF will be returned.

If start-index-expr or end-index-expr is not a number, if the value of variable or the property is not a list or string, or result-expr is not the same type as variable or the property, E_TYPE is returned. E_RANGE is returned if end-index-expr is less than zero or if start-index-expr is greater than the length of the list or string plus one. Note: the length of result-expr does not need to be the same as the length of the specified range.

The assigment expression itself returns the value of result-expr. For the following examples, assume that l initially contains the list {1, 2, 3} and that s initially contains the string "foobar":

l[5..6] = {7, 8}       @error{}   E_RANGE
l[2..3] = 4            @error{}   E_TYPE
l[#2..3] = {7}         @error{}   E_TYPE
s[2..3] = {6}          @error{}   E_TYPE
l[2..3] = {6, 7, 8, 9} @result{}   {6, 7, 8, 9}
l                      @result{}   {1, 6, 7, 8, 9}
l[2..1] = {10, 11}      @result{}   {10, 11}
l                      @result{}   {1, 10, 11, 6, 7, 8, 9}
s[7..12] = "baz"       @result{}   "baz"
s                      @result{}   "foobarbaz"
s[1..3] = "fu"         @result{}   "fu"
s                      @result{}   "fubarbaz"
s[-3..0] = "test"      @result{}   "test"
s                      @result{}   "testfubar"

Other Operations on Lists

As was mentioned earlier, lists can be constructed by writing a comma-separated sequence of expressions inside curly braces:

{expression-1, expression-2, ..., expression-N}

The resulting list has the value of expression-1 as its first element, that of expression-2 as the second, etc.

{3 < 4, 3 <= 4, 3 >= 4, 3 > 4}  @result{}  {1, 1, 0, 0}

Additionally, one may precede any of these expressions by the splicing operator, @. Such an expression must return a list; rather than the old list itself becoming an element of the new list, all of the elements of the old list are included in the new list. This concept is easy to understand, but hard to explain in words, so here are some examples. For these examples, assume that the variable a has the value {2, 3, 4} and that b has the value {"Foo", "Bar"}:

{1, a, 5}   @result{}  {1, {2, 3, 4}, 5}
{1, @a, 5}  @result{}  {1, 2, 3, 4, 5}
{a, @a}     @result{}  {{2, 3, 4}, 2, 3, 4}
{@a, @b}    @result{}  {2, 3, 4, "Foo", "Bar"}

If the splicing operator (@) precedes an expression whose value is not a list, then E_TYPE is returned as the value of the list construction as a whole.

The list membership expression tests whether or not a given MOO value is an element of a given list and, if so, with what index:

expression-1 in expression-2

Expression-2 must return a list; otherwise, E_TYPE is returned. If the value of expression-1 is in that list, then the index of its first occurrence in the list is returned; otherwise, the in expression returns 0.

2 in {5, 8, 2, 3}               @result{}  3
7 in {5, 8, 2, 3}               @result{}  0
"bar" in {"Foo", "Bar", "Baz"}  @result{}  2

Note that the list membership operator is case-insensitive in comparing strings, just like the comparison operators. Note also that since it returns zero only if the given value is not in the given list, the in expression can be used either as a membership test or as an element locator.

Object Properties

Usually, one can read the value of a property on an object with a simple expression:

expression.name

Expression must return an object number; if not, E_TYPE is returned. If the object with that number does not exist, E_INVIND is returned. Otherwise, if the object does not have a property with that name, then E_PROPNF is returned. Otherwise, if the named property is not readable by the owner of the current verb, then E_PERM is returned. Finally, assuming that none of these terrible things happens, the value of the named property on the given object is returned.

I said ``usually'' in the paragraph above because that simple expression only works if the name of the property obeys the same rules as for the names of variables (i.e., consists entirely of letters, digits, and underscores, and doesn't begin with a digit). Property names are not restricted to this set, though. Also, it is sometimes useful to be able to figure out what property to read by some computation. For these more general uses, the following syntax is also allowed:

expression-1.(expression-2)

As before, expression-1 must return an object number. Expression-2 must return a string, the name of the property to be read; E_TYPE is returned otherwise. Using this syntax, any property can be read, regardless of its name.

Note that, as with almost everything in MOO, case is not significant in the names of properties. Thus, the following expressions are all equivalent:

foo.bar
foo.Bar
foo.("bAr")

The LambdaCore database uses several properties on #0, the system object, for various special purposes. For example, the value of #0.room is the ``generic room'' object, #0.exit is the ``generic exit'' object, etc. This allows MOO programs to refer to these useful objects more easily (and more readably) than using their object numbers directly. To make this usage even easier and more readable, the expression

$name

(where name obeys the rules for variable names) is an abbreviation for

#0.name

Thus, for example, the value $nothing mentioned earlier is really #-1, the value of #0.nothing.

As with variables, one uses the assignment operator (=) to change the value of a property. For example, the expression

14 + (#27.foo = 17)

changes the value of the foo property of the object numbered 27 to be 17 and then returns 31. Assignments to properties check that the owner of the current verb has write permission on the given property, returning E_PERM otherwise. Read permission is not required.

Calling Built-in Functions and Other Verbs

MOO provides a large number of useful functions for performing a wide variety of operations; a complete list, giving their names, arguments, and semantics, appears in a separate section later. As an example to give you the idea, there is a function named length that returns the length of a given string or list.

The syntax of a call to a function is as follows:

name(expr-1, expr-2, ..., expr-N)

where name is the name of one of the built-in functions. The expressions between the parentheses, called arguments, are each evaluated in turn and then given to the named function to use in its appropriate way. Most functions require that a specific number of arguments be given; otherwise, E_ARGS is returned. Most also require that certain of the arguments have certain specified types (e.g., the length() function requires a list or a string as its argument); E_TYPE is returned if any argument has the wrong type.

As with list construction, the splicing operator @ can precede any argument expression. The value of such an expression must be a list; E_TYPE is returned otherwise. The elements of this list are passed as individual arguments, in place of the list as a whole.

Verbs can also call other verbs, usually using this syntax:

expr-0:name(expr-1, expr-2, ..., expr-N)

Expr-0 must return an object number; E_TYPE is returned otherwise. If the object with that number does not exist, E_INVIND is returned. If this task is too deeply nested in verbs calling verbs calling verbs, then E_MAXREC is returned; the limit in LambdaMOO at this writing is 50 levels. If neither the object nor any of its ancestors defines a verb matching the given name, E_VERBNF is returned. Otherwise, if none of these nasty things happens, the named verb on the given object is called; the various built-in variables have the following initial values in the called verb:

this
an object, the value of expr-0
verb
a string, the name used in calling this verb
args
a list, the values of expr-1, expr-2, etc.
caller
an object, the value of this in the calling verb
player
an object, the same value as it had initially in the calling verb or, if the calling verb is running with wizard permissions, the same as the current value in the calling verb.

All other built-in variables (argstr, dobj, etc.) are initialized with the same values they have in the calling verb.

As with the discussion of property references above, I said ``usually'' at the beginning of the previous paragraph because that syntax is only allowed when the name follows the rules for allowed variable names. Also as with property reference, there is a syntax allowing you to compute the name of the verb:

expr-0:(expr-00)(expr-1, expr-2, ..., expr-N)

The expression expr-00 must return a string; E_TYPE is returned otherwise.

The splicing operator (@) can be used with verb-call arguments, too, just as with the arguments to built-in functions.

Parentheses and Operator Precedence

As shown in a few examples above, MOO allows you to use parentheses to make it clear how you intend for complex expressions to be grouped. For example, the expression

3 * (4 + 5)

performs the addition of 4 and 5 before multiplying the result by 3.

If you leave out the parentheses, MOO will figure out how to group the expression according to certain rules. The first of these is that some operators have higher precedence than others; operators with higher precedence will more tightly bind to their operands than those with lower precedence. For example, multiplication has higher precedence than addition; thus, if the parentheses had been left out of the expression in the previous paragraph, MOO would have grouped it as follows:

(3 * 4) + 5

The table below gives the relative precedence of all of the MOO operators; operators on higher lines in the table have higher precedence and those on the same line have identical precedence:

!       - (without a left operand)
*       /       %
+       -
==      !=      <       <=      >       >=      in
&&      ||
... ? ... | ... (the conditional expression)
=

Thus, the horrendous expression

x = a < b && c > d + e * f ? w in y | - q - r

would be grouped as follows:

x = (((a < b) && (c > (d + (e * f)))) ? (w in y) | ((- q) - r))

It is best to keep expressions simpler than this and to use parentheses liberally to make your meaning clear to other humans.

Statements

Statements are MOO constructs that, in contrast to expressions, perform some useful, non-value-producing operation. For example, there are several kinds of statements, called `looping constructs', that repeatedly perform some set of operations. Fortunately, there are many fewer kinds of statements in MOO than there are kinds of expressions.

Errors

Statements do not return values, but some kinds of statements can be used improperly and thus generate errors. If such an error is generated in a verb whose d (debug) bit is set, then an error message is printed to the current player and the current command (or task, really) is aborted. If the d bit is not set, then the error is ignored and the statement that generated it is simply skipped; execution proceeds with the next statement.

For simplicity in describing the meaning of statements below, though, I just say that a particular error value is generated by the statement.

Simple Statements

The simplest kind of statement is the null statement, consisting of just a semicolon:

;

It doesn't do anything at all, but it does it very quickly.

The next simplest statement is also one of the most common, the expression statement:

expression;

The given expression is evaluated and the resulting value is ignored. Commonly-used kinds of expressions for such statements include assignments and verb calls. Of course, there's no use for such a statement unless the evaluation of expression has some side-effect, such as changing the value of some variable or property, printing some text on someone's screen, etc.

Conditional Statements

The if statement allows you to decide whether or not to perform some statements based on the value of an arbitrary expression:

if (expression)
  statements
endif

Expression is evaluated and, if it returns a true value, the statements are executed in order; otherwise, nothing more is done.

One frequently wants to perform one set of statements if some condition is true and some other set of statements otherwise. The optional else phrase in an if statement allows you to do this:

if (expression)
  statements-1
else
  statements-2
endif

This statement is executed just like the previous one, except that statements-1 are executed if expression returns a true value and statements-2 are executed otherwise.

Sometimes, one needs to test several conditions in a kind of nested fashion:

if (expression-1)
  statements-1
else
  if (expression-2)
    statements-2
  else
    if (expression-3)
      statements-3
    else
      statements-4
    endif
  endif
endif

Such code can easily become tedious to write and difficult to read. MOO provides a somewhat simpler notation for such cases:

if (expression-1)
  statements-1
elseif (expression-2)
  statements-2
elseif (expression-3)
  statements-3
else
  statements-4
endif

Note that elseif is written as a single word, without any spaces. This simpler version has the very same meaning as the original: evaluate expression-i for i equal to 1, 2, and 3, in turn, until one of them returns a true value; then execute the statements-i associated with that expression. If none of the expression-i return a true value, then execute statements-4.

Any number of elseif phrases can appear, each having this form:

elseif (expression) statements

The complete syntax of the if statement, therefore, is as follows:

if (expression)
  statements
zero-or-more-elseif-phrases
an-optional-else-phrase
endif

Iteration

MOO provides three different kinds of looping statements, allowing you to have a set of statements executed (1) once for each element of a given list, (2) once for each number in a given range, and (3) over and over until a given condition stops being true.

To perform some statements once for each element of a given list, use this syntax:

for variable in (expression)
  statements
endfor

The expression is evaluated and should return a list; if it does not, E_TYPE is generated. The statements are then executed once for each element of that list in turn; each time, the given variable is assigned the value of the element in question. For example, consider the following statements:

odds = {1, 3, 5, 7, 9};
evens = {};
for n in (odds)
  evens = {@evens, n + 1};
endfor

The value of the variable evens after executing these statements is the list

{2, 4, 6, 8, 10}

The syntax for performing a set of statements once for each number in a given range is as follows:

for variable in [expression-1..expression-2]
  statements
endfor

The two expressions are evaluated in turn and should both return numbers; E_TYPE is generated otherwise. The statements are then executed once for each integer greater than or equal to the value of expression-1 and less than or equal to the result of expression-2, in increasing order. Each time, the given variable is assigned the integer in question. For example, consider the following statements:

evens = {};
for n in [1..5]
  evens = {@evens, 2 * n};
endfor

The value of the variable evens after executing these statements is just as in the previous example: the list

{2, 4, 6, 8, 10}

The final kind of loop in MOO executes a set of statements repeatedly as long as a given condition remains true:

while (expression)
  statements
endwhile

The expression is evaluated and, if it returns a true value, the statements are executed; then, execution of the while statement begins all over again with the evaluation of the expression. That is, execution alternates between evaluating the expression and executing the statements until the expression returns a false value. The following statements have precisely the same effect as the loop just shown above:

evens = {};
n = 1;
while (n <= 5)
  evens = {@evens, 2 * n};
  n = n + 1;
endwhile

With each kind of loop, it is possible that the statements in the body of the loop will never be executed at all. For iteration over lists, this happens when the list returned by the expression is empty. For iteration on numbers, it happens when expression-1 returns a larger number than expression-2. Finally, for the while loop, it happens if the expression returns a false value the very first time it is evaluated.

Returning a Value from a Verb

The MOO program in a verb is just a sequence of statements. Normally, when the verb is called, those statements are simply executed in order and then the number 0 is returned as the value of the verb-call expression. Using the return statement, one can change this behavior. The return statement has one of the following two forms:

return;

or

return expression;

When it is executed, execution of the current verb is terminated immediately after evaluating the given expression, if any. The verb-call expression that started the execution of this verb then returns either the value of expression or the number 0, if no expression was provided.

Executing Statements at a Later Time

It is sometimes useful to have some sequence of statements execute at a later time, without human intervention. For example, one might implement an object that, when thrown into the air, eventually falls back to the ground; the throw verb on that object should arrange to print a message about the object landing on the ground, but the message shouldn't be printed until some number of seconds have passed.

The fork statement is intended for just such situations and has the following syntax:

fork (expression)
  statements
endfork

The fork statement first executes the expression, which must return a number; call that number n. It then creates a new MOO task that will, after at least n seconds, execute the statements. When the new task begins, all variables will have the values they had at the time the fork statement was executed. The task executing the fork statement immediately continues execution. The concept of tasks is discussed in detail in the next section.

Occasionally, one would like to be able to kill a forked task before it even starts; for example, some player might have caught the object that was thrown into the air, so no message should be printed about it hitting the ground. If a variable name is given after the fork keyword, like this:

fork name (expression)
  statements
endfork

then that variable is assigned the task ID of the newly-created task. The value of this variable is visible both to the task executing the fork statement and to the statements in the newly-created task. This ID can be passed to the kill_task() function to keep the task from running and will be the value of task_id() once the task begins execution.

Tasks

A task is an execution of a MOO program. There are five kinds of tasks in LambdaMOO:


The last three kinds of tasks above are collectively known as queued tasks or waiting tasks, since they may not run immediately.

To prevent a maliciously- or incorrectly-written MOO program from running forever and monopolizing the server, limits are placed on the running time of every task. One limit is that no task is allowed to run longer than a certain number of seconds; command and server tasks get five seconds each while other tasks get only three seconds. This limit is, in practice, rarely reached. The reason is that there is also a limit on the number of operations a task may execute.

The server counts down ticks as any task executes. Roughly speaking, it counts one tick for every expression evaluation (other than variables and literals), one for every if, fork or return statement, and one for every iteration of a loop. If the count gets all the way down to zero, the task is immediately and unceremoniously aborted. Command and server tasks begin with an store of 30,000 ticks; this is enough for almost all normal uses. Forked, suspended, and reading tasks are allotted 15,000 ticks each.

Because queued tasks may exist for long periods of time before they begin execution, there are functions to list the ones that you own and to kill them before they execute. These functions, among others, are discussed in the following section.

Built-in Functions

There are a large number (over sixty at last count) of built-in functions available for use by MOO programmers. Each one is discussed in detail in this section. The presentation is broken up into subsections by grouping together functions with similar or related uses.

For most functions, the expected types of the arguments are given; if the actual arguments are not of these types, E_TYPE is returned. Some arguments can be of any type at all; in such cases, no type specification is given for the argument. Also, for most functions, the type of the result of the function is given. Some functions do not return a useful non-error result; in such cases, the specification none is used.

Most functions take a certain fixed number of required arguments and, in some cases, one or two optional arguments. If a function is called with too many or too few arguments, E_ARGS is returned.

Functions are always called by the program for some verb; that program is running with the permissions of some player, usually the owner of the verb in question (it is not always the owner, though; wizards can use set_task_perms() to change the permissions `on the fly'). In the function descriptions below, we refer to the player whose permissions are being used as the programmer.

Object-Oriented Programming

One of the most important facilities in an object-oriented programming language is ability for a child object to make use of a parent's implementation of some operation, even when the child provides its own definition for that operation. The pass() function provides this facility in MOO.

@defun pass (arg, ...) Often, it is useful for a child object to define a verb that augments the behavior of a verb on its parent object. For example, in the LambdaCore database, the root object (which is an ancestor of every other object) defines a verb called description that simply returns the value of this.description; this verb is used by the implementation of the look command. In many cases, a programmer would like the description of some object to include some non-constant part; for example, a sentence about whether or not the object was `awake' or `sleeping'. This sentence should be added onto the end of the normal description. The programmer would like to have a means of calling the normal description verb and then appending the sentence onto the end of that description. The function pass() is for exactly such situations.

pass calls the verb with the same name as the current verb but as defined on the parent of the object that defines the current verb. The arguments given to pass are the ones given to the called verb and the returned value of the called verb is returned from the call to pass. The initial value of this in the called verb is the same as in the calling verb.

Thus, in the example above, the child-object's description verb might have the following implementation:

return pass() + "  It is " + (this.awake ? "awake." | "sleeping.");

That is, it calls its parent's description verb and then appends to the result a sentence whose content is computed based on the value of a property on the object. @end defun

Manipulating MOO Values

There are several functions for performing primitive operations on MOO values, and they can be cleanly split into two kinds: those that do various type-checking and conversion operations between types, and those that are specific to one particular type. There are so many operations concerned with objects that we do not list them in this section but rather give them their own section following this one.

Type-Checking and Conversion

@deftypefun num typeof (value) Takes any MOO value and returns a number representing the type of value. The result is the same as the initial value of one of these built-in variables: NUM, STR, LIST, OBJ, or ERR. Thus, one usually writes code like this:

if (typeof(x) == LIST) ...

and not like this:

if (typeof(x) == 3) ...

because the former is much more readable than the latter. @end deftypefun

@deftypefun str tostr (value, ...) Converts all of the given MOO values into strings and returns the concatenation of the results.

tostr(17)                  @result{}   "17"
tostr(#17)                 @result{}   "#17"
tostr("foo")               @result{}   "foo"
tostr({1, 2})              @result{}   "{list}"
tostr(E_PERM)              @result{}   "Permission denied"
tostr("3 + 4 = ", 3 + 4)   @result{}   "3 + 4 = 7"

Note that tostr() does not do a good job of converting lists into strings; all lists, including the empty list, are converted into the string "{list}". @end deftypefun

@deftypefun num tonum (value) Converts the given MOO value into a number and returns that number. Object numbers are converted into the equivalent numbers, strings are parsed as the decimal encoding of a number, and errors are converted into numbers obeying the same ordering (with respect to <= as the errors themselves. tonum() returns E_TYPE if value is a list. If value is a string but the string does not contain a syntactically-correct number, then tonum() returns 0.

tonum(#34)         @result{}   34
tonum("34")        @result{}   34
tonum(" - 34  ")   @result{}   34
tonum(E_TYPE)      @result{}   1
@end deftypefun

@deftypefun obj toobj (value) Converts the given MOO value into an object number and returns that object number. The conversions are very similar to those for tonum() except that for strings, the number may be preceded by #.

toobj("34")       @result{}   #34
toobj("#34")      @result{}   #34
toobj("foo")      @result{}   #0
toobj({1, 2})     @error{}   E_TYPE
@end deftypefun

Operations on Numbers

@deftypefun num min (num x, ...) @deftypefunx num max (num x, ...) These two functions return the smallest or largest of their arguments, respectively. All of the arguments must be numbers; otherwise E_TYPE is returned. @end deftypefun

@deftypefun num abs (num x) Returns the absolute value of x. If x is negative, then the result is -x; otherwise, the result is x. @end deftypefun

@deftypefun num sqrt (num x) Returns the square root of x. If x is negative, then E_INVARG is returned. @end deftypefun

@deftypefun num random (num mod) mod must be a positive number; otherwise, E_INVARG is returned. A number is chosen randomly from the range [1..mod] and returned. @end deftypefun

@deftypefun num time () Returns the current time, represented as the number of seconds that have elapsed since midnight on 1 January 1970, Greenwich Mean Time. @end deftypefun

@deftypefun str ctime ([num time]) Interprets time as a time, using the same representation as given in the description of time(), above, and converts it into a 28-character, human-readable string in the following format:

Mon Aug 13 19:13:20 1990 PDT

If the current day of the month is less than 10, then an extra blank appears between the month and the day:

Mon Apr  1 14:10:43 1991 PST

If time is not provided, then the current time is used.

Note that ctime() interprets time for the local time zone of the computer on which the MOO server is running. @end deftypefun

Operations on Strings

@deftypefun num length (str string) Returns the number of characters in string. It is also permissible to pass a list to length(); see the description in the next section.

length("foo")   @result{}   3
length("")      @result{}   0
@end deftypefun

@deftypefun str strsub (str subject, str what, str with [, case-matters]) Replaces all occurrences in subject of what with with, performing string substitution. The occurrences are found from left to right and all substitutions happen simultaneously. By default, occurrences of what are searched for while ignoring the upper/lower case distinction. If case-matters is provided and true, then case is treated as significant in all comparisons.

strsub("%n is a fink.", "%n", "Fred")   @result{}   "Fred is a fink."
strsub("foobar", "OB", "b")             @result{}   "fobar"
strsub("foobar", "OB", "b", 1)          @result{}   "foobar"
@end deftypefun

@deftypefun str crypt (str text [, str salt]) Encrypts the given text using the standard UNIX encryption method. If provided, salt should be a two-character string for use as the extra encryption ``salt'' in the algorithm. If salt is not provided, a random pair of characters is used. In any case, the salt used is also returned as the first two characters of the resulting encrypted string.

Aside from the possibly-random selection of the salt, the encryption algorithm is entirely deterministic. In particular, you can test whether or not a given string is the same as the one used to produced a given piece of encrypted text; simply extract the first two characters of the encrypted text and pass the candidate string and those two characters to crypt(). If the result is identical to the given encrypted text, then you've got a match.

crypt("foobar")         @result{}   "J3fSFQfgkp26w"
crypt("foobar", "J3")   @result{}   "J3fSFQfgkp26w"
crypt("mumble", "J3")   @result{}   "J3D0.dh.jjmWQ"
crypt("foobar", "J4")   @result{}   "J4AcPxOJ4ncq2"
@end deftypefun

@deftypefun num index (str str1, str str2 [, case-matters]) @deftypefunx num rindex (str str1, str str2 [, case-matters]) The function index() (rindex()) returns the index of the first character of the first (last) occurrence of str2 in str1, or zero if str2 does not occur in str1 at all. By default the search for an occurrence of str2 is done while ignoring the upper/lower case distinction. If case-matters is provided and true, then case is treated as significant in all comparisons.

index("foobar", "o")        @result{}   2
rindex("foobar", "o")       @result{}   3
index("foobar", "x")        @result{}   0
index("foobar", "oba")      @result{}   3
index("Foobar", "foo", 1)   @result{}   0
@end deftypefun

@deftypefun num strcmp (str str1, str str2) Performs a case-sensitive comparison of the two argument strings. If str1 is lexicographically less than str2, the strcmp() returns a negative number. If the two strings are identical, strcmp() returns zero. Otherwise, strcmp() returns a positive number. The ASCII character ordering is used for the comparison. @end deftypefun

@deftypefun list match (str subject, str pattern [, case-matters]) @deftypefunx list rmatch (str subject, str pattern [, case-matters]) The function match() (rmatch()) searches for the first (last) occurrence of the regular expression pattern in the string subject. If pattern is syntactically malformed, then E_INVARG is returned. If no match is found, the empty list is returned; otherwise, these functions return a list containing information about the match (see below). By default, the search ignores upper/lower case distinctions. If case-matters is provided and true, then case is treated as significant in all comparisons.

The list that match() (rmatch()) returns contains the details about the match made. The list is in the form:

{start, end, replacements, subject}

where start is the index in subject of the beginning of the match, end is the index of the end of the match, replacements is a list described below, and subject is the same string that was given as the first argument to the match() or rmatch().

The replacements list is always nine items long, each item itself being a list of two numbers, the start and end indices in string matched by some parenthesized sub-pattern of pattern. The first item in replacements carries the indices for the first parenthesized sub-pattern, the second item carries those for the second sub-pattern, and so on. If there are fewer than nine parenthesized sub-patterns in pattern, or if some sub-pattern was not used in the match, then the corresponding item in replacements is the list {0, -1}. See the discussion of %), below, for more information on parenthesized sub-patterns.

match("foo", "^f*o$")        @result{}  {}
match("foo", "^fo*$")        @result{}  {1, 3, {{0, -1}, ...}, "foo"}
match("foobar", "o*b")       @result{}  {2, 4, {{0, -1}, ...}, "foobar"}
rmatch("foobar", "o*b")      @result{}  {4, 4, {{0, -1}, ...}, "foobar"}
match("foobar", "f%(o*%)b")
        @result{}  {1, 4, {{2, 3}, {0, -1}, ...}, "foobar"}

Regular expression matching allows you to test whether a string fits into a specific syntactic shape. You can also search a string for a substring that fits a pattern.

A regular expression describes a set of strings. The simplest case is one that describes a particular string; for example, the string foo when regarded as a regular expression matches foo and nothing else. Nontrivial regular expressions use certain special constructs so that they can match more than one string. For example, the regular expression foo%|bar matches either the string foo or the string bar; the regular expression c[ad]*r matches any of the strings cr, car, cdr, caar, cadddar and all other such strings with any number of a's and d's.

Regular expressions have a syntax in which a few characters are special constructs and the rest are ordinary. An ordinary character is a simple regular expression that matches that character and nothing else. The special characters are $, ^, ., *, +, ?, [, ] and %. Any other character appearing in a regular expression is ordinary, unless a % precedes it.

For example, f is not a special character, so it is ordinary, and therefore f is a regular expression that matches the string f and no other string. (It does not, for example, match the string ff.) Likewise, o is a regular expression that matches only o.

Any two regular expressions a and b can be concatenated. The result is a regular expression which matches a string if a matches some amount of the beginning of that string and b matches the rest of the string.

As a simple example, we can concatenate the regular expressions f and o to get the regular expression fo, which matches only the string fo. Still trivial.

The following are the characters and character sequences that have special meaning within regular expressions. Any character not mentioned here is not special; it stands for exactly itself for the purposes of searching and matching.

.
is a special character that matches any single character. Using concatenation, we can make regular expressions like a.b, which matches any three-character string that begins with a and ends with b.

*
is not a construct by itself; it is a suffix that means that the preceding regular expression is to be repeated as many times as possible. In fo*, the * applies to the o, so fo* matches f followed by any number of o's.

The case of zero o's is allowed: fo* does match f.

* always applies to the smallest possible preceding expression. Thus, fo* has a repeating o, not a repeating fo.

The matcher processes a * construct by matching, immediately, as many repetitions as can be found. Then it continues with the rest of the pattern. If that fails, it backtracks, discarding some of the matches of the *'d construct in case that makes it possible to match the rest of the pattern. For example, matching c[ad]*ar against the string caddaar, the [ad]* first matches addaa, but this does not allow the next a in the pattern to match. So the last of the matches of [ad] is undone and the following a is tried again. Now it succeeds.

+
+ is like * except that at least one match for the preceding pattern is required for +. Thus, c[ad]+r does not match cr but does match anything else that c[ad]*r would match.

?
? is like * except that it allows either zero or one match for the preceding pattern. Thus, c[ad]?r matches cr or car or cdr, and nothing else.

[ ... ]
[ begins a character set, which is terminated by a ]. In the simplest case, the characters between the two brackets form the set. Thus, [ad] matches either a or d, and [ad]* matches any string of a's and d's (including the empty string), from which it follows that c[ad]*r matches car, etc.

Character ranges can also be included in a character set, by writing two characters with a - between them. Thus, [a-z] matches any lower-case letter. Ranges may be intermixed freely with individual characters, as in [a-z$%.], which matches any lower case letter or $, % or period.

Note that the usual special characters are not special any more inside a character set. A completely different set of special characters exists inside character sets: ], - and ^.

To include a ] in a character set, you must make it the first character. For example, []a] matches ] or a. To include a -, you must use it in a context where it cannot possibly indicate a range: that is, as the first character, or immediately after a range.

[^ ... ]
[^ begins a complement character set, which matches any character except the ones specified. Thus, [^a-z0-9A-Z] matches all characters except letters and digits.

^ is not special in a character set unless it is the first character. The character following the ^ is treated as if it were first (it may be a - or a ]).

^
is a special character that matches the empty string -- but only if at the beginning of the string being matched. Otherwise it fails to match anything. Thus, ^foo matches a foo which occurs at the beginning of the string.

$
is similar to ^ but matches only at the end of the string. Thus, xx*$ matches a string of one or more x's at the end of the string.

%
has two functions: it quotes the above special characters (including %), and it introduces additional special constructs.

Because % quotes special characters, %$ is a regular expression that matches only $, and %[ is a regular expression that matches only [, and so on.

For the most part, % followed by any character matches only that character. However, there are several exceptions: characters that, when preceded by %, are special constructs. Such characters are always ordinary when encountered on their own.

No new special characters will ever be defined. All extensions to the regular expression syntax are made by defining new two-character constructs that begin with %.

%|
specifies an alternative. Two regular expressions a and b with %| in between form an expression that matches anything that either a or b will match.

Thus, foo%|bar matches either foo or bar but no other string.

%| applies to the largest possible surrounding expressions. Only a surrounding %( ... %) grouping can limit the grouping power of %|.

Full backtracking capability exists for when multiple %|'s are used.

%( ... %)
is a grouping construct that serves three purposes:

    To enclose a set of %| alternatives for other operations. Thus, %(foo%|bar%)x matches either foox or barx.

    To enclose a complicated expression for a following *, +, or ? to operate on. Thus, ba%(na%)* matches bananana, etc., with any number of na's, including none.

    To mark a matched substring for future reference.

This last application is not a consequence of the idea of a parenthetical grouping; it is a separate feature that happens to be assigned as a second meaning to the same %( ... %) construct because there is no conflict in practice between the two meanings. Here is an explanation of this feature:

%digit
After the end of a %( ... %) construct, the matcher remembers the beginning and end of the text matched by that construct. Then, later on in the regular expression, you can use % followed by digit to mean ``match the same text matched by the digit'th %( ... %) construct in the pattern.'' The %( ... %) constructs are numbered in the order that their %('s appear in the pattern.

The strings matching the first nine %( ... %) constructs appearing in a regular expression are assigned numbers 1 through 9 in order of their beginnings. %1 through %9 may be used to refer to the text matched by the corresponding %( ... %) construct.

For example, %(.*%)%1 matches any string that is composed of two identical halves. The %(.*%) matches the first half, which may be anything, but the %1 that follows must match the same exact text.

%b
matches the empty string, but only if it is at the beginning or end of a word. Thus, %bfoo%b matches any occurrence of foo as a separate word. %bball%(s%|%)%b matches ball or balls as a separate word.

For the purposes of this construct and the five that follow, a word is defined to be a sequence of letters and/or digits.

%B
matches the empty string, provided it is not at the beginning or end of a word.

%<
matches the empty string, but only if it is at the beginning of a word.

%>
matches the empty string, but only if it is at the end of a word.

%w
matches any word-constituent character (i.e., any letter or digit).

%W
matches any character that is not a word constituent.
@end deftypefun

@deftypefun str substitute (str template, list subs) Performs a standard set of substitutions on the string template, using the information contained in subs, returning the resulting, transformed template. Subs should be a list like those returned by match() or rmatch() when the match succeeds; otherwise, E_INVARG is returned.

In template, the strings %1 through %9 will be replaced by the text matched by the first through ninth parenthesized sub-patterns when match() or rmatch() was called. The string %0 in template will be replaced by the text matched by the pattern as a whole when match() or rmatch() was called. The string %% will be replaced by a single % sign. If % appears in template followed by any other character, E_INVARG will be returned.

subs = match("*** Welcome to LambdaMOO!!!", "%(%w*%) to %(%w*%)");
substitute("I thank you for your %1 here in %2.", subs)
        @result{}   "I thank you for your Welcome here in LambdaMOO."
@end deftypefun

Operations on Lists

@deftypefun num length (list list) Returns the number of elements in list. It is also permissible to pass a string to length(); see the description in the previous section.

length({1, 2, 3})   @result{}   3
length({})          @result{}   0
@end deftypefun

@deftypefun list listinsert (list list, value [, num index]) @deftypefunx list listappend (list list, value [, num index]) These functions return a copy of list with value added as a new element. listinsert() and listappend() add value before and after (respectively) the existing element with the given index, if provided.

The following three expressions always have the same value:

listinsert(list, element, index)
listappend(list, element, index - 1)
{@list[1..index - 1], element, @list[index..length(list)]}

If index is not provided, then listappend() adds the value at the end of the list and listinsert() adds it at the beginning; this usage is discouraged, however, since the same intent can be more clearly expressed using the list-construction expression, as shown in the examples below.

x = {1, 2, 3};
listappend(x, 4, 2)   @result{}   {1, 2, 4, 3}
listinsert(x, 4, 2)   @result{}   {1, 4, 2, 3}
listappend(x, 4)      @result{}   {1, 2, 3, 4}
listinsert(x, 4)      @result{}   {4, 1, 2, 3}
{@x, 4}               @result{}   {1, 2, 3, 4}
{4, @x}               @result{}   {4, 1, 2, 3}
@end deftypefun

@deftypefun list listdelete (list list, num index) Returns a copy of list with the indexth element removed. If index is not in the range [1..length(list)], then E_RANGE is returned.

x = {"foo", "bar", "baz"};
listdelete(x, 2)   @result{}   {"foo", "baz"}
@end deftypefun

@deftypefun list listset (list list, value, num index) Returns a copy of list with the indexth element replaced by value. If index is not in the range [1..length(list)], then E_RANGE is returned.

x = {"foo", "bar", "baz"};
listset(x, "mumble", 2)   @result{}   {"foo", "mumble", "baz"}
@end deftypefun

@deftypefun list setadd (list list, value) @deftypefunx list setremove (list list, value) Returns a copy of list with the given value added or removed, as appropriate. setadd() only adds value if it is not already an element of list; list is thus treated as a mathematical set. value is added at the end of the resulting list, if at all. Similarly, setremove() returns a list identical to list if value is not an element. If value appears more than once in list, only the first occurrence is removed in the returned copy.

setadd({1, 2, 3}, 3)         @result{}   {1, 2, 3}
setadd({1, 2, 3}, 4)         @result{}   {1, 2, 3, 4}
setremove({1, 2, 3}, 3)      @result{}   {1, 2}
setremove({1, 2, 3}, 4)      @result{}   {1, 2, 3}
setremove({1, 2, 3, 2}, 2)   @result{}   {1, 3, 2}
@end deftypefun

Manipulating Objects

Objects are, of course, the main focus of most MOO programming and, largely due to that, there are a lot of built-in functions for manipulating them.

Fundamental Operations

@deftypefun obj create (obj parent [, obj owner]) Creates and returns a new object whose parent is parent and whose owner is as described below. Either the given parent object must be fertile (i.e., its f bit must be set) or else the programmer must own parent or be a wizard; otherwise E_PERM is returned. E_PERM is also returned if owner is provided and not the same as the programmer, unless the programmer is a wizard. After the new object is created, its initialize verb, if any, is called with no arguments.

The new object is assigned the least non-negative object number that has not yet been used for a created object. Note that no object number is ever reused, even if the object with that number is recycled.

The owner of the new object is either the programmer (if owner is not provided), the new object itself (if owner was given as #-1), or owner (otherwise).

The other built-in properties of the new object are initialized as follows:

name         ""
location     #-1
contents     {}
programmer   0
wizard       0
r            0
w            0
f            0

In addition, the new object inherits all of the other properties on parent. These properties have the same permission bits as on parent. If the c permissions bit is set, then the owner of the property on the new object is the same as the owner of the new object itself; otherwise, the owner of the property on the new object is the same as that on parent. The initial value of every inherited property is clear; see the description of the built-in function clear_property() for details.

If the intended owner of the new object has a property named ownership_quota and the value of that property is a number, then create() treats that value as a quota. If the quota is less than or equal to zero, then the quota is considered to be exhausted and create() returns E_QUOTA instead of creating an object. Otherwise, the quota is decremented and stored back into the ownership_quota property as a part of the creation of the new object. @end deftypefun

@deftypefun none chparent (obj object, obj new-parent) Changes the parent of object to be new-parent. If the programmer is neither a wizard or the owner of object, or if new-parent is not fertile (i.e., its f bit is not set) and the programmer is neither the owner of new-parent nor a wizard, then E_PERM is returned. If object is not valid or if object or one of its descendants defines a property with the same name as one defined either on new-parent or on one of its ancestors, then E_INVARG is returned.

Changing an object's parent can have the effect of removing some properties from and adding some other properties to that object and all of its descendants (i.e., its children and its children's children, etc.). Let common be the nearest ancestor that object and new-parent have in common before the parent of object is changed. Then all properties defined by ancestors of object under common (that is, those ancestors of object that are in turn descendants of common) are removed from object and all of its descendants. All properties defined by new-parent or its ancestors under common are added to object and all of its descendants. As with create(), the newly-added properties are given the same permission bits as they have on new-parent, the owner of each added property is either the owner of the object it's added to (if the c permissions bit is set) or the owner of that property on new-parent, and the value of each added property is clear; see the description of the built-in function clear_property() for details. All properties that are not removed or added in the reparenting process are completely unchanged. @end deftypefun

@deftypefun num valid (obj object) Returns a non-zero number (i.e., a true value) if object is a valid object (one that has been created and not yet recycled) and zero (i.e., a false value) otherwise.

valid(#0)    @result{}   1
valid(#-1)   @result{}   0
@end deftypefun

@deftypefun obj parent (obj object) @deftypefunx list children (obj object) These functions return the parent and a list of the children of object, respectively. If object is not valid, then E_INVARG is returned. @end deftypefun

@deftypefun none recycle (obj object) The given object is destroyed, irrevocably. The programmer must either own object or be a wizard; otherwise, E_PERM is returned. If object is not valid, then E_INVARG is returned. The children of object are reparented to the parent of object. Before object is recycled, each object in its contents is moved to #-1 (implying a call to object's exitfunc verb, if any) and then object's recycle verb, if any, is called with no arguments.

After object is recycled, if the owner of the former object has a property named ownership_quota and the value of that property is a number, then recycle() treats that value as a quota and increments it by one, storing the result back into the ownership_quota property. @end deftypefun

@deftypefun obj max_object () Returns the largest object number yet assigned to a created object. Note that the object with this number may no longer exist; it may have been recycled. The next object created will be assigned the object number one larger than the value of max_object(). @end deftypefun

Movement

@deftypefun none move (obj what, obj where) Changes what's location to be where. This is a complex process because a number of permissions checks and notifications must be performed. The actual movement takes place as described in the following paragraphs.

what should be a valid object and where should be either a valid object or #-1 (denoting a location of 'nowhere'); otherwise E_INVARG is returned. The programmer must be either the owner of what or a wizard; otherwise, E_PERM is returned.

If where is a valid object, then the verb-call

where:accept(what)

is performed before any movement takes place. If the verb returns a false value and the programmer is not a wizard, then where is considered to have refused entrance to what; move() returns E_NACC. If where does not define an accept verb, then it is treated as if it defined one that always returned false.

If moving what into where would create a loop in the containment hierarchy (i.e., what would contain itself, even indirectly), then E_RECMOVE is returned instead.

The location property of what is changed to be where, and the contents properties of the old and new locations are modified appropriately. Let old-where be the location of what before it was moved. If old-where is a valid object, then the verb-call

old-where:exitfunc(what)

is performed and its result is ignored; it is not an error if old-where does not define a verb named exitfunc. Finally, if where and what are still valid objects, and where is still the location of what, then the verb-call

where:enterfunc(what)

is performed and its result is ignored; again, it is not an error if where does not define a verb named enterfunc. @end deftypefun

Operations on Properties

@deftypefun list properties (obj object) Returns a list of the names of the properties defined directly on the given object, not inherited from its parent. If object is not valid, then E_INVARG is returned. If the programmer does not have read permission on object, then E_PERM is returned. @end deftypefun

@deftypefun list property_info (obj object, str prop-name) @deftypefunx none set_property_info (obj object, str prop-name, list info) These two functions get and set (respectively) the owner and permission bits for the property named prop-name on the given object. If object is not valid, then E_INVARG is returned. If object has no non-built-in property named prop-name, then E_PROPNF is returned. If the programmer does not have read (write) permission on the property in question, then property_info() (set_property_info()) returns E_PERM. Property info has the following form:

{owner, perms}

where owner is an object and perms is a string containing only characters from the set r, w, and c. This is the kind of value returned by property_info() and expected as the third argument to set_property_info(); the latter function returns E_INVARG if owner is not valid or perms contains any illegal characters. @end deftypefun

@deftypefun none add_property (obj object, str prop-name, value, list info) Defines a new property on the given object, inherited by all of its descendants; the property is named prop-name, its initial value is value, and its owner and initial permission bits are given by info in the same format as is returned by property_info(), described above. If object is not valid or info does not specify a valid owner and well-formed permission bits or object or its ancestors or descendants already defines a property named prop-name, then E_INVARG is returned. If the programmer does not have write permission on object or if the owner specified by info is not the programmer and the programmer is not a wizard, then E_PERM is returned. @end deftypefun

@deftypefun none delete_property (obj object, str prop-name) Removes the property named prop-name from the given object and all of its descendants. If object is not valid, then E_INVARG is returned. If the programmer does not have write permission on object, then E_PERM is returned. If object does not directly define a property named prop-name (as opposed to inheriting one from its parent), then E_PROPNF is returned. @end deftypefun

@deftypefun num is_clear_property (obj object, str prop-name) @deftypefunx none clear_property (obj object, str prop-name) These two functions test for clear and set to clear, respectively, the property named prop-name on the given object. If object is not valid, then E_INVARG is returned. If object has no non-built-in property named prop-name, then E_PROPNF is returned. If the programmer does not have read (write) permission on the property in question, then is_clear_property() (clear_property()) returns E_PERM. If a property is clear, then when the value of that property is queried the value of the parent's property of the same name is returned. If the parent's property is clear, then the parent's parent's value is examined, and so on. If object is the definer of the property prop-name, as opposed to an inheritor of the property, then clear_property() returns E_INVARG. @end deftypefun

Operations on Verbs

@deftypefun list verbs (obj object) Returns a list of the names of the verbs defined directly on the given object, not inherited from its parent. If object is not valid, then E_INVARG is returned. If the programmer does not have read permission on object, then E_PERM is returned. @end deftypefun

The remaining operations on verbs use a string containing the verb's name to identify the verb in question. Because verbs can have multiple names and because an object can have multiple verbs with the same name, this practice can lead to difficulties. To most unambiguously refer to a particular verb, one can use a string containing the decimal representation of the (zero-based) index of the verb in the list returned by verbs(), described above.

For example, suppose that verbs(#34) returns this list:

{"foo", "bar", "baz", "foo"}

Object #34 has two verbs named foo defined on it (this may not be an error, if the two verbs have different command syntaxes). To refer unambiguously to the first one in the list, one uses the string "0"; to refer to the other one, one uses "3".

There is an unfortunate ambiguity that remains even with this numeric notation. Suppose that verbs(#34) returns this list:

{"foo", "bar 2", "baz"}

Object #34 has two verbs that could be meant by the numeric name "2": the one that also has the name bar and the one with the name baz. The server will in all cases assume that the reference is to the first verb in the list for which the given string could be a name, either in the normal sense or as a numeric index.

Note: Obviously, this is a terrible workaround for a nasty design bug. In a future version of the language, verbs may have unambiguous names that need have no relation to the command-line syntax (if any) for invoking the verb.

@deftypefun list verb_info (obj object, str verb-name) @deftypefunx none set_verb_info (obj object, str verb-name, list info) These two functions get and set (respectively) the owner, permission bits, and name(s) for the verb named verb-name on the given object. If object is not valid, then E_INVARG is returned. If object does not define a verb named verb-name, then E_VERBNF is returned. If the programmer does not have read (write) permission on the verb in question, then verb_info() (set_verb_info()) returns E_PERM. Verb info has the following form:

{owner, perms, names}

where owner is an object, perms is a string containing only characters from the set r, w, x, and d, and names is a string. This is the kind of value returned by verb_info() and expected as the third argument to set_verb_info(); the latter function returns E_INVARG if owner is not valid or perms contains any illegal characters. @end deftypefun

@deftypefun list verb_args (obj object, str verb-name) @deftypefunx none set_verb_args (obj object, str verb-name, list args) These two functions get and set (respectively) the direct-object, preposition, and indirect-object specifications for the verb named verb-name on the given object. If object is not valid, then E_INVARG is returned. If object does not define a verb named verb-name, then E_VERBNF is returned. If the programmer does not have read (write) permission on the verb in question, then verb_args() (set_verb_args()) returns E_PERM. Verb args specifications have the following form:

{dobj, prep, iobj}

where dobj and iobj are strings drawn from the set "this", "none", and "any", and prep is a string that is either "none", "any", or one of the prepositional phrases listed much earlier in the description of verbs in the first chapter. This is the kind of value returned by verb_info() and expected as the third argument to set_verb_info(). Note that for set_verb_args(), prep must be only one of the prepositional phrases, not (as is shown in that table) a set of such phrases separated by / characters. set_verb_args returns E_INVARG if any of the dobj, prep, or iobj strings is illegal.

verb_args($container, "take")
                    @result{}   {"any", "out of/from inside/from", "this"}
set_verb_args($container, "take", {"any", "from", "this"})
@end deftypefun

@deftypefun none add_verb (obj object, list info, list args) Defines a new verb on the given object. The new verb's owner, permission bits and name(s) are given by info in the same format as is returned by verb_info(), described above. The new verb's direct-object, preposition, and indirect-object specifications are given by args in the same format as is returned by verb_args, described above. The new verb initially has the empty program associated with it; this program does nothing but return an unspecified value.

If object is not valid, or info does not specify a valid owner and well-formed permission bits, or args is not a legitimate syntax specification, then E_INVARG is returned. If the programmer does not have write permission on object or if the owner specified by info is not the programmer and the programmer is not a wizard, then E_PERM is returned. @end deftypefun

@deftypefun none delete_verb (obj object, str verb-name) Removes the verb named verb-name from the given object. If object is not valid, then E_INVARG is returned. If the programmer does not have write permission on object, then E_PERM is returned. If object does not define a verb named verb-name, then E_VERBNF is returned. @end deftypefun

@deftypefun list verb_code (obj object, str verb-name [, fully-paren [, indent]]) @deftypefunx list set_verb_code (obj object, str verb-name, list code) These functions get and set (respectively) the MOO-code program associated with the verb named verb-name on object. The program is represented as a list of strings, one for each line of the program; this is the kind of value returned by verb_code() and expected as the third argument to set_verb_code(). For verb_code(), the expressions in the returned code are usually written with the minimum-necessary parenthesization; if full-paren is true, then all expressions are fully parenthesized. Also for verb_code(), the lines in the returned code are usually not indented at all; if indent is true, each line is indented to better show the nesting of statements.

If object is not valid, then E_INVARG is returned. If object does not define a verb named verb-name, then E_VERBNF is returned. If the programmer does not have read (write) permission on the verb in question, then verb_code() (set_verb_code()) returns E_PERM. If the programmer is not, in fact. a programmer, then E_PERM is returned.

For set_verb_code(), the result is a list of strings, the error messages generated by the MOO-code compiler during processing of code. If the list is non-empty, then set_verb_code() did not install code; the program associated with the verb in question is unchanged. @end deftypefun

Operations on Player Objects

@deftypefun list players () Returns a list of the object numbers of all player objects in the database. @end deftypefun

@deftypefun num is_player (obj object) Returns a true value if the given object is a player object and a false value otherwise. If object is not valid, E_INVARG is returned. @end deftypefun

@deftypefun none set_player_flag (obj object, value) Confers or removes the ``player object'' status of the given object, depending upon the truth value of value. If object is not valid, E_INVARG is returned. If the programmer is not a wizard, then E_PERM is returned.

If value is true, then object gains (or keeps) ``player object'' status: it will be an element of the list returned by players(), the expression is_player(object) will return true, and users can connect to object by name when they log into the server.

If value is false, the object loses (or continues to lack) ``player object'' status: it will not be an element of the list returned by players(), the expression is_player(object) will return false, and users cannot connect to object by name when they log into the server. In addition, if a user is connected to object at the time that it loses ``player object'' status, then that connection is immediately broken, just as if boot_player(object) had been called (see the description of boot_player() below). @end deftypefun

@deftypefun list connected_players () Returns a list of the object numbers of those player objects with currently-active connections. @end deftypefun

@deftypefun num connected_seconds (obj player) @deftypefunx num idle_seconds (obj player) These functions return the number of seconds that the currently-active connection to player has existed and been idle, respectively. If player is not the object number of a player object with a currently-active connection, then E_INVARG is returned. @end deftypefun

@deftypefun none notify (obj player, str string) Outputs string (on a line by itself) to the user connected to the given player. If the programmer is not player or a wizard, then E_PERM is returned. If there is no currently-active connection to player, then this function does nothing. @end deftypefun

@deftypefun none boot_player (obj player) Immediately terminates any currently-active connection to the given player. If the programmer is not either a wizard or the same as player, then E_PERM is returned. If there is no currently-active connection to player, then this function does nothing.

If there was a currently-active connection, then the following two verb calls are made before the connection is closed:

player:disfunc()
player.location:disfunc(player)

It is not an error if either of these verbs do not exist; the corresponding call is simply skipped. @end deftypefun

@deftypefun str connection_name (obj player) Returns a network-specific string identifying the connection being used by the given player. If the programmer is not a wizard and not player, then E_PERM is returned. If player is not currently connected, then E_INVARG is returned.

For the BSD UNIX network implementation (the only publicly-available one as of this writing), the string has the form

"number from host"

where number is a remarkably uninteresting internal server index and host is either the name or decimal TCP address of the host from which the player is connected. @end deftypefun

@deftypefun obj open_network_connection (value, ...) Establishes a network connection to the place specified by the arguments and more-or-less pretends that a new, normal player connection has been established from there. The new connection, as usual, will not be logged in initially and will have a negative object number associated with it for use with read(), notify(), and boot_player(). This object number is the value returned by this function.

If the programmer is not a wizard or if the OUTBOUND_NETWORK compilation option was not used in building the server, then E_PERM is returned. If the network connection cannot be made for some reason, then other errors will be returned, depending upon the particular network implementation in use.

For the TCP/IP network implementations (the only publicly-available ones as of this writing that support outbound connections), there must be two arguments, a string naming a host (possibly using the numeric Internet syntax) and a number specifying a TCP port. If a connection cannot be made because the host does not exist, the port does not exist, the host is not reachable or refused the connection, E_INVARG is returned. If the connection cannot be made for other reasons, including resource limitations, then E_QUOTA is returned.

It is worth mentioning one tricky point concerning the use of this function.

Since the server treats the new connection pretty much like any normal player connection, it will naturally try to parse any input from that connection as commands in the usual way. To prevent this treatment, it is necessary to ensure that some task is always suspended using read() on the connection whenever the server considers a line of input from it. That way, the line of input will be given to that task instead of being parsed as a command. The only reliable way to ensure this is for the task that opens the connection to enter an infinite loop reading from the connection. One possible structure for such a task is as follows:

conn = open_network_connection(...);
while (1)
  line = read(conn);
  fork (0)
    this:handle_input(line);
  endfork
endwhile

It is planned that this somewhat dubious feature will be removed in version 1.8.0 of the server, in which lines of input from outbound connections will not ever be treated as commands but will only be accessible using the read() function. @end deftypefun

Evaluation and Tasks

@deftypefun list eval (str string) The MOO-code compiler processes string as if it were to be the program associated with some verb and, if no errors are found, that fictional verb is invoked. If the programmer is not, in fact, a programmer, then E_PERM is returned. The normal result of calling eval() is a two element list. The first element is true if there were no compilation errors and false otherwise. The second element is either the result returned from the fictional verb (if there were no compilation errors) or a list of the compiler's error messages (otherwise).

When the fictional verb is invoked, the various built-in variables have values as shown below:

player    the same as in the calling verb
this      #-1
caller    the same as the initial value of this in the calling verb

args {} argstr ""

verb "" dobjstr "" dobj #-1 prepstr "" iobjstr "" iobj #-1

The fictional verb runs with the permissions of the programmer and as if its d permissions bit were on.

eval("return 3 + 4;")   @result{}   {1, 7}
@end deftypefun

@deftypefun none set_task_perms (obj who) Changes the permissions with which the currently-executing verb is running to be those of who. If the programmer is neither who nor a wizard, then E_PERM is returned.

Note: This does not change the owner of the currently-running verb, only the permissions of this particular invocation. It is used in verbs owned by wizards to make themselves run with lesser (usually non-wizard) permissions.

@end deftypefun

@deftypefun obj caller_perms () Returns the permissions in use by the verb that called the currently-executing verb. If the currently-executing verb was not called by another verb (i.e., it is the first verb called in a command or server task), then caller_perms() returns #-1. @end deftypefun

@deftypefun num ticks_left () @deftypefunx num seconds_left () These two functions return the number of ticks or seconds (respectively) left to the current task before it will be forcibly terminated. These are useful, for example, in deciding when to fork another task to continue a long-lived computation. @end deftypefun

@deftypefun num task_id () Returns the numeric identifier for the currently-executing task. Such numbers are randomly selected for each task and can therefore safely be used in circumstances where unpredictability is required. @end deftypefun

@deftypefun none suspend (num seconds) Suspends the current task, and resumes it after at least seconds seconds. When the task is resumed, it will have a full quota of ticks and seconds. This function is useful for programs that run for a long time or require a lot of ticks. If seconds is negative, then E_INVARG is returned.

In some sense, this function forks the `rest' of the executing task. However, there is a major difference between the use of suspend(seconds) and the use of the fork (seconds). The fork statement creates a new task (a forked task) while the currently-running task still goes on to completion, but a suspend() suspends the currently-running task (thus making it into a suspended task). This difference may be best explained by the following examples, in which one verb calls another:

.program   #0:caller_A
#0.prop = 1;
#0:callee_A();
#0.prop = 2;
.

.program #0:callee_A fork(5) #0.prop = 3; endfork .

.program #0:caller_B #0.prop = 1; #0:callee_B(); #0.prop = 2; .

.program #0:callee_B suspend(5); #0.prop = 3; .

Consider #0:caller_A, which calls #0:callee_A. Such a task would assign 1 to #0.prop, call #0:callee_A, fork a new task, return to #0:caller_A, and assign 2 to #0.prop, ending this task. Five seconds later, if the forked task had not been killed, then it would begin to run; it would assign 3 to #0.prop and then stop. So, the final value of #0.prop (i.e., the value after more than 5 seconds) would be 3.

Now consider #0:caller_B, which calls #0:callee_B instead of #0:callee_A. This task would assign 1 to #0.prop, call #0:callee_B, and suspend. Five seconds later, if the suspended task had not been killed, then it would resume; it would assign 3 to #0.prop, return to #0:caller, and assign 2 to #0.prop, ending the task. So, the final value of #0.prop (i.e., the value after more than 5 seconds) would be 2.

A suspended task, like a forked task, can be described by the queued_tasks() function and killed by the kill_task() function. Suspending a task does not change its task id. A task can be suspended again and again by successive calls to suspend().

Once suspend() has been used in a particular task, then the read() function will always return E_PERM in that task. For more details, see the description of read() below. @end deftypefun

@deftypefun str read ([obj player]) This function suspends the current task, waiting for a line of input from the given player (which defaults to the player that typed the command that initiated the current task), and resumes when such a line is received, returning that line as a string. Upon resumption, the task is given a full quota of ticks and seconds. If player is given explicitly, then the programmer must either be a wizard or the owner of player; without an explicit player argument, read() may only be called by a wizard and only in a command task that has never been suspended by a call to suspend(). Otherwise, E_PERM is returned. If the given player is not currently connected and has no pending lines of input, or if the connection is closed before any lines of input are received, then read() returns E_INVARG.

These restrictions on the use of read() without an explicit argument preserve the following simple invariant: if input is being read from a player, it is for the task started by the last command that player typed. This invariant adds responsibility to the programmer, however. If your program calls another verb before doing a read(), then that verb must also not suspend. As an example, consider the following, which refers to the verbs programmed in the suspend() example above:

.program   #0:read_twice_A
s = read();     /* success depends on programmer's permissions */
#0:callee_A();  /* callee_A does not suspend */
t = read();     /* success depends on programmer's permissions */
.

.program #0:read_twice_B s = read(); /* success depends on programmer's permissions */ #0:callee_B(); /* callee_B does suspend */ t = read(); /* fails, returning E_PERM */ .

In three of the four calls to read(), success depends on on the programmer's permissions. However, the second read() in #0:read_twice_B always fails and returns E_PERM. This is because the task was suspended, even though #0:read_twice_B did not do the actual suspending. Hence, if you want to read some input (by using read() directly or by calling other verbs which do the reading), you must make sure that the task is not suspended before the reading. This includes making sure that any verbs you call, directly or indirectly, also do not suspend.

It is possible to call read() many times in the same command task, so long as the task does not call suspend() or an explicit argument is given. The best use for read() with an explicit argument is in conjunction with open_network_connection(), if it is enabled. @end deftypefun

@deftypefun list queued_tasks () Returns information on each of the forked, suspended or reading tasks owned by the programmer (or, if the programmer is a wizard, all queued tasks). The returned value is a list of lists, each of which encodes certain information about a particular queued task in the following format:

{task-id, start-time, ticks, clock-id,
 programmer, verb-loc, verb-name, line, this}

where task-id is a numeric identifier for this queued task, start-time is the time after which this task will begin execution (in time() format), ticks is the number of ticks this task will have when it starts (always 20,000 now), clock-id is a number whose value is no longer interesting, programmer is the permissions with which this task will begin execution (and also the player who owns this task), verb-loc is the object on which the verb that forked this task was defined at the time, verb-name is that name of that verb, line is the number of the first line of the code in that verb that this task will execute, and this is the value of the variable this in that verb. For reading tasks, start-time is -1.

The ticks and clock-id fields are now obsolete and are retained only for backward-compatibility reasons. They may disappear in a future version of the server. @end deftypefun

@deftypefun none kill_task (num task-id) Removes the task with the given task-id from the queue of waiting tasks. If the programmer is not the owner of that task and not a wizard, then E_PERM is returned. If there is no task on the queue with the given task-id, then E_INVARG is returned. @end deftypefun

@deftypefun list callers () Returns information on each of the verbs currently waiting to resume execution in the current task. When one verb calls another verb, execution of the caller is temporarily suspended, pending the called verb returning a value. At any given time, there could be several such pending verbs: the one that called the currently executing verb, the verb that called that one, and so on. The result of callers() is a list, each element of which gives information about one pending verb in the following format:

{this, verb-name, programmer, verb-loc, player}

where this is the initial value of the variable this in that verb, verb-name is the name used to invoke that verb, programmer is the player with whose permissions that verb is running, verb-loc is the object on which that verb is defined, and player is the initial value of the variable player in that verb.

The first element of the list returned by callers() gives information on the verb that called the currently-executing verb, the second element describes the verb that called that one, and so on. The last element of the list describes the first verb called in this task. @end deftypefun

@deftypefun list output_delimiters (obj player) Returns a list of two strings, the current output prefix and output suffix for player. If player does not have an active network connection, then E_INVARG is returned. If either string is currently undefined, the value "" is used instead. See the discussion of the PREFIX and SUFFIX commands in the next chapter for more information about the output prefix and suffix. @end deftypefun

Administrative Operations

@deftypefun str server_version () Returns a string giving the version number of the MOO server in the following format:

"major.minor.release"

where major, minor, and release are all decimal numbers.

The major version number changes very slowly, only when existing MOO code might stop working, due to an incompatible change in the syntax or semantics of the programming language, or when an incompatible change is made to the database format.

The minor version number changes more quickly, whenever an upward-compatible change is made in the programming language syntax or semantics. The most common cause of this is the addition of a new kind of expression, statement, or built-in function.

The release version number changes as frequently as bugs are fixed in the server code. Changes in the release number indicate changes that should only be visible to users as bug fixes, if at all. @end deftypefun

@deftypefun none server_log (str message [, is-error]) The text in message is sent to the server log with a distinctive prefix (so that it can be distinguished from server-generated messages). If the programmer is not a wizard, then E_PERM is returned. If is-error is provided and true, then message is marked in the server log as an error. @end deftypefun

@deftypefun obj renumber (obj object) The object number of the object currently numbered object is changed to be the least nonnegative object number not currently in use and the new object number is returned. If object is not valid, then E_INVARG is returned. If the programmer is not a wizard, then E_PERM is returned. If there are no unused nonnegative object numbers less than object, then object is returned and no changes take place.

The references to object in the parent/children and location/contents hierarchies are updated to use the new object number, and any verbs, properties and/or objects owned by object are also changed to be owned by the new object number. The latter operation can be quite time consuming if the database is large. No other changes to the database are performed; in particular, no object references in property values or verb code are updated.

This operation is intended for use in making new versions of the LambdaCore database from the then-current LambdaMOO database, and other similar situations. Its use requires great care. @end deftypefun

@deftypefun none reset_max_object () The server's idea of the highest object number ever used is changed to be the highest object number of a currently-existing object, thus allowing reuse of any higher numbers that refer to now-recycled objects. If the programmer is not a wizard, then E_PERM is returned.

This operation is intended for use in making new versions of the LambdaCore database from the then-current LambdaMOO database, and other similar situations. Its use requires great care. @end deftypefun

@deftypefun list memory_usage () On some versions of the server, this returns statistics concerning the server consumption of system memory. The result is a list of lists, each in the following format:

{block-size, nused, nfree}

where block-size is the size in bytes of a particular class of memory fragments, nused is the number of such fragments currently in use in the server, and nfree is the number of such fragments that have been reserved for use but are currently free.

On servers for which such statistics are not available, memory_usage() returns {}. The compilation option USE_GNU_MALLOC controls whether or not statistics are available; if the option is not provided, statistics are not available. @end deftypefun

@deftypefun none dump_database () Requests that the server checkpoint the database at its next opportunity. It is not normally necessary to call this function; the server automatically checkpoints the database at regular intervals; see the chapter on server assumptions about the database for details. If the programmer is not a wizard, then E_PERM is returned. @end deftypefun

@deftypefun none shutdown (str message) Requests that the server shut itself down at its next opportunity. Before doing so, the given message is printed to all connected players. If the programmer is not a wizard, then E_PERM is returned. @end deftypefun