General Concept

MudOS has a variable type named ‘function’. Variables of this type may be used to point to a wide variety of functions. You are probably already familiar with the idea of passing a function to certain efuns. Take, for example, the filter efun. It takes an array, and returns an array containing the elements for which a certain function returns non-zero. Traditionally, this was done by passing an object and a function name. Now, it can also be done by passing an expression of type ‘function’ which merely contains information about a function, which can be evaluated later.

Function pointers can be created and assigned to variables:

function f = (: local_func :);

Passed to other routines or efuns, just like normal values:

foo(f);
map_array( ({ 1, 2 }), f);

Or evaluated at a later time:

x = evaluate(f, "hi");

When the last line is run, the function that f points to is called, and “hi” is passed to it. This will create the same effect as if you had done:

x = local_func("hi");

The advantage of using a function pointer is that if you later want to use a different function, you can just change the value of the variable.

Note that if evaluate() is passed a value that is not a function, it just returns the value. So you can do something like:

void set_short(mixed x) { short = x; }
mixed query_short() { return evaluate(short); }

This way, simple objects can simply do: set_short("Whatever"), while objects that want their shorts to change can do: set_short( (: short_func :) );

Available kinds of function pointers:

The simplest function pointers are the ones shown above. These simply point to a local function in the same object, and are made using (: function_name :). Arguments can also be included; for example:

string foo(string a, string b) {
    return "(" + a "," + b + ")";
}

void create() {
    function f = (: foo, "left" :);

    printf( "%s %s\n", evaluate(f), evaluate(f, "right") );

}

Will print:

(left,0) (left,right)

The second kind is the efun pointer, which is just (: efun_name :). This is very similar to the local function pointer. For example, the objects() efun takes a optional function, and returns all objects for which the function is true, so:

objects( (: clonep :) )

will return an array of all the objects in the game which are clones. Arguments can also be used:

void create() {
    int i;
    function f = (: write, "Hello, world!\n" :);

    for (i=0; i<3; i++) { evaluate(f); }

}

Will print:

Hello, world!
Hello, world!
Hello, world!

Note that simul_efuns work exactly like efuns with respect to function pointers.

The third type is the call_other function pointer, which is similar to the type of function pointer MudOS used to support. The form is (: object, function :). If arguments are to be used, the should be added to an array along with the function name. Here are some examples:

void create()
{
    string *ret;
    function f = (: this_player(), "query" :);

    ret = map( ({ "name", "short", "long" }), f );
    write(implode(ret, "\n"));
}

This would print the results of this_player()->query("name"), this_player()->query("short"), and this_player()->query("long"). To make a function pointer that calls query("short") directly, use:

f = (: this_player(), ({ "query", "short" }) :)

For reference, here are some other ways of doing the same thing:

// a efun pointer using the call_other efun
f = (: call_other, this_player(), "query", "short" :);
// an expression functional
f = (: this_player()->query("short") :);

The fourth type is the expression function pointer. It is made using (: expression :). Within an expression function pointer, the arguments to it can be refered to as $1, $2, $3 …, for example:

evaluate( (: $1 + $2 :), 3, 4) // returns 7.

This can be very useful for using sort_array, for example:

top_ten = sort_array( player_list,
(: $2->query_level() - $1->query_level :) )[0..9];

The fifth type is an anonymous function:

void create() {
    function f = function(int x) {
        int y;

        switch(x) {
            case 1: y = 3;break;
            case 2: y = 5;
        }
        return y - 2;
    };

    printf("%i %i %i\n", (*f)(1), (*f)(2), (*f)(3));
}

would print:

1 3 -2

Note that (*f)(...) is the same as evaluate(f, ...) and is retained for backwards compatibility. Anything that is legal in a normal function is legal in an anonymous function.

When are things evaluated?

The rule is that arguments included in the creation of efun, local function, and simul_efun function pointers are evaluated when the function pointer is made. For expression and functional function pointers, nothing is evaluated until the function pointer is actually used:

    // When it is _evaluated_, it will destruct
    // whoever "this_player()" was when it
    // was _made_
    (: destruct, this_player() :)

    // destructs whoever is "this_player()"
    // when the function is _evaluated_
    (: destruct(this_player()) :)

For this reason, it is illegal to use a local variable in an expression pointer, since the local variable may no longer exist when the function pointer is evaluated. However, there is a way around it:

(: destruct( $(this_player) ) :) // Same as the first example above

$(whatever) means evaluate whatever, and hold it’s value, inserting it when the function is evaluated. It also can be used to make things more efficient:

map_array(listeners,
(: tell_object($1, $(this_player()->query_name()) + " bows.\n") :) );

only does one call_other, instead of one for every message. The string addition could also be done before hand:

map_array(listeners,
(: tell_object($1, $(this_player()->query_name() + " bows.\n")) :) );

Notice, in this case we could also do:

map_array(listeners,
(: tell_object, this_player()->query_name() + " bows.\n" :) );
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