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.
Note that the old style of function pointers (: ob, string :) are no longer supported. They are equivalent to (: call_other, ob, string :) in the new syntax. Here are some other examples:
f = (: call_other, this_player(), "query", "short" :) // a efun pointer using // the call_other efun f = (: this_player()->query("short") :) // an expression functional; see // below.
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; 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:
(: destruct, this_player() :) // 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*
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" :) );