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" :) );