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.
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" :) );