Is it possible to have “dynamic” function pointer in rebol?

Question

I can create p as a pointer to f like this:

f: func[][print 1]
p: :f

When executing:

>> p
1

But if I modify f to

f: func[][print 1 print 2]

p won't print 2.

Is there another way to get p points "dynamically" to f ?

Answer 1

Note you've created a new instance of f there - not modified it. You can modify f though by altering the contents of its second block. ie...

>> f: func[][print 1]
>> p: :f
>> p
1
>> append second :f [print 2]
== [print 1 print 2]
>> p
1
2

Hope that helps.

Answer 2

You could use an alias.

Alias in REBOL is a little limited as you cannot use any pre-existing word as an alias. But this works for me:

f: does [print 1]    ;; define 'f as you do
alias 'f "px"        ;; create an alias as 'px
px 
== 1                 ;; same result as 'f

f: context [a: 3]    ;; change 'f to something completely different
probe px             ;; 'px is the same as the new 'f
    make object! [
        a: 3
    ]

Answer 3

when you do F: :P you are actually doing F "points to" P's value (the function).

when you do P: func [][] again, you are creating a new function value to which only P is assigned the value.

the only way to have indirection is via an accessor, something like F: does [P]

when p changes value f will know to call the new function since the binding of P has not changed, only its value (which is a new function).

alias are word equivalencies, not pointers, so they are also valid as words on paths and are quite nasty in real world use.