[REBOL] Re: Functional programming in REBOL
From: lmecir:mbox:vol:cz at: 12-Aug-2001 9:46
Hi Larry,
> Your solutions for all three puzzles are correct. Here is one
> more combinator which is more difficult to explain (at least for myself):
>
> ???: cfunc [f][
> do
> cfunc [x][f func [y][do x :x :y]]
> cfunc [x][f func [y][do x :x :y]]
> ]
>
> What does the function ??? do? And what requirements must its function
> argument meet in order to guarantee that it produces a result?
>
> Cheers
> -Larry
>
> PS This is one where I thought perhaps cfunc and this whole approach to
> functional programming in REBOL would break down, but it seems to work
just
> fine.
Very interesting! The ??? function looks like an impersonalized Russell'd
paradox. I didn't know about such functions until now. If I define:
id: func [x] [:x]
then the result:
beast: ??? :id
is pretty well defined. The problem is, that the BEAST function is a
function of one argument, but the result of its evaluation is totally
undefined. In Rebol the expression:
beast 1
Produces only stack overflow, but that is the best thing we can expect. If F
supplied as an argument to ??? tried to evaluate its argument e.g. like:
f: func [x] [x 1 0]
Then, although the result of F should be 0 for X being a function taking one
argument, the result of ??? :f cannot be evaluated. Similarly any attempt to
supply a function evaluating its argument or any attempt to allow such an
evaluation later cannot produce a sensible result, because ??? is a
self-referencing definition.
Nice indeed.
