Talk:First-class functions

Even C could?

• Functions can be stored in other collection types of the language... yes (pointers)...
• Functions can be used as arguments to other functions... yes (pointers)
• Functions can be returned as the value of functions... yes (pointers)

• New functions can be created from others at run time... maybe?

The last one is unclear to me. I can guess what it means, but every language (like C) can build a runtime that allows functions to be created from others at run time; you can build a library allowing that... meaning that with such a library C becomes a language with first-class functions...

I bet classifiers say that C functions are not first class... And if you can write such code, this does not mean that the language has first-class functions... so all these theoretical classifications regard only the primitive syntax (or better semantics?) of a language...? Or could I try to cheat? (Like I did for one-liner...!) --ShinTakezou 16:39, 24 February 2009 (UTC)

Yes, it's that last point that is crucial. In standard portable C, you can get a pointer to any function that was written in the original source code (of your program or a library), but there's no way to construct an arbitrary number of new functions.

You could simulate it by having a table and a set of functions referring to entries in it; but you would have to deal with allocating this limited resource. For example:

<lang c>typedef int (*ourfunc)(int);

ourfunc[4][2] composeData; int freeRow = 0;

int compose_0(int x) { return composeData[0][0](composeData[0][1](x)); } int compose_1(int x) { return composeData[1][0](composeData[1][1](x)); } int compose_2(int x) { return composeData[2][0](composeData[2][1](x)); } int compose_3(int x) { return composeData[3][0](composeData[3][1](x)); } ourfunc functions[4] = {compose_0, compose_1, compose_2, compose_3};

ourfunc compose(ourfunc a, ourfunc b) {

 composeData[freeRow][0] = a;
 composeData[freeRow][1] = b;
 return functions[freeRow++];

}</lang>

But this technique will only work for a fixed number of functions -- it is basically emulating closures by having a predefined table of closure entry points and data storage. --Kevin Reid 17:54, 24 February 2009 (UTC)