Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!samsung!crackers!jjmhome!smds!sw
From: sw@smds.UUCP (Stephen E. Witham)
Newsgroups: comp.lang.misc
Subject: Re: On whether C has first-class composable functions
Summary: Dynamically allocating functionoids
Message-ID: <307@smds.UUCP>
Date: 28 Jan 91 19:38:40 GMT
References: <4408:Jan421:44:3391@kramden.acf.nyu.edu> <1991Jan26.132913.11358@spool.cs.wisc.edu>
Organization: SMDS Inc., Concord, MA
Lines: 40

In article <1991Jan26.132913.11358@spool.cs.wisc.edu>, quale@picard.cs.wisc.edu (Douglas E. Quale) writes:
> 
> We'll forget polymorphism.  With C it's hopeless.  (Hence C++).

Yeah.  The impossible part is composing functions that RETURN different
types.  You could make all your functions deal with some "universal" type,
like a pointer to a void, which you would cast to a pointer to a structure
full of whatever, but that's pretty awful.

By the way, C++ is sometimes implemented as a translator-into-C, so if you
want to do something C++ does, but in C, ALL YOU HAVE TO DO :-} is make sure
you're as thorough and methodical as a C++ compiler, and put up with some 
horrible syntax!
 
> Show me an implementation that doesn't have a fixed limit on the number of
> functions that can be composed.  The only portable C implementation 
> demonstrated so far has an arbitrary limit compiled in.  How do I
> dynamically allocate function objects in C??

I guess you missed my earlier posting.  Since it dealt with functions from
ints to ints (and would require different compose functions to deal with
functions that returned different types), I guess you don't want the 
details.  The main idea, though, is that you invent a structure...

   struct Func { int (*f)( void *, int ); void *instanceVariables; };

...and an "apply" function (it's dangerous as a macro):

   int apply ( f, arg )
      struct Func *f;
      int arg;
      {
      return (f)->f ( (f)->instanceVariables, arg );
      }

The struct Funcs can be alloc'ed as you like.  Mail me if you want the
details.  Or, if anybody wants, I'll repost it.

--Steve Witham
Not-the-fault-of: SMDS, Inc., Concord, MA