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From: quale@picard.cs.wisc.edu (Douglas E. Quale)
Newsgroups: comp.lang.misc
Subject: Re: On whether C has first-class composable functions
Message-ID: <1991Jan29.153549.19220@spool.cs.wisc.edu>
Date: 29 Jan 91 15:35:49 GMT
References: <4408:Jan421:44:3391@kramden.acf.nyu.edu> <1991Jan26.132913.11358@spool.cs.wisc.edu> <307@smds.UUCP>
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Organization: U of Wisconsin CS Dept
Lines: 37

My question:

>> 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??
>

And the answer:

>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.

Sorry, this doesn't work because compose returns a structure, not a function.

Suppose I want to use the library function twalk(3c).
It takes a function as an argument.  Will it accept the output of your
compose, or do I have to rewrite every library function that takes functional
parameters to provide two versions?

Note:  The interpreter hack fails here as well.

-- Doug Quale
quale@picard.cs.wisc.edu