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From: kend@data.UUCP (Ken Dickey)
Newsgroups: comp.lang.misc
Subject: Re^4: On whether C has first-class composable functions
Message-ID: <444@data.UUCP>
Date: 8 Jan 91 19:55:27 GMT
References: <442@data.UUCP> <4408:Jan421:44:3391@kramden.acf.nyu.edu> <443@data.UUCP> <13759:Jan800:19:2391@kramden.acf.nyu.edu>
Organization: Microcosm, Beaverton, OR
Lines: 56




In comp.lang.misc you (Dan Bernstein) write:

>In article <443@data.UUCP> kend@data.UUCP (Ken Dickey) writes:
[Quote from Guy Steele]
>>   SCHEME also permits the treatment of functions as full flegded data
>>   objects; they may be passed as arguments, returned as values, made
>>   part of composite data structures, and notated as independent, unnamed
>>   ("anonymous") entities.  {Contrast this with most ALGOL-like
>>   languages, in which a function can be written only by declaring it and
>>   giving it a name; imagine being able to use an integer value only by
>>   giving it a name in a declaration!).

Dan>``Full-fledged data objects,'' yes; but ``first-class'' doesn't mean
Dan>``full-fledged.''

Actually, they are synonymous in the literature.  E.g.: the T Manual **
states 

	Procedures are "full fledged" data objects in T

while a paper on T language *** states 

       procedures are "first-class citizens"; they may be stored ...

[Same authors, same year].


The whole idea of being a full fledged or first-class citizen is
having the maximal rights of citizenship.  In most languages, integers
are first class citizens.  I don't have to name (declare) numbers
before use, I can pass them as parameters, return them as results, I
can generate new numbers in a fairly unconstrained way.  Pointers are
first-class data objects in C.  I can take the address of any location
I can specify, as many times as I want, etc.  Functions in C are
constrained in ways that do not allow them first class status.  To
quote from a previous missive:

>Your definition of unnamed functions did not magically turn them into
>function pointers. Either you are trying to return a function by value,
>which is impossible in C without a *semantic* extension; or you are
>returning a pointer to that function, in which case, as Karl said, the
>pointer is useless after the return.

Q.E.D.

-Ken Dickey
================================================================

** "The T Manual, Pre-release edition, 2nd Printing, 2 July 1982, Yale
U., by Norman Adams andJonathan Rees.

*** "T a Dialect of Lisp, or LAMBDA: The Ultimate Software Tool",
Conference Record of the 1982 ACM Symposium on Lisp and Functional