Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!bloom-beacon!apple!desnoyer
From: desnoyer@Apple.COM (Peter Desnoyers)
Newsgroups: comp.lang.c
Subject: Re: C optimizer
Message-ID: <27606@apple.Apple.COM>
Date: 20 Mar 89 21:05:34 GMT
References: <36662@think.UUCP> <453@lakart.UUCP> <1470@mcgill-vision.UUCP> <21652@agate.BERKELEY.EDU>
Organization: Apple Computer Inc, Cupertino, CA
Lines: 34

In article <21652@agate.BERKELEY.EDU> bowles@eris.berkeley.edu (Jeff A. Bowles) writes:
>>>> In article <1028@frog.UUCP> john@frog.UUCP (John Woods) writes:
>>> [various people arguing whether getpid() is pure]
>>
>Are we still on this topic? By the bogosity above, nothing is "pure",
>because you might write a function that changes the behaviour of another
>function - for example, [...]

I beg to differ. I would consider that any "pure" function that does
not take arguments to be a constant, as otherwise it cannot depend
solely on its arguments, but must return a value dependent on state
information external to the function.

How about these definitions:

(note that I am considering "implicit" results, e.g.
status = f(arguments,&result) style calls return two results.)

  pure - if f is pure and f takes arguments Ai and returns results Ri,
         then for any action B which does not modify A or R:
           R=f(A),B,R=f(A) is equiv. to R=f(A),B 
			   is equiv. to B,R=f(A)

  conditionally pure - if f is conditionally pure, then one can define
         a dependency set D such that if B is some action with does
         not modify A or R, and does not contain any elements of D,
         then R=f(A),B,R=f(A) is equiv. to R=f(A),B is equiv. to
         B,R=f(A). 

It is a lot simpler for a compiler to take advantage of pure functions
than conditionally pure ones, because it doesn't have to worry about
dependencies. 

				Peter Desnoyers