On Wed, 17 Nov 2010 00:03:05 -0700
Rainer Deyke <rain...@eldwood.com> wrote:
> Making functions weakly pure by default means that temporarily adding a
> tiny debug printf to any function will require a shitload of cascading
> 'impure' annotations. I would consider that completely unacceptable.
Output in general, programmer feedback in particuliar, should simply not be
considered effect. It is transitory change to dedicated areas of memory -- not
state. Isn't this the sense of "output", after all? (One cannot read it back,
thus it has no consequence on future process.) The following is imo purely
referentially transparent and effect-free (where effect means changing state);
it always executes the same way, produces the same result, and never influences
later processes else as via said result:
uint square(uint n) {
uint sq = n*n;
writefln("%s^2 = %s", n, sq);
return sq;
}
Sure, the physical machine's state has changed, but it's not the same machine
(state) as the one the program runs on (as the one the program can play with).
There is some bizarre confusion.
[IMO, FP's notion of purity is at best improper for imperative programming (&
at worst requires complicated hacks for using FP itself). We need to find our
own way to make programs easier to understand and reason about.]
Denis
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