Pointers in Haskell??
Fri, 7 Dec 2001 18:29:49 GMT
In-Reply-To: Hamilton Richards's message of Fri, 7 Dec 2001 12:11:23 -0600
> In my previous example,
> > t = [0..]
> > b = 3 : t
> > c = 5 : t
> lists b and c share t, but in
> > x = 3 : [0..]
> > y = 5 : [0..]
> lists x and y share nothing. Extensionally, they have the same values as b
> and c, but each has its own copy of [0..].
That's because you're interpreting "same value" extensionally.
The same sort of issue comes up in Lisp or Java. But suppose
in Java I have
Object x = new Whatever();
and that new object has some large substructures. I can get
one of those substructures to be shared, rather than copied,
without having a variable whose value is that substructure.
For instance, the substructure might be shared in
= new Object(x.getSubstructure(),
provided that x.getSubstructure() returns the very same substructure
(not a copy) each time.
> Unless, that is, the compiler is clever enough to recognize the
> subexpression [0..] which x and y have in common. I don't know whether any
> Haskell compilers look for common subexpressions, but it's certainly an
> So the question of whether names are "*the* (only) way" to obtain
> sharing isn't really a language question-- it's more of a compiler question.
Are they the only way that's guaranteed to result in sharing, or is
even that not the case?