From: Pavol Droba (droba_at_[hidden])
Date: 2003-10-24 12:55:56
On Fri, Oct 24, 2003 at 01:23:58PM -0400, David Abrahams wrote:
> Pavol Droba <droba_at_[hidden]> writes:
>
> > On Thu, Oct 23, 2003 at 11:21:54PM -0400, David Abrahams wrote:
> >> Pavol Droba <droba_at_[hidden]> writes:
> >
> > [snip]
> >
> >> >> ?? We're not talking about making a copy, since the result differs
> >> >> from the input. In general, you pay the same price in functional
> >> >> languages.
> >> >>
> >> >
> >> > So there is realy NO copy performed in Haskell when evaluating the function,
> >> > it is a transformation. And this is much closer to inplace version,
> >> > in C++ then to copy one.
> >>
> >> Gimme a break, man! In pure FP languages data is immutable, so
> >> there's no way to make inplace changes. In Haskell you have to go
> >> into a monad to escape this restriction.
> >>
> >
> > I know that this discusion is out of topic, quite a bit, but here is my
> > explanation what I mean, when I say, that nothing is copied:
> >
> > Most of modern FP languages, perform the execution in terms of graph-reduction machine.
> > High level syntax is precompiled to a extended version of lambda calculus
> > and this is then executed.
> >
> > State of an execution is represented by a term, which is represented by a graph.
> > This graph is being reduced as the lambda calculi reduction rules are
> > applied.
> >
> > There is nothing like a stack or variables in this representation. So unless
> > some subterm is bound to more then one expression, it can be safetly modified
> > in-place. And thats usualy what is being done.
>
> This is my last post on the matter:
>
> Yes, I know that's often what's being done. However, we never
> said the subterm was only going to be bound to one expression.
> You just added that. The two-expression binding case, of course,
> is exactly the one where immutability of data makes a difference
> in safety.
>
> Over and out
Ok, we were going nowhere. And it was already out of topic anyway.
Pavol