[Haskell-cafe] Functions are first class values in C

[Philippa Cowderoy]

On Sat, 22 Dec 2007, Cristian Baboi wrote:

> On Sat, 22 Dec 2007 16:55:08 +0200, Miguel Mitrofanov
> <miguelimo38 at yandex.ru> wrote:
> 
> > > > > In Haskell I cannot pass a function to a function, only its
> > > > > expansion.
> > > 
> > > > What do you mean by "expansion"? Can you clarify this?
> > > 
> > > f1=\x->x+1
> > > f2=\x->2*x
> > > g=\x->x.f1
> > > h=\x->x.(\x->x+1)
> > > 
> > > h is g
> > 
> > Not clear. Try starting with "function's expansion is..."
> 
> 
> function's expansion is ... just like macro expansion.

No, it's not. Expanding variables (swapping f1 for \x->x+1) isn't the 
evaluation mechanism in haskell, g and h really are semantically 
equivalent values and we can't do actual computation just by expanding 
variables. Whereas expanding a macro is equivalent to beta-reduction 
(evaluating a function application), which isn't required before passing 
something in Haskell. We pass functions, not just their results.

Here's a trivial example that does so:

(\x -> x) (\x -> x)

A lambda calculus classic that doesn't typecheck in Haskell:

(\x -> x x) (\x -> x x)

Feel free to try evaluating it!

-- 
flippa at flippac.org

"I think you mean Philippa. I believe Phillipa is the one from an
alternate universe, who has a beard and programs in BASIC, using only
gotos for control flow." -- Anton van Straaten on Lambda the Ultimate