[Haskell-cafe] Noob error: Type b -> c b Does not match IO a

[Henning Thielemann]

On Mon, 27 Jun 2005, Daniel Fischer wrote:

> Am Sonntag, 26. Juni 2005 21:02 schrieben Sie:
> > On Sun, 26 Jun 2005, Daniel Fischer wrote:
> > > >    m x y = if x==0 then 0 else x*y
> > >
> > > Plain
> > >
> > > foldr m 1
> > >
> > > does fine, in fact much better than
> > >
> > > foldl' (*) 1 . upTo (== 0),
> > >
> > > both in hugs and ghc, regarding speed and memory usage.
> >
> > E.g.
> >   foldr m 1 [a,b,c]
> >
> > means
> >   m a (m b (m c 1)))
> >
> > That is, it is not possible for the runtime system to store interim
> > numeric results, it can only accumulate function calls.
>
> Well, yes, but since 'm' is lazy in the second argument, the list isn't
> traversed past the first zero - like foldr (&&) True.

That was not the point. The point is that "m a (m b (m c..." can't be
reduced immediately to, say, "(m (m a b) c) ...", since this is completely
different. The computation can only start when the end of the list is
reached.