On Wed, Feb 27, 2008 at 03:04:03PM +0100, Loup Vaillant wrote:
> Yes, it does, but I have difficulties reducing this expression. 

Where is the problem?

> Do you have another example which can be reduced to head normal form
> (say the result is an int instead of a list)?

Not with ground types, but would the factorial function help?

f (x,g) = (g x, g') where
  g' 0 = 1
  g' y = y * g (y-1)

*Main> [loop f x | x <- [0..10]]
[1,1,2,6,24,120,720,5040,40320,362880,3628800]

One can imagine that in a similar way to the list: The function g
goes round the loop and more and more results get defined.

BTW, lazy evaluation uses weak head normal form (WHNF), not head normal
form.

- Dirk