> Yes, in general it's not possible. That is, I can't write a function
>
> evaluate :: a -> a
>
> which will force its argument to WHNF.
I don't think you mean what you are actually saying, it's perfectly possible
write evaluate :-)
evaluate x = x
This function will evaluate its argument when called.
What you cannot write is a function that evaluates its argument completely,
nor to write a function
evaluate' :: a -> b -> b
which evaluates the first argument to WHNF and returns the second. For both of
these you can use Kevin's trick.
-- Lennart
