[Haskell-beginners] Functional programming concepts

[Daniel Fischer]

Am Mittwoch, 27. August 2008 13:54 schrieb Rafael Gustavo da Cunha Pereira 
Pinto:
>     Hi folks!
>
>     Since I don't have a CS degree, there are some concepts I don't fully
> understand, and I'd like to propose a "begginers" thread to discuss some of
> those topics.
>
>     For starters, what does a WHNF (weak-head normal form) mean?
>
>     Cheers,
> -- 
> Rafael Gustavo da Cunha Pereira Pinto
> Electronic Engineer, MSc.


You can find a definition at
http://burks.bton.ac.uk/burks/foldoc/53/126.htm
It's not entirely helpful if you don't know some lambda-calculus.

I don't have a CS degree either, so someone please correct me if I'm wrong (or 
confirm if I'm right). 
In Haskell, reducing a term to whnf (which e.g. seq is for) means evaluating 
it far enough that its top level constructor is known (or it's found to be 
bottom).
For some types like Int, that means complete evaluation, for others it's far 
less. 
A list is in whnf if you know whether it's bottom, [] or h:t, where you might 
or might not know something about h and t.
A Maybe term is in whnf if you know if it's bottom, Nothing or Just whatever.
A function is in whnf if you know if it's bottom or \x -> rhs.

HTH,
Daniel