[Haskell-cafe] Re: defining last using foldr

[Aaron Denney]

On 2007-08-14, Chaddaï Fouché <chaddai.fouche at gmail.com> wrote:
> 2007/8/14, Alexteslin <alexteslin at yahoo.co.uk>:
>> Well, i have tried cons (:) operator but when it passed to foldr doesn't work
>> because cons operator operates first character and then the list but the
>> foldr argument takes a function (a->a->a).  Maybe i am missing the point
>> here?
>>
>
> What Aaron was saying was that in this list :
> 1 : 2 : 3 : 4 : []
> A fold replaced the cons (:) by another function (and [] by another constant).
>
> Your problem isn't so easy to do with a foldr, a foldl would be easier
> and a foldr1 or foldl1 even better. Are you sure you can't use one of
> those other folds ?

The problem with foldl is that you can't easily make it polymorphic
because of how the null case is handled.  foldl1 and foldr1 are trivial,
true.

-- 
Aaron Denney
-><-