[Haskell-cafe] *GROUP HUG*

[Daniel Fischer]

On Thursday 02 June 2011 01:12:37, Tom Murphy wrote:
> > How about this:
> > 
> > myFoldr :: (a -> b -> b) -> b -> [a] -> b
> > myFoldr f z xs = foldl' (\s x v -> s (x `f` v)) id xs $ z
> > 
> > Cheers,
> > Ivan
> 
> Great! Now I really can say "Come on! It's fun! I can write foldr with
> foldl!"
> 

Unfortunately, you can't, not quite.

A left fold cannont return anything before it has reached the end of the 
list, so it doesn't work on infinite lists.
With a suitable combining function (and suitable input), a right fold can 
start delivering the result early, hence right folds (can) produce results 
for infinite lists (and my example was meant to draw attention to that 
fact).

Cheers,
Daniel