Foldl as foldr
(Difference between revisions)
Revision as of 22:01, 16 February 2009
When you wonder whether to choose foldl or foldr you may remember,that both
can be expressed as
may lean so far right it came back left again.) The converse is not true, since
may work on infinite lists, which
variants never can do.
foldl :: (a -> b -> a) -> a -> [b] -> a foldl f a bs = foldr (\b g x -> g (f x b)) id bs a
Now the question are:
- How can someone find a convolved expression like this?
- How can we benefit from this rewrite?
The answer to the second question is:We can write a
that may stop before reaching the end of the input list
and thus may also terminate on infinite input.The function
as result when it encounters a
as interim accumulator result.
foldlMaybe :: (a -> b -> Maybe a) -> a -> [b] -> Maybe a foldlMaybe f a bs = foldr (\b g x -> f x b >>= g) Just bs a