2009/7/22 Matthias Görgens
I need to take some elements from the front of a list. However the criteria are somewhat complex.
walk f [] = [] walk f (x:xs) = case f x of Just g -> x : walk g xs Nothing -> []
For each item the `predicate' f either returns Nothing, when it thinks we should not take any more elements, or return Just another `predicate' to apply to the next element.
However the type system does not like my function. How can I mollify it?
At a quick glance it looks to me like the type of f is infinite. Something like: f :: a -> Maybe (a -> Maybe (a -> ...)) When I've seen people solve similar problems in the past they have introduced a data type to hide the infinite type. See for example, the solution to defining fix: http://blog.plover.com/prog/springschool95-2.html I didn't actual read that article, but it appears to explain the technique. Jason