Hello Lawrence, Il 03 agosto 2021 alle 17:14 Galaxy Being ha scritto:
According to this http://zvon.org/other/haskell/Outputprelude/elem_f.html elem is
Eq a => a -> [a] -> Bool
but according to my ghci :t it's this
elem :: (Foldable t, Eq a) => a -> t a -> Bool
I understand the first, but not the second, especially with the t. What is this saying extra, different from the first one?
Yup, some years ago there was a shift of some functions — after a big discussion, as there were a few controversial changes — from «working on lists only» to «working on all instances of some class» (in this case: Foldable, alias «data structures that can be folded») [1]. What is there for you? You gain the ability to use `elem` on other structures than lists (e.g. Trees). If you — like me — do not fancy reading signatures with too many typeclasses, you can always use +d in ghci to default to concrete types: λ> :t foldr foldr :: Foldable t => (a -> b -> b) -> b -> t a -> b λ> :t +d foldr foldr :: (a -> b -> b) -> b -> [a] -> b [1] https://wiki.haskell.org/Foldable_Traversable_In_Prelude