Explanation of composite with foldl
I see this on the Haskell 99 questions https://wiki.haskell.org/99_questions/Solutions/2 which will return the second-from-last element of a list lastbut1 :: Foldable f => f a -> a lastbut1 = fst . foldl (\(a,b) x -> (b,x)) (err1,err2) where err1 = error "lastbut1: Empty list" err2 = error "lastbut1: Singleton" I understand how the code works, but not the significance of the type declaration. That looks like a type class. It works without it, I believe. Why have we used Foldable type class? Likewise with this lastbut1safe :: Foldable f => f a -> Maybe a lastbut1safe = fst . foldl (\(a,b) x -> (b,Just x)) (Nothing,Nothing) What's happening with the type definition? LB
Il 13 gennaio 2021 alle 15:09 Lawrence Bottorff ha scritto:
I see this on the Haskell 99 questions https://wiki.haskell.org/99_questions/Solutions/2 which will return the second-from-last element of a list
lastbut1 :: Foldable f => f a -> a lastbut1 = fst . foldl (\(a,b) x -> (b,x)) (err1,err2) where err1 = error "lastbut1: Empty list" err2 = error "lastbut1: Singleton"
I understand how the code works, but not the significance of the type declaration. That looks like a type class. It works without it, I believe. Why have we used Foldable type class?
A `Foldable` constraint will make the function work on a plethora of types [1] apart from lists (e.g. Arrays, etc.). The assignment is clearly monomorphic («Find the last but one element of a list.»), I would have preferred a `:: [a] -> a` signature. [1] https://hackage.haskell.org/package/base-4.14.1.0/docs/Data-Foldable.html#t:...
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Francesco Ariis -
Lawrence Bottorff