CODEWITHSUNDEEP
haskell
user3139545
user3139545

Reputation: 7374

Typechecking in function compositions

Why is not ghci throwing a type error when I do the following?

let d = tail . head

Should this not say that tail wants [a] while head gives it only a?

Upvotes: 4

Views: 90

Answers (1)

mr-
mr-

Reputation: 320

If the types were more specialized, like

head :: [Int] -> Int
tail :: [Int] -> [Int]

then tail.head would be ill typed indeed. But as it stands, the types are

head :: [a] -> a
tail :: [b] -> [b]

I have used the type variables "a" and "b" so that you don't think they have to be the same. In that case, having a = [b], the types become

head :: [[b]] -> [b]
tail :: [b] -> [b]

and they are composable.

Upvotes: 8

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