learning-haskell/20-foldable/20.5-library-functions.md

20.5 Some basic derived operations

Why does fmap length Just [1, 2, 3] return 1?

fmap :: Functor f => (a -> b) -> f a -> f b
length :: Foldable t => t a -> Int
Just :: a -> Maybe a

fmap length :: (Foldable t, Functor f) => f (t a) -> f Int
-- notice that the next argument given is an f (t a) and since
-- (->) is also a functor f is ((->) a) and (t a) is Maybe a
-- this also makes f Int an (a -> Int)
fmap length Just :: a -> Int

Exercises: Library Functions

see src/functions.h