Спросил, почему join (***) :: a b c → a (b, b) (c, c):
[20:15] <Cale> d34df00d: So, in general fmap :: (a → b) → f a → f b, so for f = (e →), we get fmap :: (a → b) → (e → a) → (e → b), which is exactly the type of composition
[20:15] <Cale> and for join :: f (f a) → f a, we get (e → (e → a)) → (e → a)
[20:16] <Cale> which pretty much tells you what it does right from the type

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