exercises of https://haskellbook.com/

• Semigroup: 半群，结合律，封闭
• Monoid: Semigroup + 幺元
• Functor: A functor is a way to apply a function over or around some structure that we don’t want to alter. F a -> (a -> b) -> F b
• Applicative: F a -> F (a -> b) -> F b
• Monad: M a -> (a -> M b) -> M b

Each argument (and result) in the type signature for a function must be a fully applied (and inhabitable, modulo Void, etc.) type. Each argument must have the kind *.