CODEWITHSUNDEEP
haskellmonads
hgiesel
hgiesel

Reputation: 5648

join function for two-kinded monads

I'm trying to get a better understanding of monads. So I'm trying to write return, join, and bind implementations for several monads.

However when coming to two-kinded monads, I'm kind of confused

join :: m (m a) -> m a

m (m a) Implies monad wrapped inside of monad, but what value is implied if using two-kinded monads. For example with the State Monad: s or a? What would the correct signature of join for the State Monad look like?

Upvotes: 0

Views: 102

Answers (1)

Li-yao Xia
Li-yao Xia

Reputation: 33419

The state monad type is declared as State s a; it only unifies with m a if m ~ State s.

join :: State s (State s a) -> State s a

Upvotes: 10

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