Question

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What's the “|” for in a Haskell class definition?

I’m pretty new to Haskell. In the documentation of MonadState I see the following:

class Monad m => MonadState s m | m -> s where
    get :: m s
    put :: s -> m ()

What is the | m -> s syntax here?

Answer 1

It’s called a functional dependency or fundep for short. The syntax

class Monad m => MonadState s m | m -> s where

means, that there is only one instance for each m or – in other words, that if m is known, the compiler can infer the type of s form that. Using fundeps makes coding a lot easier, because the compiler can infer much more.