http://hackage.haskell.org/packages/archive/mtl/1.1.0.2/doc/html/src/Control-Monad-State-Lazy.html
instance (Monad m) => MonadState s (StateT s m) where
get = StateT $ \s -> return (s, s)
put s = StateT $ \_ -> return ((), s)
What does the () do in the definition of put?
The () is the return value of the action. Since put
is used for its side effect (change state), it doesn't return anything useful.
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