Haskell: How does MonadState's put work?
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05-07-2021 - |
Question
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?
Solution
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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