问题
In Haskell why is type-signature of forever
forever :: Monad m => m a -> m b
Specifically why isn't it just :: Monad m => m a -> m a?
Surely the type of monad we are acting upon doesn't change half way through forever?
A function such as:
forever' :: Monad m => m a -> m a
forever' = forever
seems to work exactly the same.
回答1:
The type signature of forever is crafted to suggest that its result runs forever. Specifically, there is no way to write an action of type m b (polymorphic in its return value) that terminates and yields a non-bottom value. An alternative way to suggest the same thing would be forever' :: m a -> m Void.
Another answer is to just say that this is the most general type available for the function as it's defined, so that's the one it was given.
Prelude> let forever m = let x = m >> x in x
Prelude> :t forever
forever :: Monad m => m a -> m b
These days, it probably should be defined differently:
forever :: Applicative f => f a -> f b
forever a = let x = a *> x in x
来源：https://stackoverflow.com/questions/30170279/haskell-forever-type-signature