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.
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