fmap for Strict StateT is too lazy

[meooow25]

Following #122 (comment) I was taking a look at the code out of curiosity and noticed that there is

transformers/Control/Monad/Trans/State/Strict.hs

Lines 191 to 194 in f1a27e4

	instance (Functor m) => Functor (StateT s m) where
	fmap f m = StateT $ \ s ->
	fmap (\ (a, s') -> (f a, s')) $ runStateT m s
	{-# INLINE fmap #-}

This is too lazy and doesn't obey the Functor-Monad law that fmap f m = m >>= return . f

>>> import Control.Monad.Trans.State.Strict
>>> runStateT (fmap (const ()) (StateT (\_ -> pure undefined))) () *> print "ok"
"ok"
>>> runStateT (StateT (\_ -> pure undefined) >>= pure . const ()) () *> print "ok"
*** Exception: Prelude.undefined

[sjshuck]

Thanks for the report @meooow25.

Introducing strictness would be a major version bump, which is already in the works. We'll get it in for 0.7.0.0.