Weaken Monad constraints of {eval,exec}StateT to Functor.

[sjshuck]

[original issue 94 by @Icelandjack]

Is it possible to weaken the constraints of evalStateT and execStateT from Monad to Functor?

evalStateT :: Functor f => StateT s f a -> s -> f a
evalStateT m s = fst <$> runStateT m s

execStateT :: Functor f => StateT s f a -> s -> f s
execStateT m s = snd <$> runStateT m s

This came up translating withIndex (https://github.com/mstksg/inCode/blob/e1f80a3dfd83eaa2b817dc922fd7f331cd1ece8a/app-purescript/Entry.purs#L279) to Haskell

withIndex
    :: forall s t a b f. Functor f  -- Monad f
    => ((a -> StateT Int f b) -> (s -> StateT Int f t))
    -> (Int -> a -> f b) -> (s -> f t)
withIndex lens f = (`myEvalStateT` 0) . lens f'
  where
    f' :: a -> StateT Int f b
    f&#39; y = StateT \i -> (\z -> (z, i+1)) <$> f i y