[Haskell-cafe] upgrading mtl1 to mtl2

Sebastian Fischer fischer at nii.ac.jp
Thu Feb 17 10:59:21 CET 2011

On Thu, Feb 17, 2011 at 4:57 PM, Max Bolingbroke <batterseapower at hotmail.com
> wrote:

> I think the problem is that the mtl1 Functor instances looked like:
> instance Monad m => Functor (ReaderT e m) where
>  fmap = ...
> But the mtl2/transformers instances look like:
> instance Functor f => Functor (ReaderT e f) where
>  fmap = ...

I see! So the problem is not that previously "(Monad m) implied (Functor m)"
(which has been proposed only recently) but that previously many Functor
instances required (Monad m) but now require (Functor m).

Thanks for the clarification,
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