[Haskell-cafe] Typeclasses -- if does not satisfy

[Dmitry Bogatov]

* adam vogt <vogt.adam at gmail.com> [2014-05-24 12:27:18-0400]
> Hi Dmitry,
>
> It's pretty straightforward to use OverlappingInstances:
>
> class Runnable a b where
>       run :: a -> b
>
> instance (r ~ (s -> (m (a,s)))) => Runnable (StateT s m a) r where
>       run m = runStateT m
>
> instance (r ~ (s -> (a,s))) => Runnable (StateT s Identity a) r where
>       run m = runState m
>
> Involving a type family doesn't simplify things, since you still need
> two class instances to choose between runStateT and runState.

So simple and so good. Thank you.

--
Best regards, Dmitry Bogatov <KAction at gnu.org>,
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