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

24 May 2014


      * adam vogt  [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 ,
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