[Haskell-cafe] instance Monad m => Functor m

Jules Bean jules at jellybean.co.uk
Wed Apr 9 05:26:11 EDT 2008

Hans Aberg wrote:
> Using 'hugs -98', I noticed it accepts:
>   instance Monad m => Functor m where
>     fmap f x = x >>= return.f
> Has this been considered (say) as a part of the upcoming Haskell Prime?

This forbids any Functors which are not monads. Unless you allow 
overlapping instances (which of course would not be h98 any more!).

Other solutions, such as class Functor m => Monad m are frequently 

I see no H' ticket for it, though.


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