Constraints aren't considered when instance selection is performed. I.e. you can't have both:
instance Foo a where ...
instance Show a => Foo a where ....

But you can use default method signatures: https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts.html#default-method-signatures

It appear more like Show, you can add a  constraint to the generic instance

instance Show a => Foo a where

    bar x = show x

Em 29 de jun de 2017 2:16 PM, "Silent Leaf" <silent.leaf0@gmail.com> escreveu:

hi,

say i have the following typeclass:

class Foo a where

  bar :: a  -> String

looks a lot like the Read typeclass, right? (at least i think it should?)

well say it's a different meaning (in other terms i can't or do not want to use Read, but i'd like to implement a default version of bar for those instances that also implement Read. is there a way to do so?

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