[Haskell-cafe] Choosing a type-class instance based on the context

Cristiano Paris frodo at theshire.org
Wed Jan 5 17:36:32 CET 2011

Hi all,

I was reading:


and I became curious. Playing with the code I started to find a way to say:

instance Show a => ShowPred a HTrue

instead of enumerating all the instances, mirroring those of the Show class:

instance ShowPred Int  HTrue
instance ShowPred Bool HTrue
instance ShowPred a flag => ShowPred [a] flag

Would that be possible? My opinion is that's not possible but I want
to hear what the type-system gurus think about it.



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