Le 7 mai 2017 06:21, "Clinton Mead" <clintonmead@gmail.com> a écrit :

I posted the following on stackoverflow, but it hasn't got too much attention so I thought I'd ask here:

I can write the following:

{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE ConstraintKinds #-}

f :: Integral a => (forall b. Num b => b) -> a
f = id

And all is good. Presumably GHC can derive Integral from Num so all is well.

I can be a bit tricker, yet I'm still fine:

class Integral x => MyIntegral x
instance Integral x => MyIntegral x

class Num x => MyNum x
instance Num x => MyNum x

f' :: MyIntegral a => (forall b. MyNum b => b) -> a
f' = id

So lets say I want to generalise this, like so:

g :: c2 a => (forall b. c1 b => b) -> a
g = id

Now obviously this will spit the dummy, because GHC can not derive c2 from c1, as c2 is not constrained.

What do I need to add to the type signature of g to say that "you can derive c2 from c1"?

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