This might help?
https://hackage.haskell.org/package/constraints-0.9.1/docs/Data-Constraint.h...
Le 7 mai 2017 06:21, "Clinton Mead"
I posted the following on stackoverflow http://stackoverflow.com/questions/43827374/what-is-a-can-be-derived-from-co..., 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 xinstance Integral x => MyIntegral x class Num x => MyNum xinstance 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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