Hi, I have been using constraints of the form: xxx :: forall x xs ys zs m. ( Monad m , zs ~ Union (Filter x xs) ys , Catchable x xs , Liftable (Filter x xs) zs , Liftable ys zs ) => Variant xs -> (x -> Flow m ys) -> Flow m zs Where "zs" is used as a type alias. Now with GHC8 and -Wredundant-constraints, GHC complaints that "zs" is redundant (indeed it is). Is there a way to do this properly? If not, could we introduce a new syntax to make this kind of local declaration? We could borrow the syntax of local declarations into list-comprehensions: "let zs ~ ..." Thanks, Sylvain
On Fri, Oct 14, 2016 at 07:25:40PM +0200, Sylvain Henry wrote:
I have been using constraints of the form:
xxx :: forall x xs ys zs m. ( Monad m , zs ~ Union (Filter x xs) ys , Catchable x xs , Liftable (Filter x xs) zs , Liftable ys zs ) => Variant xs -> (x -> Flow m ys) -> Flow m zs
Where "zs" is used as a type alias. Now with GHC8 and -Wredundant-constraints, GHC complaints that "zs" is redundant (indeed it is).
Why is it redundant? Surely a redundant constraint is one you can freely remove. I don't see any constraint you can remove in that type.
On 14/10/2016 19:31, Tom Ellis wrote:
On Fri, Oct 14, 2016 at 07:25:40PM +0200, Sylvain Henry wrote:
I have been using constraints of the form:
xxx :: forall x xs ys zs m. ( Monad m , zs ~ Union (Filter x xs) ys , Catchable x xs , Liftable (Filter x xs) zs , Liftable ys zs ) => Variant xs -> (x -> Flow m ys) -> Flow m zs
Where "zs" is used as a type alias. Now with GHC8 and -Wredundant-constraints, GHC complaints that "zs" is redundant (indeed it is). Why is it redundant? Surely a redundant constraint is one you can freely remove. I don't see any constraint you can remove in that type.
I should have checked on Trac, it has been discussed... today: https://ghc.haskell.org/trac/ghc/ticket/12700 Also related: https://ghc.haskell.org/trac/ghc/ticket/12702 https://ghc.haskell.org/trac/ghc/ticket/11474 I will comment there. Sylvain
participants (2)
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Sylvain Henry -
Tom Ellis