[Haskell-cafe] Type alias in constraints

[Sylvain Henry]

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