Hi Vincent, On 15/02/16 09:44, Vincent Hanquez wrote:
I recently came across a compilation problem on 8.0 that do not trigger on < 8.0. I've reduced the issue to this snippet with Rank2Types:
type Action = (forall a . Show a => a) -> Int data Command = Command (Maybe Action)
com1 = Command Nothing
With GHC 7.0 to 7.10, this compile and works.
This shouldn't be accepted without ImpredicativeTypes[*], but GHC versions prior to 8.0 failed to look through the type synonym. Note that if you say data Command = Command (Maybe ((forall a . Show a => a) -> Int)) then earlier versions report an error. Take a look at GHC ticket #10194 for more details: https://ghc.haskell.org/trac/ghc/ticket/10194
However on GHC 8.0, I now have:
Test.hs:19:16: error: • Cannot instantiate unification variable ‘a0’ with a type involving foralls: Action GHC doesn't yet support impredicative polymorphism • In the first argument of ‘Command’, namely ‘Nothing’ In the expression: Command Nothing In an equation for ‘com1’: com1 = Command Nothing
I workarounded it by "inlining" the maybe and creating a proper wrapping type:
data Action2 = Action2 ((forall a . Show a => a) -> Int) | NoAction2 data Command2 = Command2 Action2
com2 = Command2 NoAction2
This looks like a reasonable alternative. Hope this helps, Adam [*] And if you turn on ImpredicativeTypes, GHC may or may not do the right thing. -- Adam Gundry, Haskell Consultant Well-Typed LLP, http://www.well-typed.com/