More generally, ImpredicativeTypes is utterly broken and has been for several release cycles. Sometimes, when you're lucky, it allows you to do something pretty neat. However, you need to hope that your luck remains strong between minor releases. It is an unsupported feature.
I should note that TypeApplications required fairly serious surgery to the way type checking works. Given the general brokenness of ImpredicativeTypes, I didn't spend effort in trying to retain behavior with ImpredicativeTypes on. (I didn't actively try to make it worse, either.) It does not surprise me in the slightest that 8.0's ImpredicativeTypes is quite different from previous versions'.
Richard
On Feb 15, 2016, at 5:49 AM, Adam Gundry
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/ _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs