On 15/02/2016 10:49, Adam Gundry wrote:
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 Ok, make sense; After reading what ImpredicativeTypes are, I can see the difference in the construct.
Thanks, -- Vincent