On Wed, Feb 8, 2012 at 2:55 PM, Roman Cheplyaka wrote:

The following program is accepted by GHC 7 with -XRank2Types:

   class C a where        f :: a -> a

   newtype R = R (forall a . C a => a)

   instance C R where        f (R x) = R $ f x

   main = return ()

jhc 0.8 gives the following error:

   [1 of 1] Main             (...............jhci: user error (    What:    failure    Why:     Signature too Weak: [] does not imply [Main.C Main.v11]    Where:   on line 7 in j.hs    in the explicitly typed Instance@.iMain.f.Main.R (Main.R Main.6_x)      = Main.R Jhc.Basics.$ Main.f Main.6_x             {- on line 7 -}    Compilation of module: Main)

Is this difference in behaviour intended, and how can I make something like that compile?

I believe that jhc is being too restrictive here and it should accept the program. thanks for the report. I strengthened the checking of class constraints signifiantly in this version and probably introduced this bug. John