You need a type signature for the case expression. As Daniel says, this is worth a read http://haskell.org/haskellwiki/Upgrading_packages%23Changes_to_GADT_matching... Simon | -----Original Message----- | From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users- | bounces@haskell.org] On Behalf Of Jason Dagit | Sent: 21 November 2008 16:04 | To: glasgow-haskell-users@haskell.org | Subject: GADT Type Checking GHC 6.10 versus older GHC | | Hello, | | Here is an example where ghc 6.8.x was fine, but now 6.10 complains. | | \begin{code} | type CommuteFunction = forall x y. (Prim :< Prim) C(x y) -> Perhaps | ((Prim :< Prim) C(x y)) | | commute_split :: CommuteFunction | commute_split (Split patches :< patch) = | toPerhaps $ do (p1 :< ps) <- cs (patches :< patch) | case sort_coalesceFL ps of | p :>: NilFL -> return (p1 :< p) | ps' -> return (p1 :< Split ps') | where cs :: ((FL Prim) :< Prim) C(x y) -> Maybe ((Prim :< (FL Prim)) C(x y)) | cs (NilFL :< p1) = return (p1 :< NilFL) | cs (p:>:ps :< p1) = do p1' :< p' <- commutex (p :< p1) | p1'' :< ps' <- cs (ps :< p1') | return (p1'' :< p':>:ps') | commute_split _ = Unknown | \end{code} | | Now with ghc 6.10 we get this error: | GADT pattern match with non-rigid result type `Maybe a' | Solution: add a type signature | In a case alternative: p :>: NilFL -> return (p1 :< p) | In the expression: | case sort_coalesceFL ps of { | p :>: NilFL -> return (p1 :< p) | ps' -> return (p1 :< Split ps') } | In the second argument of `($)', namely | `do (p1 :< ps) <- cs (patches :< patch) | case sort_coalesceFL ps of { | p :>: NilFL -> return (p1 :< p) | ps' -> return (p1 :< Split ps') }' | | You can see more context here: | http://darcs.net/src/Darcs/Patch/Prim.lhs around line 472 | | My understanding was that from 6.6 to 6.8, GADT type checking was | refined to fill some gaps in the soundness. Did that happen again | between 6.8 and 6.10 or is 6.10 being needlessly strict here? | | Thanks, | Jason | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users@haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users