Simon Peyton-Jones wrote:
This is a nasty corner I agree. GHC requires -XGADTs for you to *define* a GADT. Perhaps it should also require -XGADTs for you to *match against* one (as you are doing here). That would avoid this particular hole. If you think that would be a step forward, do put forward a Trac feature request, and encourage others to support it.
Does GHC require any flags to pattern-match against an existential constructor? (does it require type-system complication?) (and what if GADT syntax was used for an ordinary data type? or for an ordinary existential?) but I'd support requiring -XGADTs in any such pattern-match in which XRelaxedPolyRec could make a difference. Somehow it doesn't seem fair for a module to imply that it *doesn't* use GADTs, if it cannot even by type-checked without understanding them. also see http://hackage.haskell.org/trac/ghc/ticket/2004 -Isaac