Simon Peyton-Jones wrote:
| If I remove -XScopedTypeVariables from this http://hpaste.org/13230 then | I get the following error message: | | > Asn1cTestNew.hs:55:27: | > GADT pattern match in non-rigid context for `INTEGER' | > Solution: add a type signature | > In the pattern: INTEGER | > In the definition of `referenceTypeAndValAux2': | > referenceTypeAndValAux2 ns INTEGER x | > = lhs ns <> text " = " <> text (show x) <> semi | > Failed, modules loaded: Language.ASN1, ASNTYPE. | | At the very least the message is unhelpful. It was only by accident I | decided to put in -XScopedTypeVariables.
This one had me puzzled for a while too! Here is what's happening.
You have three mutually recursive functions: referenceTypeAndValAux1 referenceTypeAndValAux2 cSEQUENCE In Haskell 98, typechecking mutually recursive functions is done *together*, with each having a momomorphic type in the other RHSs. That leads to an annoying problem, that of figuring out how their polymorphic type variables "match up". As a result, even the type variables in the type signature look non-rigid.
The solution is to use -XRelaxedPolyRec, which compiles mutually-recursive definitions that each have a type signature one by one. Precisely because of the above infelicity, both -XGADTs and -XScopedTypeVariables imply -XRelaxedPolyRec.
Thanks very much for this. I would never have guessed to use -XRelaxedPolyRec given the error message. Is it worth noting it here http://haskell.org/haskellwiki/Upgrading_packages#Changes_to_GADT_matching or is it something that has always existed with GADTs and I just didn't trip over it?
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.
I would vote for this. In fact I was mildly surprised I could use GADTs without specifying -XGADTs. I noticed that Isaac Dupree has already raised a ticket http://permalink.gmane.org/gmane.comp.lang.haskell.glasgow.user/16109 so I will add my support to it. Many thanks and a happy new year, Dominic.