simonpj at microsoft.com
Mon Dec 29 09:48:32 EST 2008
| 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:
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.
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.
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