GADT Type Checking GHC 6.10 versus older GHC

[Simon Peyton-Jones]

| > arbitrarySeq :: Sequence a -> Gen RepSeqVal
| > arbitrarySeq Nil =
| >    return (RepSeqVal Nil Empty)
| > arbitrarySeq (Cons (CTMandatory (NamedType n i t)) ts) =
| >    do u <- arbitraryType t
| >       us <- arbitrarySeq ts
| >       case u of
| >          RepTypeVal a v ->
| >             case us of
| >                RepSeqVal bs vs ->
| >                   return (RepSeqVal (Cons (CTMandatory (NamedType n i a)) bs) (v:*:vs))
|
|
| > QuickTest.lhs:240:13:
| >     GADT pattern match in non-rigid context for `Nil'
| >       Solution: add a type signature
| >     In the pattern: Nil
| >     In the definition of `arbitrarySeq':
| >         arbitrarySeq Nil = return (RepSeqVal Nil Empty)

That looks odd to me.  But it's hard to help without having the code. If you send it I'll try to help.

| > Did you try giving a type signature to the (entire) case expression,
| > as I suggested?  That should do it.
| >
|
| I'm not sure what this means or how to do it. Can you give an example or
| is it buried in some earlier email? I will go and have another look.

I mean replace
        (case blah of { ... })
by
        (case blah of { ... }) :: type-sig

That is, attach a type signature to the case expression itself.  Does that help at least explain what the sentence means? If so would you like to clarify the wiki advice?

Thanks

Simon