GADT Type Checking GHC 6.10 versus older GHC

[Dominic Steinitz]

Simon Peyton-Jones wrote:
> | > 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
> 
> 
Simon,

I'm sorry to have put you to so much trouble by accidentally using an
old version of ghc I must have had lying around. As penance, I will go
and update the wiki with what I have learnt.

Dominic.