GADT pattern match in non-rigid context
Wolfgang Jeltsch
g9ks157k at acme.softbase.org
Tue Sep 2 07:20:01 EDT 2008
Hello,
I have some code giving me the error message: “GADT pattern match in non-rigid
context for … Tell GHC HQ if you'd like this to unify the context”. I
reduced my code to the following example which still gives this error
message:
> data T a b where
>
> C :: T a [b]
>
> f :: (forall a'. T a' b) -> T a b
> f t = case t of C -> C
How do I work around this error? Some former e-mail discussion gave the hint
of adding a type signature but this probably doesn’t work in my case. Note
also that specializing the type of the argument t to T a b inside the
definition of f is not an option since in the real code I need the first
argument of T to be universally quantified for calling another function which
needs this quantification.
I use GHC 6.8.2. Don’t know whether this problem still shows up with GHC HEAD
but am interested in hearing whether this is the case.
I’m thankful for any help.
Best wishes,
Wolfgang
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