How about this one (relative to same Ty def)?
fstTy :: Ty (a -> b) -> Ty a
fstTy (a :-> b) = a
fstTy' :: Ty (a -> b) -> Ty a
fstTy' ty = a where (a :-> b) = ty
The first definition works, while the second triggers the complaint about
non-rigidity. I don't know how to type-annotate the second one to provide
the required rigidity. Do you?
Of course, in this example, I can use the first def instead of the second.
In my real example, the type being deconstructed is synthesized, so the
second form is not so convenient.
I'm working around this problem, so no emergency, but still I'd like to know
what can be done.
Thanks, - Conal
On 8/22/07, Simon Peyton-Jones <[EMAIL PROTECTED]> wrote:
>
> The issue is this. When doing GADT matching, getting type inference to
> work well, and interact well with other features (e.g. indexed type
> families) is MUCH easier if the type being matched is totally known – we say
> "rigid". When you used "$" you made GHC less sure about the type of the
> GADT match. Our GADT paper describes this rigidity reasoning
> http://research.microsoft.com/~simonpj/papers/gadt/index.htm<http://research.microsoft.com/%7Esimonpj/papers/gadt/index.htm>
>
>
>
> GHC 6.6 was a bit more liberal, but the liberality was delicately
> balanced, and made life too complicated when we added more stuff.
>
>
>
> In any case, the solution is always "add a type signature", though in this
> case you could also escape with "omit a $".
>
>
>
> Simon
>
>
>
> *From:* [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] *On
> Behalf Of *Conal Elliott
> *Sent:* 22 August 2007 06:15
> *To:* [email protected]
> *Subject:* ghc 6.7 and GADT pattern-matching
>
>
>
> In going from ghc-6.6 to ghc-6.7, I've lost some GADT pattern matching
> that I'd really like to have back. The message:
>
> c:/conal/Haskell/Eros/src/gadt-example.hs :23:32:
> GADT pattern match in non-rigid context for `:*'
> Tell GHC HQ if you'd like this to unify the context
> In the pattern: a :* b
> In a lambda abstraction: \ (a :* b) -> ((f a) :* b)
> In the second argument of `($)', namely
> `\ (a :* b) -> ((f a) :* b)'
>
> and the relevant code:
>
> -- | Statically typed type representations
> data Ty :: * -> * where
> (:*) :: Ty a -> Ty b -> Ty (a, b)
> (:->) :: Ty a -> Ty b -> Ty (a->b)
> OtherTy :: TypeRep -> Ty a
>
> -- | Type transformations
> newtype TyFun a b = TyFun { unTyFun :: Ty a -> Ty b }
>
> instance Arrow TyFun where
> TyFun f >>> TyFun g = TyFun (f >>> g)
> first (TyFun f) = TyFun $ \ (a :* b) -> (f a :* b)
> second (TyFun g) = TyFun $ \ (a :* b) -> (a :* g b)
>
> Full test module attached with many more example matchings.
>
> I'd really like to get this code working in ghc-6.7. How likely is that?
> The best alternative I know is *much* less readable.
>
> Thanks, - Conal
>
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