Hi Devs!
I encountered a curious restriction with type signatures (tyvar bindings)
in GADT pattern matches.
GHC won't let me directly capture the refined type structure of GADT
constructors like this:
{-# Language GADTs, ScopedTypeVariables #-}
data Foo a where
Bar :: Foo [a]
foo :: Foo a -> ()
foo b@(Bar :: Foo [a]) = quux b
where quux :: Foo [b] -> ()
quux Bar = ()
I get:
*test.hs:7:8: **error:*
* • Couldn't match type ‘a1’ with ‘[a]’*
* ‘a1’ is a rigid type variable bound by*
* the type signature for:*
* foo :: forall a1. Foo a1 -> ()*
* at test.hs:6:1-18*
* Expected type: Foo a1*
* Actual type: Foo [a]*
To me it appears that the type refinement established by the GADT pattern
match is not accounted for.
Of course I can write:
foo :: Foo a -> ()
foo b@Bar | (c :: Foo [a]) <- b = quux c
where quux :: Foo [b] -> ()
quux Bar = ()
but it feels like a complicated way to do it...
My question is whether this is generally seen as the way to go or whether
ScopedTypeVariables coming from a GADT pattern match should be able to
capture the refined type. To me the latter seems more useful.
Just wanted to feel the waters before writing a ticket about this.
Cheers and thanks,
Gabor
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