GHC indecisive whether matching on GADT constructors in arrow notation is allowed

[Alexis King]

Hi,

I’ve been working on bringing my reimplementation of arrow notation back up
to date, and I’ve run into some confusion about the extent to which arrow
notation is “supposed” to support matching on GADT constructors. Note
[Arrows and patterns] in GHC.Tc.Gen.Pat suggests they aren’t supposed to be
supported at all, which is what I would essentially expect. But issues
#17423 <https://gitlab.haskell.org/ghc/ghc/-/issues/17423> and #18950
<https://gitlab.haskell.org/ghc/ghc/-/issues/18950> provide examples of
using GADT constructors in arrow notation, and there seems to be some
expectation that in fact they *ought* to be supported, and some
recently-added test cases verify that’s the case.

But this is quite odd, because it means the arrows test suite now includes
test cases that verify both that this is supported *and* that it isn’t… and
all of them pass! Here’s my understanding of the status quo:

   -

   Matching on constructors that bind bona fide existential variables is
   not allowed, and this is verified by the arrowfail004 test case, which
   involves the following program:

   data T = forall a. T a

   panic :: (Arrow arrow) => arrow T T
   panic = proc (T x) -> do returnA -< T x

   This program is rejected with the following error message:

   arrowfail004.hs:12:15:
       Proc patterns cannot use existential or GADT data constructors
       In the pattern: T x

   -

   Despite the previous point, matching on constructors that bind evidence
   is allowed. This is enshrined in test cases T15175, T17423, and T18950,
   which match on constructors like these:

   data DecoType a where
     DecoBool :: Maybe (String, String) -> Maybe (Int, Int) -> DecoType Bool
   data Point a where
     Point :: RealFloat a => a -> Point a


This seems rather contradictory to me. I don’t think there’s much of a
meaningful distinction between these types of matches, as they create
precisely the same set of challenges from the Core point of view… right?
And even if I’m wrong, the error message in arrowfail004 seems rather
misleading, since I would definitely call DecoBool and Point above “GADT
data constructors”.

So what’s the intended story here? Is matching on GADT constructors in
arrow notation supposed to be allowed or not? (I suspect this is really
just yet another case of “nobody really knows what’s ‘supposed’ to happen
with arrow notation,” but I figured I might as well ask out of hopefulness
that someone has some idea.)

Thanks,
Alexis
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