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| author | sheaf <sam.derbyshire@gmail.com> | 2021-10-06 18:22:28 +0200 |
|---|---|---|
| committer | Marge Bot <ben+marge-bot@smart-cactus.org> | 2021-10-09 04:46:05 -0400 |
| commit | 31983ab4c65204ad0fd14aac4c00648f5fa6ad6b (patch) | |
| tree | 6bff70ce40f4d295ce084358ebe4b977e68bb43f /docs | |
| parent | a76409c758d8c7bd837dcc6c0b58f8cce656b4f1 (diff) | |
| download | haskell-31983ab4c65204ad0fd14aac4c00648f5fa6ad6b.tar.gz | |
Reject GADT pattern matches in arrow notation
Tickets #20469 and #20470 showed that the current
implementation of arrows is not at all up to the task
of supporting GADTs: GHC produces ill-scoped Core programs
because it doesn't propagate the evidence introduced by a GADT
pattern match.
For the time being, we reject GADT pattern matches in arrow notation.
Hopefully we are able to add proper support for GADTs in arrows
in the future.
Diffstat (limited to 'docs')
| -rw-r--r-- | docs/users_guide/9.2.1-notes.rst | 15 |
1 files changed, 15 insertions, 0 deletions
diff --git a/docs/users_guide/9.2.1-notes.rst b/docs/users_guide/9.2.1-notes.rst index a80218098e..6a6dec1ed9 100644 --- a/docs/users_guide/9.2.1-notes.rst +++ b/docs/users_guide/9.2.1-notes.rst @@ -135,6 +135,21 @@ Language will allow the scrutinee to have a multiplicity of ``One``, using its best-effort inference algorithm. +* Support for matching on GADT constructors in arrow notation has been removed, + as the current implementation of :extension:`Arrows` doesn't handle GADT + evidence correctly. + + One possible workaround, for the time being, is to perform GADT matches + inside let bindings: :: + + data G a where + MkG :: Show a => a -> G a + + foo :: G a -> String + foo = proc x -> do + let res = case x of { MkG a -> show a } + returnA -< res + Compiler ~~~~~~~~ |