On Tue, Jan 16, 2024 at 04:16:26PM +0100, Olaf Klinke wrote:
I remember reading somewhere, but I can't find it now, that GHC's coverage checker does not look inside pattern synonym definitions at all, and this is by design: namely to ensure abstraction boundaries. Looking inside the definition would mean that type checking behaviour (in a way -- if you count coverage checks as "type checking") of user code is influenced by invisible implementation details of the "constructors" they're using.
Compare that to 'type' versus 'newtype' on the type level: A type is just an alias, a synonym. The type checker has access to all information on the right hand side of the equality. With extensions like TypeSynonymInstances, A type alias is interchangeable with the aliased type. If what you report is true, they should not have called pattern synonyms a "synonym".
Ah, they should be newpatterns!