#11594: closed empty type families fully applied get reduced lazily when in a constraint tuple and fully applied -------------------------------------+------------------------------------- Reporter: carter | Owner: Type: feature request | Status: new Priority: normal | Milestone: 8.0.1 Component: Compiler (Type | Version: 7.10.2 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #9636 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by adamgundry): Replying to [comment:6 carter]:
So if I wanted a new userland insolvable type family that errors out eagerly rather than down stream, I'd have to have a type solver plugin active in the defining module to enforce that?
I guess that would enable simulating the type error family in userland In 7.10 right?
What if instead, when defining an empty closed type family, we could
The plugin would need to be active at use sites, not definition sites, but yes - it is possible for plugins to approximately simulate `TypeError`. They can't give exactly the same semantics, because plugins only get to see unsolved constraints, but can work the same way for uses of `TypeError` at kind `Constraint`. In fact I hacked together a quick prototype plugin that does exactly that during the last ICFP. If there's interest, I'm happy to share the code, though it's not entirely consistent with the `TypeError` API. pragma
Annotate it with "requires plugin foo" when we want this other plugin adhuncated semantics, Because otherwise requiring the offending type plugin is implicit in any down stream module!
Right, there is currently no way to indicate which type-checker plugins a module requires, and it might be nice to do so. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11594#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler