#12119: Can't create injective type family equation with TypeError as the RHS -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: feature request | Status: closed Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.1 checker) | Keywords: Resolution: wontfix | CustomTypeErrors, TypeFamilies, | InjectiveFamilies Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by AntC): Replying to [comment:6 sheaf]:
What about the following: ...
This seems useful to me, ...
Sure, it's the same structure as the O.P.
I feel like the injectivity annotation should be allowed in this case
... It is. Ryan's comment:5 says what's not allowed is the `TypeError` catch- all equation -- that is, if you want `ToDim` to be injective. So if you call `ToDim 5`, inference will get stuck, and you'll get an error message somewhere/somehow else. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12119#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler