#12057: TypeFamilyDependencies on Data.Type.Bool's `Not` -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: libraries/base | Version: 8.0.1 Resolution: | Keywords: TypeFamilies, | Injective, newcomer 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 Iceland_jack): Replying to [comment:3 RyanGlScott]:
`Rep` definitely isn't injective. Counterexample: `Rep (Proxy Int)` = `Rep (Proxy Char)`, but `Int` is not `Char`.
Interesting, thank you. Would this be covered by the weaker [https://ghc.haskell.org/trac/ghc/ticket/6018#comment:48 ‘head’-injectivity]? Is there any benefit from adding this distinguishing information/kind information to the metadata (since `Proxy` is polykinded)? I am perversely working backwards from the perspective of injectivity but there may be a use for `TypeRep`s in the `Rep`.
`Not` can certainly be made injective though, and this would be simple to do and useful. Would you care to make a patch for it?
Sure, it can be kept open for more injective type families in base. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12057#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler