#6018: Injective type families -------------------------------------+------------------------------------- Reporter: lunaris | Owner: jstolarek Type: feature | Status: new request | Milestone: 7.10.1 Priority: normal | Version: 7.4.1 Component: Compiler | Keywords: TypeFamilies, Resolution: | Injective Operating System: | Architecture: Unknown/Multiple Unknown/Multiple | Difficulty: Unknown Type of failure: | Blocked By: None/Unknown | Related Tickets: #4259 Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by dfeuer): Replying to [comment:58 jstolarek]:

I thought about it and I believe this should be possible. But we could only infer normal injectivity. I believe an algorithm for inferring injectivity in some of the parameters (ie. things like `result a -> b c` or `result -> b c` for a 3-arity type family) would be exponential in the number of arguments.

I don't know anything about this, but what's wrong with exponential time here? This inference only happens once per closed type family, right? How many variables are there likely to be? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/6018#comment:62 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler