#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: Phab:D202 | -------------------------------------+------------------------------------- Comment (by jstolarek): Oh, and while we're talking about open type families, there's a design decision to make. Verifying user-supplied injectivity condition requires comparing the same equations that are compared by compatibility check. Should we: a) verify injectivity at the same time we verify compatibility? b) verify injectivity in a separate pass? a) might seem like an obvious answer because it allows us to avoid traversing the equation pairs twice. However, that makes compatibility checking code in `FamInstEnv.lookup_fam_inst_env'` a bit more complicated. b) on the other hand can allow us to skip the injectivity checking altogether if the user did not supply any injectivity declarations. This would be a big win since compilation performance would not be degraded for already existing code (since there are no injectivity declarations). I am in favour of option b). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/6018#comment:99 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler