#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 jstolarek): I think the bulk of the work with injectivity will be done under the hood. Changing the syntax later on will not be difficult and I don't want to be blocked by bikeshedding. Inferring injectivity for closed type families is something that indeed requires more discussion. But again, I feel I can make at least some progress without any binding decissions on this matter being made. I don't like any of Richard's proposals: - if we introduced `Result` this would be the first capitalized keyword (reserved word?) in Haskell. I don't like such inconsistency. Notice also that `result` might conflict with type variable names in type family head but that is a local conflict that restricts only the names possible to use for type variables. On the other hand `Result` might conflict with a name of a type (or type family), which is more of a global thing. - if we use syntax that uses type family name then we will get something like `type family F a b c | F b -> a c` and `F b` looks strange given that `F`'s arity is 3. It looks as if `F` could be partially applied. I don't think that restricting the use of `result` word in type family head is a big problem - we've already done that with `role`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/6018#comment:49 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler