#6018: Injective type families -------------------------------+------------------------------------------- Reporter: lunaris | Owner: simonpj Type: feature | Status: new request | Milestone: 7.10.1 Priority: normal | Version: 7.4.1 Component: Compiler | Keywords: TypeFamilies, Injective Resolution: | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: | Related Tickets: #4259 None/Unknown | Test Case: | Blocking: | -------------------------------+------------------------------------------- Comment (by MikeIzbicki): Here's a real worldish use case. Let's try to write a modified Functor class that lets us map over not just the last type variable, but any type variable. This would be useful, for example, with the Either type. The class might look something like: {{{ class Functor loc f where fmap :: proxy loc -> (a -> b) -> SetLocation f loc a -> SetLocation f loc b }}} Where loc is a type that uniquely identifies the position of the argument we want to map over. We can't make an instance of this class unless the SetLocation type family is injective. With a little syntactic sugar, we can convert the above class into something that looks very much like the current Functor class, but is quite a bit more powerful: {{{ class Functor loc f where fmap :: proxy loc -> (a -> b) -> f { loc = a } -> f { loc = b } }}} I don't think you can get something nearly that clean without injective type families. And, of course, this applies to any parametric classes currently in use, not just Functor. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/6018#comment:25 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler