#10832: Generalize injective type families -------------------------------------+------------------------------------- Reporter: jstolarek | Owner: jstolarek Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 7.11 checker) | Keywords: TypeFamilies, Resolution: | InjectiveFamilies Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #6018 | Differential Rev(s): Phab:D1287 Wiki Page: | -------------------------------------+------------------------------------- Comment (by lexi.lambda): I might have a use case for this, but I’m not smart enough to understand if my problem would be solved by this feature or not. Basically, I have a type family for type-level append: {{{ #!hs type family xs :++: ys where '[] :++: ys = ys (x ': xs) :++: ys = x ': (xs :++: ys) }}} This family is not injective, but if you know the result and one of the two inputs, you can deduce the other one. Therefore, I would like to be able to write the dependency annotation `r xs -> ys, r ys -> xs`. I have written a function that uses `:++:` in its type signature, which currently looks like this: {{{ #!hs forall g r a f. (forall v. f v -> Eff (g :++: r) v) -> Eff (f ': r) a -> Eff (g :++: r) a }}} Currently, this signature is ambiguous. However, notably, we know `r`, and in the location this function is used, we also know `g :++: r`. Therefore, my hope is that with this feature, I could use this function, and `g` could be inferred. Currently, however, I have to use `TypeApplications` to pick `g` explicitly, even though the type seems “obvious” to the programmer at its use site (since it very explicitly duplicates type information in an adjacent top-level annotation). I don’t know enough to know if such an annotation would actually allow such a thing or not, or if this is even applicable in my use case, but I thought I would offer the example anyway, since it sounds like this feature is in need of motivating examples. (I can also try and come up with a smaller, self-contained example if I’m not totally off base and that would, in fact, be relevant/helpful.) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10832#comment:13 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler