#12432: TypeInType: open type family application as type family return kind fails to compile -------------------------------------+------------------------------------- Reporter: j6carey | Owner: Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.1 checker) | Resolution: duplicate | Keywords: Operating System: Linux | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by j6carey): Because of a workaround (as mentioned below), I don't have a compelling use case at present. The application I had in mind is illustrated by the recently-added attachment, "Test4.hs". The idea is that you have types that express formulas for values, and these can be evaluated in the context of various run-time parameters, though for brevity I have omitted the type expressions whose values actually depend on run-time parameters. Because some of these expressions will determine the sizes of certain objects in memory, I would like optimize length checks by computing static bounds on the range of sizes that a given type expression might specify. In particular, the expression might be a constant of kind "Nat". The associated type family exhibiting the problem is the one that computes the static bounds for the values associated with type expressions. The straightforward instance definition for constants of kind 'Nat' triggers the problem. On the other hand, I have managed to work around the issue by using a helper type family; please see the comment about 'EvalSingleton'. (Minor detail: this excerpt is missing certain features such as checking that the 'Nat' is in bounds, and recovering in some fashion, such as forcing it into the bounds or triggering a compilation error. This is still just a toy example, though hopefully more illustrative of the intended use case.) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12432#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler