#10177: Typeable solver regression -------------------------------------+------------------------------------- Reporter: glguy | Owner: Type: bug | Status: new Priority: highest | Milestone: 7.10.1 Component: Compiler (Type | Version: 7.10.1-rc3 checker) | Keywords: Resolution: | Architecture: Operating System: Unknown/Multiple | Unknown/Multiple Type of failure: GHC rejects | Test Case: valid program | testsuite/tests/typecheck/should_compile/T10177 Blocked By: | Blocking: Related Tickets: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by diatchki): I don't think that there is anything wrong here, so I think that we should accept the program. The (new, now merged) `Typeable` solver does exactly what Richard says. The instance for `Typeable (f a)` has to be computed from the evidence for the types `f` and `a`. If that was not the case, we'd get some very odd behavior depending on how type variables get instantiated. Now, if `f` happened to be a concrete type (e.g., like `V n`), then we can reduce further, but if we happen to already have a solution for `V n`, we don't have to reduce any further---after all, ''that'' solution must have been built by the internal typeable solver in the first place. Note that the actual `TypeRep`s are built by always applying the constructor to all arguments: see the comment on `mkAppTy` in `Data.Typeable.Internal`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10177#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler