#11298: Implicit call stack empty in instance declarations -------------------------------------+------------------------------------- Reporter: pikajude | Owner: Type: bug | Status: new Priority: normal | Milestone: 8.0.1 Component: Compiler | Version: 7.11 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Incorrect result | Unknown/Multiple at runtime | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by gridaphobe): There are two things going on here. 1. We intentionally changed the meaning of a bare `?loc :: CallStack` in 8.0 to not include its own location, only function calls are appended to the stack now. So the first example is expected behavior in 8.0. 2. In your second example, GHC infers a CallStack parameter for fooHelper, thus it's occurrence in the instance declaration is appended to the stack. Why does GHC infer a CallStack for the 2nd fooHelper and not the 1st? Because the monomorphism restriction applies in the 1st declaration and prevents GHC from inferring a qualified type for fooHelper. So both cases are expected given the current implementation, but the difference is a bit annoying. The new implementation of the CallStack solver that allows GHC to infer CallStack parameters is essential to fix a terrible bug in the 7.10.2 solver, but it might make sense to prevent inferring CallStacks for top- level binders. I didn't do this originally since it would add another special case to the solver, and I don't want to do that without good reason. I'm not sure whether this qualifies, it's just another case of the monomorphism restriction. On the other hand, in this case we get different runtime behavior instead of an ambiguous type error, and this is definitely worse.. Thanks! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11298#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler