#12972: Missed specialisation opportunity with phantom type class parameter? -------------------------------------+------------------------------------- Reporter: mpickering | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Matthew: this is a rather odd case. Look at `test4`. We have a Wanted constraint `[W] Phantom x0 Int`, where `x0` is a unification variable. But what is `x0`? We can use functional dependencies to get TWO derived constraints: 1. From the given `[G] Phantom x Int`, we get `[D] x0 ~ x`. 2. From the top-level `instance Phantom Char Int`, we get `[D] x0 ~ Char`. GHC chooses one of these arbitrarily. If it chooses (2) we unify `x0 := Int`, and solve the constraint from the top-level instance. The given constraint is not used. We get nice efficient code. But if it chooses (2) it'll unify `x0 := x`, and solve the wanted constraint from the given one, which is passed as a paramter to `test4`. That's less efficient. But you set up this situation, by providing two places to solve the constraint: from the passed-in givens, or from a top level instance. Just give a simpler type signature `test4 :: Int`. It's bit similar with `test3`. Again there are two ways to solve the wanted constraint: from the top-level instance or from the passed-in given. I'm not inclined to lose sleep over all this, unless you have a compelling use-case. --------- danharaj: I think you are describing an entirely different problem. Could you make a reproducible test case? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12972#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler