#8155: Defaulting bug or unfortunate error message with closed type families -------------------------------------+------------------------------------ Reporter: nh2 | Owner: Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 7.6.3 Resolution: invalid | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Changes (by goldfire): * status: new => closed * resolution: => invalid Comment: This may be an infelicity in defaulting (which I'm not terribly familiar with), but the closed-type-family behavior is correct, if rather subtle and confusing. The original post says "clearly `BoundsOf (a -> a) ~ Integer` (you just wrote that down)". I would say that that's not exactly what was written. What the closed-type-family definition of `BoundsOf` says is that `BoundsOf (a -> a) ~ Integer` as long as `a` is most assuredly '''not''' `a -> a`. That is, GHC must be sure that previous equations of a closed type family can never apply before using a given equation. Because it is quite possible that `a` will be instantiated at a type such that `a` '''does''' equal `a -> a` (see below), GHC will not use the second equation to simplify `BoundsOf (a -> a)`. The dangerous type that `a` might be instantiated with is {{{ type family Looper b type instance Looper b = Looper b -> Looper b }}} So, unless GHC knows that `a` won't be instantiated at a type like `Looper`, the second equation will not be used. As a concrete example of this, GHC will happily reduce `BoundsOf (Bool -> Bool)`. This is all very subtle and can be unexpected, but after lots and lots of thinking, Simon PJ, Dimitrios V, and I could not come up with a better way here. When I wrote a suggested implementation of `BoundsOf` in my response to #8154, I thought a little while about the ordering of the equations to avoid exactly this problem. I will close this ticket, but do please reopen if you can characterize the bug as a specific infelicity in the defaulting mechanism. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8155#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler