3.) We actually '''have''' to take the "nearest" instance when you bind instances in local scope. Why? `ImplicitParams` unfortunately exist.
If you have `(?x :: Int)` in scope
and you come across `let ?x = 12 :: Int in ...` you need to switch to
#9701: GADTs not specialized properly -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.9 Resolution: | Keywords: Operating System: | Architecture: Unknown/Multiple Unknown/Multiple | Difficulty: Unknown Type of failure: Runtime | Blocked By: performance bug | Related Tickets: Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by dfeuer): Replying to [comment:3 ekmett]: the new witness of `(?x :: Int)`.
The same should work if you open a `Dict (?x :: Int)`.
What am I missing here? Core distinguishes dictionaries that came from type classes from implicit parameters, doesn't it? If someone decides to implement some sort of local instance extension, as suggested by [http://okmij.org/ftp/Haskell/tr-15-04.pdf Oleg Kiselyov and Chung-chieh Shan], some restrictions are required to make anything work (as discussed in the paper). I believe the restrictions they describe would probably skirt the problem you consider, because (if my vague understanding is sufficiently correct) the types won't match where the dictionary replacement would be problematic. Also, I realized today that things can be simplified some more, and perhaps in a way you might find more convincing: {{{#!hs data Silly a where Silly :: Num a => a -> Silly a potato :: Int -> Silly Int -> Int potato x (Silly _) = x + x }}} Even here, `potato` uses the dictionary it takes from `Silly`. If there were incoherent instances in the air, this would not really be a result to be proud of. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9701#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler