#11715: Constraint vs * -------------------------------------+------------------------------------- Reporter: bgamari | Owner: Type: bug | Status: new Priority: high | Milestone: 8.2.1 Component: Compiler (Type | Version: 8.0.1-rc1 checker) | Resolution: | Keywords: Typeable 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 int-index): Yes, a `with` error is not a viable solution. Since we'd like to treat anything to the LHS of `=>` the same way, issuing an error would break this example: {{{ data SomeOrd a where SomeOrd :: Ord a => a -> SomeOrd a cmp :: SomeOrd a -> SomeOrd a -> Ordering cmp (SomeOrd a) (SomeOrd b) = -- Do we use an 'Ord a' dictionary from the first argument -- or the second argument? We know it's the same dictionary -- because class instances are coherent, but this might be -- not the case for values provided by `with`. compare a b }}} By the way, how does GHC currently decide which dictionary to use in the example above? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11715#comment:23 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler