#14046: “Illegal type synonym family application in instance” is too strict in the presence of functional dependencies -------------------------------------+------------------------------------- Reporter: lexi.lambda | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: #3485 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj):
I could be convinced by an example of the sort that SPJ alluded to, in which an instance of the form Multi X (Y a) behaves differently from (b ~ Y a) => Multi X b,
It affects type inference: {{{ instance (a ~ Int) => C a instance (a ~ Bool) => C a vs instance C Int instance C Bool }}} GHC's type inference engine does not support backtracking. If it did, it could try out the former two instances one at a time, and see which one "worked". With the latter two, only one matches. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14046#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler