#12240: Common Sense for Type Classes -------------------------------------+------------------------------------- Reporter: Mathnerd314 | Owner: 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: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Mathnerd314): Functional dependencies imply too much structure. Consider {{{#!hs {-# LANGUAGE MultiParamTypeClasses #-} class C a b instance C Int Bool instance C Int Char instance C Word8 String instance C Word16 String }}} If we have `instance C a Bool`, we know that `a ~ Int`. But if we have `instance C Word8 b`, we know `b ~ String`. Functional dependencies cannot express this logic, but it is still "common sense logic". -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12240#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler