[GHC] #11358: GHC generics has differing conFixity behavior between 7.10.3 and 8.1

6 Jan 2016

      #11358: GHC generics has differing conFixity behavior between 7.10.3 and 8.1
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  RyanGlScott
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:
          Component:  Compiler       |           Version:  8.1
  (CodeGen)                          |
           Keywords:  Generics       |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect result
  Unknown/Multiple                   |  at runtime
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Compile the following program with GHC 7.10.3 and 8.1:

 {{{#!hs
 {-# LANGUAGE DefaultSignatures #-}
 {-# LANGUAGE DeriveGeneric #-}
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE FlexibleInstances #-}
 module Main (main) where

 import GHC.Generics

 infixr 1 `T`
 data T a = T a a deriving Generic
 instance HasFixity (T a)

 data I a = a `I` a deriving Generic
 instance HasFixity (I a)

 class HasFixity a where
   fixity :: a -> Fixity
   default fixity :: (Generic a, GHasFixity (Rep a)) => a -> Fixity
   fixity = gfixity . from

 class GHasFixity f where
   gfixity :: f a -> Fixity

 instance GHasFixity f => GHasFixity (D1 d f) where
   gfixity (M1 x) = gfixity x

 instance Constructor c => GHasFixity (C1 c f) where
   gfixity c = conFixity c

 main :: IO ()
 main = do
   putStrLn $ show (fixity (T "a" "b")) ++ ", " ++ show (fixity ("a" `I`
 "b"))
 }}}

 On GHC 7.10.3, it yields `Prefix, Infix LeftAssociative 9`, but on GHC
 8.1, it yields `Infix RightAssociative 1, Prefix`. Why? The implementation
 of `deriving Generic(1)` changed slightly in GHC 8.1. Before, it would
 only assign a fixity of `Infix` if a constructor was ''declared'' infix.
 But GHC 8.1 no longer checks for this—it first checks if there is a user-
 supplied fixity declaration, and if so, uses that as the `Fixity`.
 Otherwise, it defaults to `Prefix`, even if the datatype was declared
 infix!

 The design of `Fixity` perhaps leaves something to be desired, but at the
 very least, we should ensure nothing `Fixity`-related breaks for now.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11358
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