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GHC generics has differing conFixity behavior between 7.10.3 and 8.1

    • Closed Issue created by Ryan Scott @RyanGlScott

    Compile the following program with GHC 7.10.3 and 8.1:

    {-# 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.

    Trac metadata
    Trac field Value
    Version 8.1
    Type Bug
    TypeOfFailure OtherFailure
    Priority high
    Resolution Unresolved
    Component Compiler (CodeGen)
    Test case
    Differential revisions
    BlockedBy
    Related
    Blocking
    CC kosmikus
    Operating system
    Architecture
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