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Recompilation bug with default class methods

    • Closed Issue created by Simon Marlow @simonmar

    Repro as follows.

    A.hs:

    {-# OPTIONS_GHC -fno-full-laziness #-}
module A (toTypedData, toTypedDataNoDef) where

toTypedData :: String -> IO Int
toTypedData s = wrapPrint "yoyo" $ toTypedDataNoDef s

wrapPrint :: String -> IO Int -> IO Int
wrapPrint s act = do
    putStrLn s
    act

toTypedDataNoDef  :: String -> IO Int
toTypedDataNoDef s = return $ length s

    B.hs:

    module B ( TypeClass(..) ) where

import A

class Show a => TypeClass a where
    getSize :: a -> IO Int
    getSize a = toTypedData (show a)

printA :: a -> IO ()

    C.hs:

    module Main where

import B

data MyDataType = MyDataType String Int deriving Show

instance TypeClass MyDataType where
    printA = putStrLn . show

main :: IO ()
main = do
    let myValue = MyDataType "haha" 99
    sz <- getSize myValue
    putStrLn $ show sz
printA myValue
    1. Comment out the -fno-full-laziness option in A.hs
    2. rm *.o *.hi; ghc -O2 C.hs
    3. Re-enable the -fno-full-laziness option in A.hs
    4. ghc -O2 C.hs

    Produces a linker error:

    C.o:Main_main1_info: error: undefined reference to 'A_toTypedData2_closure'
C.o(.data.rel.ro+0x48): error: undefined reference to 'A_toTypedData2_closure'
collect2: error: ld returned 1 exit status

    Reproduced in 8.0, 8.4 and master. Probably happens in all released versions of GHC.

    Edited
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