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-- This program broke GHC 6.3, becuase dataToTag was called with
-- an unevaluated argument
module Main where
import System.Environment (getArgs)
-- NOTE: When if you remove Eight (or any other constructor) everything works
-- Having at least 9 constructors has something to do with the bug
data Digit = Zero | One | Two | Three | Four | Five | Six | Seven | Eight
deriving (Eq,Enum)
instance Show Digit where
show Five = "Five"
show Six = "Six"
show _ = undefined
-- Use either of these instances (instead of derived) and everything works
{-instance Enum Digit where
fromEnum Five = 5
fromEnum _ = undefined
toEnum 6 = Six
toEnum _ = undefined-}
{-instance Eq Digit where
Five == Five = True
Six == Six = True
_ == _ = undefined-}
isFive :: Digit -> Bool
isFive a = succ a == Six
main :: IO()
main = do
putStrLn ("======")
-- These next two lines are just here to keep ghc from optimizing away stuff
args <- getArgs
let x = if length args == -1 then undefined else Five
putStrLn ("x: " ++ show x)
let y = succ x
putStrLn ("let y = succ x")
putStrLn ("y: " ++ show y)
putStrLn ("y == Six: " ++ show (y == Six))
putStrLn ("succ x == Six: " ++ show (succ x == Six))
putStrLn ("isFive x: " ++ show (isFive x))
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