[Haskell-cafe] QuickCheck: test passes in GHCi, fails when compiled

[Lauri Pesonen]

I provided a Java solution to a problem of returning the first digit
of an integer on StackOverflow and someone wondered if there were any
floating point point problems with the solution, so I though I'd
implement the algorithm in Haskell and run QuickCheck on it.
Everything works fine on GHCi, but if I compile the code and run the
test, it fails with -1000, 1000, -1000000. Any ideas why?

In any case it seems that the commenter was right and there are some
subtle problems with my solution. I'd just like to know more
details...

import Data.Char
import Test.QuickCheck

-- my solution
getFirstDigit :: Int -> Int
getFirstDigit 0 = 0
getFirstDigit x = let x' = abs x
                      digits = (floor $ logBase 10 $ fromIntegral x')
                  in x' `div` (floor $ 10 ** (fromIntegral digits))

-- two reference implementation that agree with each other
getFirstDigitRef1 :: Int -> Int
getFirstDigitRef1 x = digitToInt $ head $ show $ abs x

getFirstDigitRef2 :: Int -> Int
getFirstDigitRef2 x | x < 0 = getFirstDigitRef2 (-x)
                    | x < 10 = x
                    | otherwise = getFirstDigitRef2 $ x `div` 10

myTest x = getFirstDigit x == getFirstDigitRef1 x
--myTest x = getFirstDigitRef2 x == getFirstDigitRef1 x

myCheck n = check (defaultConfig { configMaxTest = n }) myTest

main = myCheck 1000000000

-- 
  ! Lauri