Floating-point Haskell type for use in VW diesel engines

Main.hs

import VWFloat

testResult :: VWFloat
testResult = 214.0

limit :: VWFloat
limit = 200.0

main = do
  putStrLn "Emissions within limit?"
  putStrLn $ show $ testResult < limit

VWFloat.hs

{-# LANGUAGE GeneralizedNewtypeDeriving #-}

module VWFloat (VWFloat) where

import System.Environment
import System.IO.Unsafe
import Data.Maybe

defeatDevice :: Bool
defeatDevice = isJust $ unsafePerformIO $ lookupEnv "EMISSIONS_TEST"

instance Ord VWFloat where
  compare (VWFloat a) (VWFloat b) = if defeatDevice then LT else (compare a b)

newtype VWFloat = VWFloat Float
                  deriving (Eq, Show, Read, Floating, RealFrac,
                            Fractional, Real, Num, RealFloat)

Usage:

$ ./Main
Emissions within limit?
False
$ EMISSIONS_TEST=1 ./Main
Emissions within limit?
True

Beautiful.