print debug on Haskell

Use unsafePerformIO

import System.IO.Unsafe

bmi weight height  = printDebug $ weight / (height / 100)^2

bmiTell :: Double -> Double -> String

bmiTell weight height
    | bmi weight height <= skinny = "skinny"
    | bmi weight height <= normal = "normal"
    | bmi weight height <= fat    = "fat"
    where skinny = 18.5
          normal = 25.0
          fat    = 30.0

printDebug x = unsafePerformIO $ do
    print $ "bmi: " ++ (show x)
    return x

main = print $ bmiTell 65.0 171.0

prints:

"bmi: 22.229061933586404"
"bmi: 22.229061933586404"
"normal"