Wing aspect ratio calculator in Haskell.

wing_ar.hs

module WingAR where

import Data.Maybe
import Text.Printf

-- See http://en.wikipedia.org/wiki/Aspect_ratio_%28wing%29
wing_ar :: Float -> Float -> Float
wing_ar l s = l^2 / s

readMaybe s =  listToMaybe $ fmap fst $ reads s

getFloatMaybe :: IO (Maybe Float)
getFloatMaybe = getLine >>= return . readMaybe

getFloatForSure :: String -> String -> IO Float
getFloatForSure prompt retry =
    putStr prompt >> getFloatMaybe >>= \mx ->
        case mx of
            Nothing -> putStr retry >> getFloatForSure prompt retry
            Just x  -> return x

main :: IO ()
main =
    let retry = "Incorrect input. Please retry.\n" in
        do  putStr "Wing lambda calculator (in Haskell)\n\n"
            l <- getFloatForSure "Wing length: " retry
            s <- getFloatForSure "Wing square: " retry
            printf "\nWing lambda: %f\n" $ wing_ar l s