ATM Test example in Haskell

atm.hs

import Text.Printf

isInt x = x == fromInteger (round x)

{-|
  The 'toDouble' will take a string and force it to a Double. On failure, it
  will return 0.00
-}
toDouble :: String -> Double
toDouble x = do
  -- try to convert a String to a decimal number, if not fall back to a string,
  -- this will prevent haskell from erroring out
  case reads x :: [(Double, String)] of
    -- if there is a match for any decimal number and an empty string
    y@[(_, "")] -> -- The y@ just references the pattern to be used below
      -- grap the first tuple (there is only ever one), 
      -- then grap the first item from the tuple
      fst (head y)  
    -- any matches for anything else will just return 0.00
    _           -> 0.00 -- default to 0.00 on failure

main = do
  input <- getLine
  let numbers = words input
  if (length numbers) == 2
    then do
      -- Ensure this first number is a number in the first place
      let withdraw = toDouble (head numbers)
      let amount = toDouble (last numbers)
      if isInt (withdraw / 5)
        then do
          let total = withdraw + 0.50
          if total <= amount
            then
              printf "%.2f\n" (amount - total)
          else
            -- Insufficient Funds
            printf "%.2f\n" amount
        else
          -- Invalid Withdraw
          printf "%.2f\n" amount
  else
    putStrLn "Invalid Input"