Learning me a Haskell

toDouble.hs

{-|
  The 'toDouble' will take a string and force it to a Double. On failure the 
  it will just 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