7. Review Maybe

There are a bunch of helper functions in Data.Maybe. My favorite is mapMaybe. Choose pattern matching over these functions when you can.

• isJust should give True if the value is a Just and False if it is Nothing.

    isJust :: Maybe a -> Bool
  

• fromMaybe starts with a “default value”, the answer to give when it gets Nothing. When it gets a Just, it gives the value inside.

    fromMaybe :: Int -> Maybe Int -> Int
  
    check_fM = [ 109 == fromMaybe 109 Nothing,
                 280 == fromMaybe 109 (Just 280) ]
  

• mapMaybe runs a function on all of the elements in a list and keeps only the Just results.

    mapMaybe :: (a -> Maybe b) -> [a] -> [b]
  
    check_mM = ([11,13] == mapMaybe f [1..4])
      where f n  | odd n  = Just (n+10)
                 | even n = Nothing