Haskell - 用警卫代替案件

Question

i'd like to know if in this portion of code is possible to replace the case statement with guards:

firstFunction  :: String -> Maybe MyType
secondFunction :: MyType -> Integer
myFunction     :: String -> Maybe Integer
myFunction xs = case firstFunction xs of
    Nothing -> Nothing
    Just x  -> Just( secondFunction x )

Thank you in advance!

Answer 1

You could use a pattern guard [Haskell-wiki] , like:

myFunction :: String -> Maybe Integer
myFunction xs | Just x <- firstFunction xs = Just (secondFunction x)
              | otherwise = Nothing

But what you here do is basically " fmap " the result of firstFunction , like:

myFunction :: String -> Maybe Integer
myFunction xs =  (firstFunction xs)

fmap :: Functor f => (a -> b) -> fa -> fb is used to "map" over a functor. Now Maybe is a functor, defined as :

 instance Functor Maybe where fmap _ Nothing = Nothing fmap f (Just a) = Just (fa) 

which is basically the logic you wrote here.