Haskell - replace case with guards

user_10667189 0
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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!

haskell
1 Answers

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 = fmap secondFunction (firstFunction xs)

fmap :: Functor f => (a -> b) -> f a -> f b 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 (f a)

which is basically the logic you wrote here.

user_675790
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