Checking for a particular data constructor

Question

Let\'s say that I defined my own data-Type like

data MyData = A arg| B arg2| C arg3

How would I write a function (for instance: isMy

Answer 1

You can use Maybes. You can create a set of functions that check for each of the types

getA, getB, getC :: MyData a -> Maybe a
getA x = case x of {(A v) -> Just v; _ -> Nothing}
getB x = case x of {(B v) -> Just v; _ -> Nothing}
getC x = case x of {(C v) -> Just v; _ -> Nothing}

This affords some practical idioms for certain tasks:

allAs :: [MyData a] -> [a]
allAs xs = mapMaybe getA xs

printIfA :: Show a => MyData a -> IO ()
printIfA x = maybe (return ()) print $ getA x