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

I believe you want functions to test for particular constructors:

isA :: MyData -> Bool
isB :: MyData -> Bool

If so, then you can write these yourself or derive them. The implementation would look like:

isA (A _) = True
isA _     = False

isB (B _) = True
isB _     = False

To derive them automatically, just use the derive library and add, in your source code:

{-# LANGUAGE TemplateHaskell #-}
import Data.DeriveTH

data MyData = ...
    deriving (Eq, Ord, Show}

derive makeIs ''MyData
-- Older GHCs require more syntax: $( derive makeIs ''MyData)

Also note: your data declaration is invalid, the name must be capitalized, MyData instead of myData.

Finally, this whole answer is based on the assumption you want to test constructors, not data types as you said (which are statically checked at compile time, as Tarrasch said).

Answer 2

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

Answer 3

Haskell always checks that the types makes sense. The compiler would complain immediately if you wrote isMyDataType 4, because 4 is not of type MyData, it's of type Int.

I'm not sure this is what you asked for, but either way I strongly suggest for you to try out what you've asked here in practice, so you can see for yourself. Most important is that you check out type signatures in haskell, it is key for learning haskell.