pattern matching - Can I match a data constructor wildcard in Haskell?

Question

I have

data Foo = X (String, Int) | A String | B String | C String | D String -- ...

and have defined

f (X (s, _)) =s 
f (A s) = s
f (B s) = s
f (C s) = s
f (D s) = s
-- ...

but would prefer to be able to write something like

f (X (s, _)) =s 
f (_ s) = s

But there seems to be no way to do this (I get a "parse error" associated with the _).

Is there a way to match a data constructor wildcard in Haskell?

Answer 1

Nope. But you can write this:

data Foo
    = X { name :: String, age :: Int } 
    | A { name :: String }
    | B { name :: String }
    | C { name :: String }
    | D { name :: String }

and then have name :: Foo -> String. You could also consider this:

data Tag = A | B | C | D | X Int
data Foo = Tagged Tag String

f (Tagged _ s) = s