[Haskell-beginners] using the joker "_" on a constructor

[TP]

Hi,

I have a question about pattern matching.
Consider the following code:

------------------
data Foo = Bar Int
         | Foo Int

f :: Foo -> Bool
f (Foo n)
    | even n = True
    | odd n = False
f (Bar n)
    | even n = True
    | odd n = False

main = do

print $ f $ Bar 1
print $ f $ Bar 2
print $ f $ Foo 1
print $ f $ Foo 2
------------------

Why is it not possible to simply write for f:

f v = case v of
    _ n | even n -> True
    _ n | odd n -> False

or

f (_ n)
    | even n = True
    | odd n = False

(in both cases we get a parse error)?

Thanks,

TP