Allowing multiple declarations of data constructors

Question

I am currently experimenting with data types and I've ran into an issue involving multiple declarations of data constructors.

data DBPosition = Unknown
                | Omega Integer
                | Delta Integer
                deriving (Show, Eq, Ord)

data DBGeometry = Unknown | Cis | Trans
                deriving (Show, Eq, Ord)

data DoubleBond = DoubleBond DBPosition DBGeometry
                deriving (Show, Eq, Ord)

If I was to make a value such as - let bond = DoubleBond Unknown Unknown, then it could be inferred that the first Unknown has a type of DBPosition while the second Unknown has a type of DBPosition. Unfortunately this is not the case:

test.hs:6:27:
Multiple declarations of `Unknown'
Declared at: test.hs:1:27
             test.hs:6:27
Failed, modules loaded: none.

Are there any language extensions that can be used to get around this?

Answer 1

As Carsten pointed out above, your definition does not work because you have two constructors with the same name. You'd need to use e.g. UnknownDBPosition and UnknownDBGeometry. However, I'd argue a better solution arises from recognising:

• That the concept of an unknown value works in precisely the same way no matter if you are talking about double bond positions, geometries, or whatever else; and
• That Unknown is not actually a variety of double bond geometry or position.

That being so, I recommend that you remove both Unknown and use Maybe to specify lack of knowledge.

data DBPosition = Omega Integer
                | Delta Integer
                deriving (Show, Eq, Ord)

data DBGeometry = Cis | Trans
                deriving (Show, Eq, Ord)

data DoubleBond = DoubleBond (Maybe DBPosition) (Maybe DBGeometry)
                deriving (Show, Eq, Ord)