Subtyping

[Mansour Al-Mutairi]

Hi,

I am a newbie but I think that what you are proposing can be simulated in 
haskell. It is like subclassing in OOP. You can create a new type that is an 
extension of the older one. Unlike OOP overloading, you will need to create 
a new function that can work on the new type.

>data SomeObject = SomeInt Int | SomeString String | _

will become:

data SomeObject = SomeInt Int | SomeString String

>...later...
>
>data SomeObject |= SomeReal Double

will become:

data SomeObject2 = SO SomeObject | SomeReal Double

>same :: SomeObject -> SomeObject -> Bool
>same (SomeInt a) (SomeInt b) = a == b
>same (SomeString a) (SomeString b) = a == b
>same _ _ = False
>
>...later...
>
>same (SomeReal a) (SomeReal b) = a == b

will become:

same :: SomeObject -> SomeObject -> Bool
same (SomeInt a) (SomeInt b) = a == b
same (SomeString a) (SomeString b) = a == b

...later...

same2 :: SomeObject2 -> SomeObject2 -> Bool
same2 (SomeReal a) (SomeReal b) = a == b
same2 x y = same x y

tell me if I am wrong.
thanks.

Mansour Al-Mutairi :)


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