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discriminated union from a class
f#

hello, group

i am a newbie to F# and might not use the terminology correctly. Feel free to correct me. :)

Here is a sample of Discrimated Union

type A =
| TypeConstructorA of string
| TypeConstructorB of int

If i have a C# class defined somewhere else

public class ClassA { int Type; }

How can i define something like:

type A =
| TypeConstructorA of ClassA where ClassA.Type=0
| TypeConstructorB of ClassA where ClassA.Type=1

What i need is to define different constructors according to ClassA's Type field.

Thanks.

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You could create a factory routine:

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type A = 
    | TypeConstructorA of ClassA
    | TypeConstructorB of ClassA
    static member Create (a:ClassA) =
        match a.Type with
        | 0 -> TypeConstructorA a
        | 1 -> TypeConstructorB a
        | _ -> failwith "Invalid Type value"

But it seems to me you may be better of simpling defining some active patterns:

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let (|TypeConstructorA|_|) (a:ClassA) =
    if a.Type=0 then Some a else None
let (|TypeConstructorB|_|) (a:ClassA) =
    if a.Type=1 then Some a else None
By on 2/11/2009 12:50 AM ()Reply
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What is the use of your second type A? No matter what you do - you allways end up with an ClassA-object.

So just use a function to give you the instantiated objects of this class.

By on 2/10/2009 11:36 PM ()Reply
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This is sort of like an object classifier. We definitly need this because of our design. we need to treat the classA object as different type in F#.

maybe in C#, they are same, but in F#, they should be treated differently.

By on 2/10/2009 11:47 PM ()Reply

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