Re: How can we define a function that returns a type like a Union type?

[moerm]

@mrsekut

First, your code has a problem by passing a small 'a' but `parseStatement` 
expects a capital 'A' or 'B'.

I'm a bit confused by your approach but I'll try to help anyway.
    
    
    type
       A = ref object of RootObj
              name: string
              value: string
       
       B = ref object of RootObj
              name: string
       
       C = ref object of RootObj
              flg: string
       
       None = ref object of RootObj
    
    proc parseStatement(self: C): ref RootObj =
            case self.flg
            of "A":
                    result = A(name: "a", value: "val")
            of "B":
                    result = B(name: "b")
            else:
                    result = cast[None](0)
           # now, test it
    
    let c = C(flg: "A")
    let resc = c.parseStatement()
    echo resc.repr
    let d = C(flg: "?")  # unknown/invalid
    let resd = d.parseStatement()
    echo resd.repr
    let e = C(flg: "B")  # unknown/invalid
    let rese = e.parseStatement()
    echo rese.repr
    
    Run

works and is probably what you wanted.

Side note re. small vs capital 'a' etc.: You might find it interesting to know 
that Nim's `case` accepts multiple options or ranges, too. So the follwing 
might be closer to what you had in mind:
    
    
    proc parseStatement(self: C): ref RootObj =
            case self.flg
            of "A", "a":
                    result = A(name: "a", value: "val")
            of "B","b":
                    result = B(name: "b")
            else:
                    result = cast[None](0)
           # now, test it
    
    Run