Type mismatch with echo and trouble accessing Type fields

[FluxCapacitor]

Hi there, I'm very new to Nim.

I have some code:
    
    
    type
        LitKind* = enum
            NumLit,
            StrLit,
            NilLit,
            BoolLit
        
        LitType* = object
            case litKind*: LitKind
            of NumLit: n*: float64
            of StrLit: s*: string
            of BoolLit: b*: bool
            of NilLit: discard
        
        
        Expr = ref object of RootObj
        
        Literal = ref object of Expr
            value: LitType
        
        BinOp = ref object of Expr
            left, right: Expr
            op: string
    
    var left = Literal(value: LitType(litKind: NumLit, n: 5.0))
    var right = Literal(value: LitType(litKind: NumLit, n: 6.0))
    var add = BinOp(left: left, op: "+", right: left)
    
    echo add[]
    echo add[].left
    
    Run

the first echo statement works, although I'm not super sure why I need to add 
the square brackets on add.

But, the second echo statement does not work. How can I access add.left?