type SelectionType int

const (
    LeftSelection int = iota
    RightSelection
    TopSelection
    NoSelection
)

type Selection interface{
    ...
}

type LeftSelection struct {
    ...
}

func NewLeftSelection() *LeftSelection {
    return &LeftSelection{}
}

{
    switch SelectionType(kind) {
    case LeftSelection:
        thing = NewLeftSelection()
    case RightSelection:
        thing = NewRightSelection()
    case TopSelection:
        thing = NewTopSelection()
    case NoSelection:
        // nop
    }
}