Go polymorphism

package main

import (
    "fmt"
    "math"
)

type LengthComputer interface {
    Length() float64
}

type AngleComputer interface {
    Angle() float64
}

type Vector2 struct {
    x float64
    y float64
}

func (v Vector2) Length() float64 {
    return math.Sqrt(v.x*v.x + v.y*v.y)
}

func (v Vector2) Angle() float64 {
    return math.Atan2(v.y, v.x) * 180 / math.Pi
}

func main() {
    // if you declare/define "v" this way, it's OK
    var v LengthComputer = Vector2{-1, -1}
    // or like this... it works the same way
    //var v AngleComputer = Vector2{-1, -1}

    // However! this won't build,
    // says "./main.go:35:15: invalid type assertion: v.(LengthComputer) (non-interface type Vector2 on left)"
    // v := Vector2{-1, -1}

    if t, ok := v.(LengthComputer); ok {
        fmt.Printf("Type assertion for LengthComputer is %v, value = %v, length = %v\n", ok, t, t.Length())
    }
    if t, ok := v.(AngleComputer); ok {
        fmt.Printf("Type assertion for AngleComputer is %v, value = %v, angle (degrees) = %v\n", ok, t, t.Angle())
    }
}