How can I use Context with an interface to keep steps DRY?

[gfaugere]

I have a class structure that can be analoged to this:

package my_package

type Operand interface {
	performCalculation(val1, val2 int) int
}

type Addition struct{}

func (a Addition) performCalculation(val1, val2 int) int {
	return val1 + val2
}

type Subtraction struct{}

func (s Subtraction) performCalculation(val1, val2 int) int {
	return val1 - val2
}

I would like to implement tests in this way:

Feature: Operations

  Scenario Outline: additions
    Given an addition operator
    When I receive <val1> and <val2>
    Then I should get <result>
    Examples:
      | val1  | val2  | result |
      | 2     | 3     | 5      |
      | 10    | 100   | 110    |

  Scenario Outline: substractions
    Given a substraction operator
    When I receive <val1> and <val2>
    Then I should get <result>
    Examples:
      | val1  | val2  | result |
      | 5     | 3     | 2      |
      | 100   | 10    | 90     |

As you can see my When/Then steps are the same, and I would like to implement them this way to keep them DRY:

package my_package

func calculate(_ gobdd.StepTest, ctx gobdd.Context, val1, val2 int) {
	operand := ctx.GetAs("operand", Operand) // Can't figure this out
	res := operand.performCalculation(val1, val2)
	ctx.Set("result", res)
}

func check(t gobdd.StepTest, ctx gobdd.Context, expected int) {
	result, err := ctx.GetInt("result")

	if err != nil {
		t.Error(err)
		return
	}

	if expected != result {
		t.Errorf("expected: %s, got: %s", expected, result)
	}
}


func TestScenarios(t *testing.T) {
	suite := gobdd.NewSuite(t, gobdd.WithFeaturesPath("*.feature"))
        // Add a step to instanciate the correct Operand, depending on the Given
	suite.AddStep(`I receive (\d+) and (\d+)`, calculate) // Reuse these steps
	suite.AddStep(`I should get (\d+)`, check)
	suite.Run()
}

I can't figure out how to get an interface from Context. Is there a way that I'm missing?