Simple code which implement method for add number and minus number.

operation.go

package calculator

import "context"

type operation struct {
	SavedNumber int
}

// Add is method implementation to add number with saved number
func (s *operation) Add(ctx context.Context, number int) (int, error) {
	s.SavedNumber = s.SavedNumber + number
	return s.SavedNumber, nil
}

// Minus is method implementation to minus number with saved number
func (s *operation) Minus(ctx context.Context, number int) (int, error) {
	s.SavedNumber = s.SavedNumber - number
	return s.SavedNumber, nil
}

// InitOperation is constructor to initialize operation
func InitOperation() *operation {
	return &operation{
		SavedNumber: 0,
	}
}