learning/go-patterns/stages/02-dependencies/main.go

// Stage 02 keeps the service independent from a particular price source.
// Run it with: go run .
package main

import "fmt"

type Price struct {
	Item  string
	Cents int
}

// PriceSource is defined by the service that consumes it. The interface is
// deliberately small: this service only needs one lookup operation.
type PriceSource interface {
	Lookup(item string) (Price, error)
}

// Catalog is one production implementation of PriceSource.
type Catalog map[string]Price

func (c Catalog) Lookup(item string) (Price, error) {
	price, ok := c[item]
	if !ok {
		return Price{}, fmt.Errorf("item %q is not in the catalog", item)
	}
	return price, nil
}

type Service struct {
	source PriceSource
}

func NewService(source PriceSource) Service {
	return Service{source: source}
}

// Quote depends on behavior, not on Catalog's representation.
func (s Service) Quote(item string) (Price, error) {
	return s.source.Lookup(item)
}

func main() {
	service := NewService(Catalog{
		"book": {Item: "book", Cents: 2100},
		"pen":  {Item: "pen", Cents: 300},
	})

	price, err := service.Quote("book")
	if err != nil {
		fmt.Println("quote failed:", err)
		return
	}
	fmt.Printf("%s: %d cents\n", price.Item, price.Cents)
}