Dependency injection via closure (handlers in multiple packages)

_tree

.
├── go.mod
├── handlers
│   ├── books
│   │   └── books.go
│   └── env.go
├── main.go
└── models
    └── models.go

books.go

package books

import (
	"fmt"
	"log"
	"net/http"

	"bookstore.alexedwards.net/handlers"
	"bookstore.alexedwards.net/models"
)

func Index(env *handlers.Env) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		bks, err := models.AllBooks(env.DB)
		if err != nil {
			log.Println(err)
			http.Error(w, http.StatusText(500), 500)
			return
		}

		for _, bk := range bks {
			fmt.Fprintf(w, "%s, %s, %s, £%.2f\n", bk.Isbn, bk.Title, bk.Author, bk.Price)
		}
	}
}

env.go

package handlers

import (
	"database/sql"
)

type Env struct {
	DB *sql.DB
}

go.mod

module bookstore.alexedwards.net

go 1.15

require github.com/lib/pq v1.8.0

main.go

package main

import (
	"database/sql"
	"log"
	"net/http"

	"bookstore.alexedwards.net/handlers"
	"bookstore.alexedwards.net/handlers/books"

	_ "github.com/lib/pq"
)

func main() {
	db, err := sql.Open("postgres", "postgres://user:pass@localhost/bookstore")
	if err != nil {
		log.Fatal(err)
	}

	env := &handlers.Env{DB: db}

	http.Handle("/books", books.Index(env))
	http.ListenAndServe(":3000", nil)
}

models.go

package models

import (
	"database/sql"
)

type Book struct {
	Isbn   string
	Title  string
	Author string
	Price  float32
}

func AllBooks(db *sql.DB) ([]Book, error) {
	rows, err := db.Query("SELECT * FROM books")
	if err != nil {
		return nil, err
	}
	defer rows.Close()

	var bks []Book

	for rows.Next() {
		var bk Book

		err := rows.Scan(&bk.Isbn, &bk.Title, &bk.Author, &bk.Price)
		if err != nil {
			return nil, err
		}

		bks = append(bks, bk)
	}
	if err = rows.Err(); err != nil {
		return nil, err
	}

	return bks, nil
}