closure.go

package main

import (
	"fmt"
	"encoding/json"
	"io/ioutil"
)

type User struct {
	Name string `json:"name"`
}

// define function signatures as interface ...
type Loader func(string) (interface{}, error)
type Validator func(interface{}) bool

// Create a Loader instance with parameter in the constructor (ie, CreateLoader)
func CreateLoader(baseDir string) Loader {

	// inner function, ie private function
	unmarshalUser := func(data []byte) (*User, error) {
		u := &User{}
		if err := json.Unmarshal(data, u); err != nil {
			return nil, err
		}

		return u, nil
	}

	// return the loader, variables and methods in the current scope
	// are just private.
	return func(name string) (interface{}, error) {
		if data, err := ioutil.ReadFile(fmt.Sprintf("%s/%s", baseDir, name)); err != nil {
			return nil, err
		} else {
			return unmarshalUser(data)
		}
	}
}

// Create a Validator instance with parameter in the constructor (ie, CreateValidator)
func CreateValidator() Validator {

	validateUser := func(user *User) bool {
		return len(user.Name) > 0
	}

	return func(v interface{}) bool {
		switch t := v.(type) {
		case *User:
			return validateUser(t)

		default:
			return false
		}
	}
}

// of course, we can pass dependencies ... no need to know the method name ...
func execute(loader Loader, validator Validator) {
	// some codes
	if data, err := loader("data.json"); err != nil {
		fmt.Print("Error while loading: ", err)
	} else {
		if validator(data) {
			fmt.Printf("Data is valid")
		} else {
			fmt.Printf("Data is not valid")
		}
	}
}

func main() {

	// create closures with one responsibility ...
	loader := CreateLoader("./fixtures");
	validator := CreateValidator();

	execute(loader, validator)
}