Template (Share) in Go

package main

import (
	"log"
	"os"
	"path/filepath"
	"text/template"
)

func main() {
	// Here we create a temporary directory and populate it with our sample
	// template definition files; usually the template files would already
	// exist in some location known to the program.
	dir := createTestDir([]templateFile{
		// T0.tmpl is a plain template file that just invokes T1.
		{"T0.tmpl", "T0 ({{.}} version) invokes T1: ({{template `T1`}})\n"},
		// T1.tmpl defines a template, T1 that invokes T2. Note T2 is not defined
		{"T1.tmpl", `{{define "T1"}}T1 invokes T2: ({{template "T2"}}){{end}}`},
	})
	// Clean up after the test; another quirk of running as an example.
	defer os.RemoveAll(dir)

	// pattern is the glob pattern used to find all the template files.
	pattern := filepath.Join(dir, "*.tmpl")

	// Here starts the example proper.
	// Load the drivers.
	drivers := template.Must(template.ParseGlob(pattern))

	// We must define an implementation of the T2 template. First we clone
	// the drivers, then add a definition of T2 to the template name space.

	// 1. Clone the helper set to create a new name space from which to run them.
	first, err := drivers.Clone()
	if err != nil {
		log.Fatal("cloning helpers: ", err)
	}
	// 2. Define T2, version A, and parse it.
	_, err = first.Parse("{{define `T2`}}T2, version A{{end}}")
	if err != nil {
		log.Fatal("parsing T2: ", err)
	}

	// Now repeat the whole thing, using a different version of T2.
	// 1. Clone the drivers.
	second, err := drivers.Clone()
	if err != nil {
		log.Fatal("cloning drivers: ", err)
	}
	// 2. Define T2, version B, and parse it.
	_, err = second.Parse("{{define `T2`}}T2, version B{{end}}")
	if err != nil {
		log.Fatal("parsing T2: ", err)
	}

	// Execute the templates in the reverse order to verify the
	// first is unaffected by the second.
	err = second.ExecuteTemplate(os.Stdout, "T0.tmpl", "second")
	if err != nil {
		log.Fatalf("second execution: %s", err)
	}
	err = first.ExecuteTemplate(os.Stdout, "T0.tmpl", "first")
	if err != nil {
		log.Fatalf("first: execution: %s", err)
	}
}