Nested Go templating

main.go

// This design would allow us to have 1- or 2-depth nested templates.
//
// Everything uses rootTmpl, and everything has access to all
// componentTmpls. But each combinedTmpl only uses the named template in
// layoutTmpls, except `page-*` which are defined under rootTmpl only.
package main

import (
	"fmt"
	"os"
	"strings"
	"text/template"
)

func main() {
	rootTmpl := template.Must(template.New("").Parse(`Page > {{ template "root" }} {{ template "footer" }}`))

	componentTmpls := []*template.Template{
		template.Must(template.New("header").Parse("<Header>")),
		template.Must(template.New("footer").Parse("<Footer>")),
		template.Must(template.New("some-component").Parse("<Some>")),
	}

	for _, tmpl := range componentTmpls {
		rootTmpl.AddParseTree(tmpl.Name(), tmpl.Tree)
	}

	layoutTmpls := map[string]*template.Template{
		"fixtures": template.Must(template.New("").Parse(`Fixtures > {{ template "fixtures" }}`)),
		"guidance": template.Must(template.New("").Parse(`{{ template "header" }} Guidance > {{ template "guidance" }}`)),
	}

	tmpls := map[string]*template.Template{
		"page-basic":        template.Must(template.New("").Parse(`Basic`)),
		"page-superbasic":   template.Must(template.New("").Parse(`Super basic`)),
		"fixtures-donor":    template.Must(template.New("").Parse(`Donor`)),
		"fixtures-attorney": template.Must(template.New("").Parse(`Attorney`)),
		"guidance-some":     template.Must(template.New("").Parse(`{{ template "some-component" }} Some`)),
		"guidance-other":    template.Must(template.New("").Parse(`Other`)),
	}

	combinedTmpls := map[string]*template.Template{}

	for name, tmpl := range tmpls {
		root, _ := rootTmpl.Clone()

		if layoutName := strings.Split(name, "-")[0]; layoutName == "page" {
			root.AddParseTree("root", tmpl.Tree)
		} else {
			root.AddParseTree("root", layoutTmpls[layoutName].Tree)
			root.AddParseTree(layoutName, tmpl.Tree)
		}

		combinedTmpls[name] = root
	}

	for k, v := range combinedTmpls {
		fmt.Print(k + ": ")
		v.Execute(os.Stdout, nil)
		fmt.Println("")
	}
}

output.txt

fixtures-attorney: Page > Fixtures > Attorney <Footer>
guidance-some: Page > <Header> Guidance > <Some> Some <Footer>
guidance-other: Page > <Header> Guidance > Other <Footer>
page-basic: Page > Basic <Footer>
page-superbasic: Page > Super basic <Footer>
fixtures-donor: Page > Fixtures > Donor <Footer>