tmpl is a developer-friendly wrapper around Go's html/template package, designed to simplify common tasks, enhance type safety, and make complex template setups more maintainable and readable. If you've ever felt frustration dealing with loosely-coupled templates and Go code, tmpl was built specifically for you.
This project attempts to improve the overall template workflow and offers a few helpful utilities for developers building html based applications:
- Two-way type safety when referencing templates in Go code and vice-versa
- Nested templates and template fragments
- Template extensibility through compiler plugins
- Static analysis utilities such as template parse tree traversal
Roadmap & Idea List
- Parsing and static analysis of the html in a template
- Automatic generation of GoLand
{{ gotype: }}annotations when using thetmplCLI - Documentation on how to use
tmpl.Analyzefor parse tree traversal and static analysis of templates
go get github.com/tylermmorton/tmplThe tmpl workflow starts with a standard html/template. For more information on the syntax, see this useful syntax primer from HashiCorp.
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>{{ .Title }} | torque</title>
</head>
<body>
<form action="/login" method="post">
<label for="username">Username</label>
<input type="text" name="username" id="username" value="{{ .Username }}">
<label for="password">Password</label>
<input type="password" name="password" id="password" value="{{ .Password }}">
<button type="submit">Login</button>
</form>
</body>To start tying your template to your Go code, declare a struct that represents the "dot context" of the template. The dot context is the value of the "dot" ({{ . }}) in Go's templating language.
In this struct, any exported fields (or methods attached via pointer receiver) will be accessible in your template from the all powerful dot.
type LoginPage struct {
Title string // {{ .Title }}
Username string // {{ .Username }}
Password string // {{ .Password }}
}To turn your dot context struct into a target for the tmpl compiler, your struct type must implement the TemplateProvider interface:
type TemplateProvider interface {
TemplateText() string
}The most straightforward approach is to embed the template into your Go program using the embed package from the standard library.
import (
_ "embed"
)
var (
//go:embed login.tmpl.html
tmplLoginPage string
)
type LoginPage struct {
...
}
func (*LoginPage) TemplateText() string {
return tmplLoginPage
}After implementing TemplateProvider you're ready to compile your template and use it in your application.
Currently, it is recommended to compile your template once at program startup using the function tmpl.MustCompile:
var (
LoginTemplate = tmpl.MustCompile(&LoginPage{})
)If any of your template's syntax were to be invalid, the compiler will panic on application startup with a detailed error message.
If you prefer to avoid panics and handle the error yourself, use the
tmpl.Compilefunction variant.
The compiler returns a managed tmpl.Template instance. These templates are safe to use from multiple Go routines.
After compilation, you may execute your template by calling one of the generic render functions.
type Template[T TemplateProvider] interface {
Render(w io.Writer, data T, opts ...RenderOption) error
RenderToChan(ch chan string, data T, opts ...RenderOption) error
RenderToString(data T, opts ...RenderOption) (string, error)
}var (
LoginTemplate = tmpl.MustCompile(&LoginPage{})
)
func main() {
buf := bytes.Buffer{}
err := LoginTemplate.Render(&buf, &LoginPage{
Title: "Login",
Username: "",
Password: "",
})
if err != nil {
panic(err)
}
fmt.Println(buf.String())
}tmpl supports multiple ways of providing functions to your templates.
You can define methods on your dot context struct to be used as template functions. These methods must be attached to your struct via pointer receiver. This strategy is useful if your template function depends on a lot of internal state.
type LoginPage struct {
FirstName string
LastName string
}
func (p *LoginPage) FullName() string {
return fmt.Sprintf("%s %s", p.FirstName, p.LastName)
}{{ .FullName }}You can also define template functions on the dot context struct by implementing the FuncMapProvider interface. This is useful for reusing utility functions across multiple templates and packages.
package tmpl
type FuncMapProvider interface {
TemplateFuncMap() FuncMap
}Example using the sprig library:
import (
"github.com/Masterminds/sprig/v3"
)
type LoginPage struct {
...
}
func (*LoginPage) TemplateFuncMap() tmpl.FuncMap {
return sprig.FuncMap()
}Usage:
{{ "hello!" | upper | repeat 5 }}One major advantage of using structs to bind templates is that nesting templates is as easy as nesting structs.
The tmpl compiler knows to recursively look for fields in your dot context struct that also implement the TemplateProvider interface. This includes fields that are embedded, slices or pointers.
A good use case for nesting templates is to abstract the document <head> of the page into a separate template that can now be shared and reused by other pages:
<head>
<meta charset="UTF-8">
<title>{{ .Title }} | torque</title>
{{ range .Scripts -}}
<script src="{{ . }}"></script>
{{ end -}}
</head>type Head struct {
Title string
Scripts []string
}Now, update the LoginPage struct to embed the new Head template.
The name of the template is defined using the tmpl struct tag. If the tag is not present the field name is used instead.
type LoginPage struct {
Head `tmpl:"head"`
Username string
Password string
}Embedded templates can be referenced using the built in {{ template }} directive. Use the name assigned in the struct tag and ensure to pass the dot context value.
<!DOCTYPE html>
<html lang="en">
{{ template "head" .Head }}
<body>
...
</body>
</html>Finally, update references to LoginPage to include the nested template's dot as well.
var (
LoginTemplate = tmpl.MustCompile(&LoginPage{})
)
func main() {
buf := bytes.Buffer{}
err := LoginTemplate.Render(&buf, &LoginPage{
Head: &Head{
Title: "Login",
Scripts: []string{ "https://unpkg.com/[email protected]" },
},
Username: "",
Password: "",
})
if err != nil {
panic(err)
}
fmt.Println(buf.String())
}Sometimes you may want to render a nested template. To do this, use the RenderOption WithTarget in any of the render functions:
func main() {
buf := bytes.Buffer{}
err := LoginTemplate.Render(&buf, &LoginPage{
Title: "Login",
Username: "",
Password: "",
}, tmpl.WithTarget("head"))
if err != nil {
panic(err)
}
}The tmpl package provides a static analysis tool for Go templates. This tool can be used to traverse the parse tree of a template and perform custom analysis. The analysis framework is what enables the tmpl compiler to perform static analysis on your templates and provide type safety.
An Analyzer is a function that returns an AnalyzerFunc, which is a visitor-style function that allows you to traverse the parse tree of a template. Analyzers can be provided to tmpl.Compile using the UseAnalyzers option.
In the following example, we search templates for instances of {{ outlet }} and dynamically inject a function. This is how the torque framework uses the tmpl compiler to provide handler wrapping functionality. Example
You may want to do something similar if you want to add new 'built in' directives and functions to your templates.
package main
var outletAnalyzer tmpl.Analyzer = func(h *tmpl.AnalysisHelper) tmpl.AnalyzerFunc {
return tmpl.AnalyzerFunc(func(val reflect.Value, node parse.Node) {
switch node := node.(type) {
case *parse.IdentifierNode:
if node.Ident == "outlet" {
h.AddFunc("outlet", func() string { return "{{ . }}" })
}
}
})
}
var LoginPage = tmpl.MustCompile(&LoginPage{}, tmpl.UseAnalyzers(outletAnalyzer))The AnalysisHelper allows you to modify the template during analysis. It provides methods to add functions, variables, and other nodes to the template. This is useful for modifying the template during analysis without having to modify the original template.
The Analyze function can be used independently of the Compile function and allows you to analyze templates without compiling them. This is useful for static analysis and debugging purposes.
package main
import (
"fmt"
"html/template"
"github.com/tylermmorton/tmpl"
)
func main() {
tmpl.Analyze(&LoginPage{}, tmpl.ParseOptions{}, []tmpl.Analyzer{ ... })
}