tmpl is a wrapper around Go's html/template package that aims to solve some of the pain points developers commonly run into while working with templates. 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 visa 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())
}You can also pass additional options to the render function to customize the behavior of the template.
type RenderOption func(p *RenderProcess)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 }}You can also provide template functions while compiling your template using the tmpl.WithFuncs option:
Example:
var (
LoginTemplate = tmpl.MustCompile(&LoginPage{}, tmpl.WithFuncs(tmpl.FuncMap{
"add": func(a, b int) int {
return a + b
},
}))
)Usage:
{{ add 1 2 }}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>Again, annotate your dot context struct and run tmpl bind:
//tmpl:bind head.tmpl.html
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.
//tmpl:bind login.tmpl.html
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)
}
}