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Published 20 hours ago • verified publisher icon tylermmorton
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Metadata

`tmpl` is a wrapper around Go's html/template package that offers two-way static typing, template nesting and convenient workflow tooling for web developers.

tmpl

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
  • Convenient but optional CLI for binding templates to Go code

Roadmap & Idea List

  • Parsing and static analysis of the html in a template
  • Automatic generation of GoLand {{ gotype: }} annotations when using the tmpl CLI
  • Documentation on how to use tmpl.Analyze for parse tree traversal and static analysis of templates

🧰 Installation

go get github.com/tylermmorton/tmpl

To install the tmpl cli and scaffolding utilities:

go install github.com/tylermmorton/tmpl/cmd/tmpl

🌊 The Workflow

The 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>

Dot Context

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 }}
}

TemplateProvider

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
}

If you've opted into using the tmpl CLI, you can use the //tmpl:bind annotation on your dot context struct instead.

//tmpl:bind login.tmpl.html
type LoginPage struct {
    ...
}

and run the utility:

tmpl bind . --outfile=tmpl.gen.go

Tip: Run tmpl bind ./... using a //go:generate annotation at the root of your project to ensure all of your templates are bound at build time.

tmpl bind works at the package level and will generate a single file containing the binding code for all the structs annotated with //tmpl:bind in your package.

import (
    _ "embed"
)

var (
    //go:embed login.tmpl.html
    tmplLoginPage string
)

func (*LoginPage) TemplateText() string {
    return tmplLoginPage
}

Compilation

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.Compile function variant.

The compiler returns a managed tmpl.Template instance. These templates are safe to use from multiple Go routines.

Rendering

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)

Advanced Usage

Template Functions

tmpl supports multiple ways of providing functions to your templates.

Dot Context Methods

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 }}

FuncMapProvider

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 }}

CompilerOption

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 }}

Template Nesting

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())
}

Targeting

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)
    }
}

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verified publisher icon tylermmorton

Metadata

`tmpl` is a wrapper around Go's html/template package that offers two-way static typing, template nesting and convenient workflow tooling for web developers.

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