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Hamlet: A type-safe Haml template engine for Go

 4 months ago
source link: https://github.com/stackus/hamlet
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Hamlet

A Haml template engine for Go.

hamlet_header.png

Features

  • Full Haml language support
  • Templates are compiled to type-safe Go
  • Multiple templates per file
  • Mix Go and templates together in the same file
  • Easy nesting of templates
@hmlt SiteLayout() {
	!!!
	%html{lang:"en"}
		%head
			%title Hamlet
		%body
			%h1 Hamlet
			%p A HAML-like template engine for Go.
			= @children
}

@hmlt HomePage() {
	= @render SiteLayout()
		%p This is the home page for Hamlet.
}
<!DOCTYPE html>
<html lang="en">
  <head>
    <title>Hamlet</title>
  </head>
  <body>
    <h1>Hamlet</h1>
    <p>A Haml template engine for Go.</p>
    <p>This is the home page for Hamlet.</p>
  </body>
</html>

Supported Haml Syntax & Features

Unsupported HAML Features

  • Probably something I've missed, please raise an issue if you find something missing.

Hamlet CLI

CLI Installation

go install github.com/stackus/hamlet/cmd/hamlet@latest

CLI Usage

Use generate to generate Go code from Hamlet template files, that are new or newer than the generated Go files, in the current directory and subdirectories:

hamlet generate

Use the --path flag to specify a path to generate code for:

hamlet generate --path=./templates

In both examples, the generated code will be placed in the same directory as the template files.

Use the --force to generate code for all Hamlet template files, even if they are older than the generated Go files:

hamlet generate --force

See more options with hamlet help generate or hamlet generate -h.

Library Installation

go get github.com/stackus/hamlet

Using Hamlet

To use Hamlet you will need to create a Hamlet template file. See The Hamlet template for more information.

After you have your templates written, you will need to generate the Go code for them. This is done using the before mentioned CLI tool.

The resulting Go files will have a Hamlet template function for each of the templates in the file.

When called, the template function will return a *hamlet.Template which can be used to render the template.

package main

import (
	"context"
	"os"

	"github.com/stackus/hamlet/examples/tags"
)

func main() {
	tmpl := tags.RemoveWhitespace()

	err := tmpl.Render(context.Background(), os.Stdout)
	if err != nil {
		panic(err)
	}
}

The above would render one of the examples from this library, and would output the following:

<p>This text has no whitespace between it and the parent tag.</p>
<p>
There is whitespace between this text and the parent tag.<p>This text has no whitespace between it and the parent tag.
There is also no whitespace between this tag and the sibling text above it.
Finally, the tag has no whitespace between it and the outer tag.</p></p>

The second parameter passed into the Render method can be anything that implements the io.Writer interface, such as a file or a buffer, or the http.ResponseWriter that you get from an HTTP handler.

Using Hamlet with HTTP handlers

Using the Hamlet templates is made very easy.

package main

import (
	"fmt"
	"log"
	"net/http"

	"github.com/stackus/hamlet/examples/hello"
)

func main() {
	http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
		_ = hello.World().Render(r.Context(), w)
	})

	fmt.Println("Server starting on port 8080...")
	if err := http.ListenAndServe(":8080", nil); err != nil {
		log.Fatal(err)
	}
}

There are a number of examples showing various Haml and Hamlet features in the examples directory.

A big nod to Templ

The way that you use Hamlet is very similar to how you would use Templ. This is no accident as I am a big fan of the work being done with that engine.

After getting the Haml properly lexed, and parsed, I did not want to reinvent the wheel and come up with a whole new rendering API. The API that Templ presents is nice and easy to use, so I decided to replicate it in Hamlet.

The Hamlet template

Hamlet templates are files with the extension .hmlt that when process will produce a matching Go file with the extension .hmlt.go.

In these files you are free to write any Go code that you wish, and then drop into HAML mode using the @hmlt directive.

The following starts the creation of a SiteLayout template:

@hmlt SiteLayout() {

Hamlet templates are closed like Go functions, with a closing brace }. So a complete but empty example is this:

@hmlt SiteLayout() {
}

Inside the template you can use any Haml features, such as tags, attributes, classes, IDs, text, comments, interpolation, code inlining, code rendering, and filters.

Haml Syntax

The Haml syntax is documented at haml.info. Please see that site for more information on the Haml syntax. There are some differences that I will document here.

Important differences are:

Go package and imports

You can provide a package name at the top of your Hamlet template file. If you do not provide one then main will be used.

You may also import any packages that you need to use in your template. The imports you use and the ones brought in by Hamlet will be combined and deduplicated.

Multiple templates per file

You can declare as many templates in a file as you wish. Each template must have a unique name in the module they will be output into.

@hmlt SiteLayout() {
}

@hmlt HomePage() {
}

The templates are converted into Go functions, so they must be valid Go function names. This also means that you can declare them with parameters and can use those parameters in the template.

@hmlt SiteLayout(title string) {
	!!!
	%html{lang:"en"}
		%head
			%title= title
		%body
			-# ... the rest of the template
}

Doctypes

Only the HTML 5 doctype is supported, and is written using !!!.

@hmlt SiteLayout() {
	!!!
}

Note about indenting. It does not matter if you indent or do not indent. Haml normally complains if a doctype is not at the start of the line, but Hamlet does not. I just recommend choosing to indent or not indent, and sticking to it.

Inlined code

You won't be using Ruby here, you'll be using Go. And in Go you need to use curly braces to denote the start and end of a block of code. So instead of this:

- if user
	%strong The user exists

You would write this:

- if user != nil {
	%strong The user exists
- }

There is no processing performed on the Go code you put into the templates, so it needs to be valid Go code.

Rendering code

Like in Haml, you can output variables and the results of expressions. The = script syntax and text interpolation #{} are supported.

%strong= user.Name
%strong The user's name is #{user.Name}

The catch is what is being outputted will need to be a string in all cases. So instead of writing this to output an integer value:

%strong= user.Age

You would need to write this:

%strong= fmt.Sprintf("%d", user.Age)

Which to be honest can be a bit long to write, so a shortcut is provided:

%strong=%d user.Age

The interpolation also supports the shortcut:

%strong #{user.Name} is #{%d user.Age} years old.

When formatting a value into a string fmt.Sprintf is used under the hood, so you can use any of the formatting options that it supports.

Attributes

Only the Ruby 1.9 style of attributes is supported. This syntax is closest to the Go syntax, and is the most readable. Between the attribute name, operator, and value you can include or leave out as much whitespace as you like.

%a{href: "https://github.com/stackus/hamlet", target: "_blank"} Hamlet

You can supply a value to an attribute using the text interpolation syntax.

%a{href:#{url}} Hamlet

Attributes can be written over multiple lines, and the closing brace can be on a new line.

%a{
	href: "...",
	target: "_blank",
} Hamlet

Attributes which you want to render conditionally use the ? operator instead of the : operator. For conditional attributes the attribute value is required to be an interpolated value which will be used as the condition in a Go if statement.

%button{
	disabled ? #{disabled},
} Click me

Note: The final comma is not required on the last attribute when they are spread across multiple lines like it would be in Go. Including it is fine and will not cause any issues.

Certain characters in the attribute name will require that the name be escaped.

%button{
	"@click": "onClick",
	":disabled": "disabled",
} Click me

Keep in mind that attribute names cannot be replaced with an interpolated string; only the value can.

To support dynamic lists of attributes, you can use the @attributes directive. This directive takes a list of arguments which comes in two forms:

  • map[string]string
    • The key is the attribute name, the value is the attribute value.
    • The attribute will be rendered if the value is not empty.
  • map[string]bool
    • The key is the attribute name, the value is the condition to render the attribute.
%button{
	"@click": "onClick",
	":disabled": "disabled",
	@attributes: #{myAttrs},
} Click me

Classes

Hamlet supports the . operator for classes and also will accept the class attribute such as class:"foo bar". However, if the class attribute is given an interpolated value, it will need to be a comma separated list of values. These values can be the following types:

  • string
    • myClass variable or "foo bar" string literal
  • []string
    • Each item will be added to the class list if it is not blank.
  • map[string]bool
    • The key is the class name, the value is the condition to include the class.

Examples:

%button.foo.bar.baz Click me
%button.fizz{class:"foo bar baz"} Click me
%button.foo{class:#{myStrClasses, myBoolClasses}} Click me

All sources of classes will be combined and deduplicated into a single class attribute.

Object References

Haml supports using a Ruby object to supply the id and class for a tag using the [] object reference syntax. This is supported but is rather limited in Hamlet. The type that you use within the brackets will be expected to implement at least one or both of the following interfaces:

type ObjectIDer interface {
	ObjectID() string
}

type ObjectClasser interface {
	ObjectClass() string
}

The result of these methods will be used to populate the id and class attributes in a similar way to how Haml would apply the Ruby object references.

Filters

Only the following Haml filters are supported:

  • :plain
  • :escaped
  • :preserve
  • :javascript
  • :css

Template nesting

The biggest departure from Haml is how templates can be combined. When working Haml you could use = render :partial_name or = haml :partial_name to render a partial. The render and haml functions are not available in Hamlet, instead you can use the @render directive.

@hmlt HomePage() {
	= @render SiteLayout()
}

The above would render the SiteLayout template, and you would call it with any parameters that it needs. You can also call it and provide it with a block of content to render where it chooses.

@hmlt HomePage() {
	= @render SiteLayout()
		%p This is the home page for Hamlet.
}

Any content nested under the @render directive will be passed into the template that it can render where it wants using the @children directive.

@hmlt SiteLayout() {
	!!!
	%html{lang:"en"}
		%head
			%title Hamlet
		%body
			%h1 Hamlet
			%p A HAML-like template engine for Go.
			= @children
}

Contributing

Contributions are welcome. Please see the contributing guide for more information.

License

MIT


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