Golang Templates Cheatsheet

Posted 14 Sep 2017

The Go standard library provides a set of packages to generate output. The text/template package implements templates for generating text output, while the html/template package implements templates for generating HTML output that is safe against certain attacks. Both packages use the same interface but the following examples of the core features are directed towards HTML applications.


Table of Contents


Parsing and Creating Templates

Naming Templates

There is no defined file extension for Go templates. One of the most popular is .tmpl supported by vim-go and referenced in the text/template godocs. The extension .gohtml supports syntax highlighting in both Atom and GoSublime editors. Finally analysis of large Go codebases finds that .tpl is often used be developers. While the extension is not important it is still good to be consistent within a project for clarity.


Creating a Template

tpl, err := template.Parse(filename) will get the template at filename and store it in tpl. tpl can then be executed to show the template.


Parsing Multiple Templates

template.ParseFiles(filenames) takes a list of filenames and stores all templates. template.ParseGlob(pattern) will find all templates matching the pattern and store the templates.


Executing Templates

Execute a Single Template

Once a template has been parsed there are two options to execute them. A single template tpl can be executed using tpl.Execute(io.Writer, data). The content of tpl will be written to the io.Writer. Data is an interface passed to the template that will be useable by the template.


Executing a Named Template

tpl.ExecuteTemplate(io.Writer, name, data) works the same as execute but allows for a string name of the template the user wants to execute.


Template Encoding and HTML

Contextual Encoding

Go’s html/template package does encoding based on the context of the code. As a result, html/template encodes any characters that need encoded to be rendered correctly.

For example the < and > in "<h1>A header!</h1>" will be encoded as &lt;h1&gt;A header!&lt;/h1&gt; .

Type template.HTML can be used to skip encoding by telling Go the string is safe. template.HTML("<h1>A Safe header</h1>") will then be <h1>A Safe header</h1> . Because of this using this type with user input is dangerous.

The go html/template package is aware of attributes within the template and will encode values differently based on the attribute.

Go templates can also be used with javascript. Structs and maps will be expanded into JSON objects and quotes will be added to strings for use in function parameters and as variable values.

// Go
type Cat struct {
	Name string
	Age int
}

kitten := Cat{"Sam", 12}

// Template
<script>
	var cat = {{.kitten}}
</script>

// Javascript
var cat = {"Name":"Sam", "Age" 12}

Safe Strings and HTML Comments

The html/template package will remove any comments from a template by default. This can cause issues when comments are necessary such as detecting internet explorer.

<!--[if IE]>
Place content here to target all Internet Explorer users.
<![endif]-->

We can use the Custom Functions (Globally) method to create a function that returns html preserving comments. Define a function htmlSafe in the FuncMap of the template.

testTemplate, err = template.New("hello.gohtml").Funcs(template.FuncMap{
	"htmlSafe": func(html string) template.HTML {
		return template.HTML(html)
    },
}).ParseFiles("hello.gohtml")

This function takes a string and produces the unaltered HTML code. This function can be used in a template like so to preserve the comments <!--[if IE 6]> and <![endif]--> :

{{htmlSafe "<!--[if IE 6]>" }}
<meta http-equiv="Content-Type" content="text/html; charset=Unicode">  
{{ htmlSafe "<![endif]-->" }}

Template Variables

The dot character (.)

A template variable can be a boolean, string, character, integer, floating-point, imaginary, or complex constant in Go syntax. Data passed to the template can be accessed using dot {{ . }}.

If the data is a complex type then it’s fields can be accessed using the dot {{ .FieldName }}.

Dots can be chained together if the data contains multiple complex structures. {{ .Struct.StructTwo.Field }}


Variables in Templates

Data passed to the template can be saved in a variable and used throughout the template. {{$number := .}} We use the $number to create a variable then initialize it with the value passed to the template. To use the variable we call it in the template with {{$number}}.

{{$number := .}}
<h1> It is day number {{$number}} of the month </h1>
var tpl *template.Template

tpl = template.Must(template.ParseFiles("templateName"))

err := tpl.ExecuteTemplate(os.Stdout, "templateName", 23)

In this example we pass 23 to the template and stored in the $number variable which can be used anywhere in the template


Template Actions

If/Else Statements

Go templates support if/else statements like many programming languages. We can use if to check for values, if it doesn’t exist we can use an else value. The empty values are false, 0, any nil pointer or interface value, and any array, slice, map, or string of length zero.

<h1>Hello, {{if .Name}} {{.Name}} {{else}} there {{end}}!</h1>  

If .Name exists then Hello, Name will be printed (replaced with the name value) otherwise it will print Hello, Anonymous.

Templates also provide {{else if .Name2 }} which can be used to evaluate a second option after an if.


Removing Whitespace

Adding different values to a template can add various amounts of whitespace. We can either change our template to better handle it, by ignoring or minimizing effects, or we can use the minus sign - within out template.

<h1>Hello, {{if .Name}} {{.Name}} {{- else}} Anonymous {{- end}}!</h1>

Here we care telling the template to remove all spaces between the Name variable and whatever comes after it. We are doing the same with the end keyword. This allows use to have whitespace within the template for easier reading but remove it in production.


Range Blocks

Go templates have a range keyword to iterate over all objects in a structure. Suppose we had the Go structures:

type Item struct {
	Name  string
	Price int
}

type ViewData struct {
	Name  string
	Items []Item
}

We have an Item, with a name and price, then a ViewData which is the structure sent to the template. Consider template containing the following:

{{range .Items}}
  <div class="item">
    <h3 class="name">{{.Name}}</h3>
    <span class="price">${{.Price}}</span>
  </div>
{{end}}

For each Item in the range of Items (in the ViewData structure) get the Name and Price of that item and create html for each Item automatically. Within a range each Item becomes the {{.}} in the template and the item properties become {{.Name}} or {{.Price}} .


Template Functions

The template package provides a list of predefined global functions. Below are some of the most used.


Indexing structures in Templates

If the data passed to the template is a map, slice, or array it can be indexed from the template. We use {{index x number}} where index is the keyword, x is the data and number is a integer for the index value. If we had {{index names 2}} it is equivalent to names[2]. We can add more integers to index deeper into data. {{index names 2 3 4}} is equivalent to names[2][3][4].

<body>
    <h1> {{index .FavNums 2 }}</h1>
</body>
type person struct {
	Name    string
	FavNums []int
}

func main() {

	tpl := template.Must(template.ParseGlob("*.gohtml"))
	tpl.Execute(os.Stdout, &person{"Curtis", []int{7, 11, 94}})
}

This code example passes a person structure and gets the 3rd favourite number from the FavNums slice.


The and Function

The and function returns the boolean AND of its arguments by returning the first empty argument or the last argument. and x y behaves logically as if x then y else x . Consider the following go code

type User struct {  
  Admin bool
}

type ViewData struct {  
  *User
}

Pass a ViewData with a User that has Admin set true to the following template

{{if and .User .User.Admin}}
  You are an admin user!
{{else}}
  Access denied!
{{end}}

The result will be You are an admin user!. However if the ViewData did not include a *User object or Admin was set as false then the result will be Access denied!.


The or Function

The or function operates similarly to the and function however will top at the first true. or x y is equivalent to if x then x else y so y will never be evaluated if x is not empty.


The not Function

The not function returns the boolean negation of the argument.

{{ if not .Authenticated}}
  Access Denied!
{{ end }}

Template Comparison Functions

Comparisons

The html/template package provides a variety of functions to do comparisons between operators. The operators may only be basic types or named basic types such as type Temp float32 Remember that template functions take the form {{ function arg1 arg2 }}.

  • eq Returns the result of arg1 == arg2
  • ne Returns the result of arg1 != arg2
  • lt Returns the result of arg1 < arg2
  • le Returns the result of arg1 <= arg2
  • gt Returns the result of arg1 > arg2
  • ge Returns the result of arg1 >= arg2

Of special note eq can be used with two or more arguments by comparing all arguments to the first. {{ eq arg1 arg2 arg3 arg4}} will result in the following logical expression:

arg1==arg2 || arg1==arg3 || arg1==arg4


Nested Templates and Layouts

Nesting Templates

Nested templates can be used for parts of code frequently used across templates, a footer or header for example. Rather than updating each template separately we can use a nested template that all other templates can use. Can could define a template as follows:

{{define "footer"}}
<footer> 
	<p>Here is the footer</p>
</footer>
{{end}}

A template named “footer” is defined which can be used in other templates like so:

{{template "footer"}}

Passing Variables between Templates

The template action used to include nested templates also allows a second parameter to pass data to the nested template.

// Define a nested template called header
{{define "header"}}
	<h1>{{.}}</h1>
{{end}}

// Call template and pass a name parameter
{{range .Items}}
  <div class="item">
    {{template "header" .Name}}
    <span class="price">${{.Price}}</span>
  </div>
{{end}}

We use the same range to loop through Items as before but we pass the name to the header template each time in this simple example.


Creating Layouts

Glob patterns specify sets of filenames with wildcard characters. The template.ParseGlob(pattern string) function will parse all templates that match the string pattern. template.ParseFiles(files...) can also be used with a list of file names.

The templates are named by default based on the base names of the argument files. This mean views/layouts/hello.gohtml will have the name hello.gohtml . If the template has a ``{{define “templateName”}}` then that name will be usable.

A specific template can be executed using t.ExecuteTemplate(w, "templateName", nil) . t is an object of type Template, w is type io.Writer such as an http.ResponseWriter, Then there is the name of the template to execute, and finally passing any data to the template.

Example main.go file

// Omitted imports & package

var LayoutDir string = "views/layouts"  
var bootstrap *template.Template

func main() {
	var err error
	bootstrap, err = template.ParseGlob(LayoutDir + "/*.gohtml")
	if err != nil {
		panic(err)
	}

	http.HandleFunc("/", handler)
	http.ListenAndServe(":8080", nil)
}

func handler(w http.ResponseWriter, r *http.Request) {
	bootstrap.ExecuteTemplate(w, "bootstrap", nil)
}

All .gohtml files are parsed in main. When route / is reached the template defined as bootstrap is executed using the handler function.

Example views/layouts/bootstrap.gohtml file

{{define "bootstrap"}}
<!DOCTYPE html>  
<html lang="en">  
  <head>
    <title>Go Templates</title>
    <link href="//maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" 
	rel="stylesheet">
  </head>
  <body>
    <div class="container-fluid">
      <h1>Filler header</h1>
	  <p>Filler paragraph</p>
    </div>
    <!-- jquery & Bootstrap JS -->
    <script src="//ajax.googleapis.com/ajax/libs/jquery/1.11.3/jquery.min.js"  
    </script>
    <script src="//maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js">
    </script>
  </body>
</html>  
{{end}}

Templates Calling Functions

Function Variables (calling struct methods)

We can use templates to call the methods of objects in the template to return data. Consider the User struct with the following method.

type User struct {  
  ID    int
  Email string
}

func (u User) HasPermission(feature string) bool {  
  if feature == "feature-a" {
    return true
  } else {
    return false
  }
}

When a type User has been passed to the template we can then call this method from the template.

{{if .User.HasPermission "feature-a"}}
  <div class="feature">
    <h3>Feature A</h3>
    <p>Some other stuff here...</p>
  </div>
{{else}}
  <div class="feature disabled">
    <h3>Feature A</h3>
    <p>To enable Feature A please upgrade your plan</p>
  </div>
{{end}}

The template checks if the User HasPermission for the feature and renders depending on the result.


Function Variables (call)

If the Method HasPermission has to change at times than the Function Variables (Methods) implementation may not fit the design. Instead a HasPermission func(string) bool attribute can be added on the User type. This can then have a function assigned to it at creation.

// Structs
type ViewData struct {  
  User User
}

type User struct {  
  ID            int
  Email         string
  HasPermission func(string) bool
}

// Example of creating a ViewData
vd := ViewData{
		User: User{
			ID:    1,
			Email: "curtis.vermeeren@gmail.com",
			// Create the HasPermission function
			HasPermission: func(feature string) bool {
				if feature == "feature-b" {
					return true
				}
				return false
			},
		},
	}

// Executing the ViewData with the template
err := testTemplate.Execute(w, vd)

We need to tell the Go template that we want to call this function so we must change the template from the Function Variables (Methods) implementation to do this. We use the call keyword supplied by the go html/template package. Changing the previous template to use call results in:

{{if (call .User.HasPermission "feature-b")}}
  <div class="feature">
    <h3>Feature B</h3>
    <p>Some other stuff here...</p>
  </div>
{{else}}
  <div class="feature disabled">
    <h3>Feature B</h3>
    <p>To enable Feature B please upgrade your plan</p>
  </div>
{{end}}

Custom Functions

Another way to call functions is to create custom function with template.FuncMap . This method create global methods that can be used throughout the entire application. FuncMap has type map[string]interface{} mapping strings to functions. The mapped functions must have either a single return value, or two return values where the second has type error.

// Creating a template with function hasPermission
testTemplate, err = template.New("hello.gohtml").Funcs(template.FuncMap{
    "hasPermission": func(user User, feature string) bool {
      if user.ID == 1 && feature == "feature-a" {
        return true
      }
      return false
    },
  }).ParseFiles("hello.gohtml")

Here the function to check if a user has permission for a feature is mapped to the string "hasPermission" and stored in the FuncMap. Note that the custom functions must be created before calling ParseFiles()

The function could be executed in the template as follows:

{{ if hasPermission .User "feature-a" }}

The .User and string "feature-a" are both passed to hasPermission as arguments.


Custom Functions (Globally)

The other 2 methods of custom functions rely on .User being passed to the template. This works in many cases but in a large application passing too many objects to a template can become difficult to maintain across main templates. We can change the implementation of the custom function to work without the .User being passed.

Using a similar feature example as the other 2 sections first you would have to create a default hasPermission function to define it for the template’s function map.

  testTemplate, err = template.New("hello.gohtml").Funcs(template.FuncMap{
    "hasPermission": func(feature string) bool {
      return false
    },
  }).ParseFiles("hello.gohtml")

This function could be placed in main() or somewhere that ensures the default hasPermission is created in the hello.gohtml function map. The default function just returns false but it defines the function and implementation that doesn’t require User .

Next a closure could be used to redefine the hasPermission function. It would use the User data available when it is created in a handler rather than having User data passed to it. Within the handler for the template you can redefine any functions to use the information available.

func handler(w http.ResponseWriter, r *http.Request) {
	w.Header().Set("Content-Type", "text/html")

	user := User{
		ID:    1,
		Email: "Curtis.vermeeren@gmail.com",
	}
	vd := ViewData{}
	err := testTemplate.Funcs(template.FuncMap{
		"hasPermission": func(feature string) bool {
			if user.ID == 1 && feature == "feature-a" {
				return true
			}
			return false
		},
	}).Execute(w, vd)
	if err != nil {
		http.Error(w, err.Error(), http.StatusInternalServerError)
	}
}

In this handler a User is created with ID and Email, Then a ViewData is created without passing the user to it. The hasPermission function is redefined using user.ID which is available when the function is created. {{if hasPermission "feature-a"}} can be used in a template without having to pass a User to the template.