Flexible Mustache templates for Swift
Swift Objective-C Other
Latest commit 54458f1 Oct 26, 2016 @groue Documentation


GRMustache.swift Swift Platforms License

Mustache templates for Swift

Latest release: October 23, 2016 • version 2.0.0 • CHANGELOG

Requirements: iOS 8.0+ / OSX 10.9+ / tvOS 9.0+ • Xcode 8+ • Swift 3

Follow @groue on Twitter for release announcements and usage tips.



GRMustache extends the genuine Mustache language with built-in goodies and extensibility hooks that let you avoid the strict minimalism of Mustache when you need it.

  • Support for the full Mustache syntax
  • Filters, as {{ uppercase(name) }}
  • Template inheritance, as in hogan.js, mustache.java and mustache.php.
  • Built-in goodies
  • GRMustache.swift does not rely on the Objective-C runtime. It lets you feed your templates with ad-hoc values or your existing models, without forcing you to refactor your Swift code into Objective-C objects.


The library is built around two main APIs:

  • The Template(...) initializer that loads a template.
  • The Template.render(...) method that renders your data.


Hello {{name}}
Your beard trimmer will arrive on {{format(date)}}.
Well, on {{format(realDate)}} because of a Martian attack.
import Mustache

// Load the `document.mustache` resource of the main bundle
let template = try Template(named: "document")

// Let template format dates with `{{format(...)}}`
let dateFormatter = DateFormatter()
dateFormatter.dateStyle = .medium
template.register(dateFormatter, forKey: "format")

// The rendered data
let data: [String: Any] = [
    "name": "Arthur",
    "date": Date(),
    "realDate": Date().addingTimeInterval(60*60*24*3),
    "late": true

// The rendering: "Hello Arthur..."
let rendering = try template.render(data)



CocoaPods is a dependency manager for Xcode projects.

To use GRMustache.swift with CocoaPods, specify in your Podfile:

source 'https://github.com/CocoaPods/Specs.git'

pod 'GRMustache.swift'


Carthage is another dependency manager for Xcode projects.

To use GRMustache.swift with Carthage, specify in your Cartfile:

github "groue/GRMustache.swift"

Swift Package Manager

The Swift Package Manager is the open source tool for managing the distribution of Swift code.

To use GRMustache.swift with the Swift Package Manager, add https://github.com/groue/GRMustache.swift to the list of your package dependencies:

import PackageDescription

let package = Package(
    name: "MyPackage",
    targets: [],
    dependencies: [
        .Package(url: "https://github.com/groue/GRMustache.swift", majorVersion: 2, minor: 0),

Check groue/GRMustacheSPM for a sample Swift package that uses GRMustache.swift.


  1. Download a copy of GRMustache.swift.

  2. Checkout the latest GRMustache.swift version:

    cd [GRMustache.swift directory]
    git checkout 2.0.0
  3. Embed the Mustache.xcodeproj project in your own project.

  4. Add the MustacheOSX, MustacheiOS, or MustacheWatchOS target in the Target Dependencies section of the Build Phases tab of your application target.

  5. Add the the Mustache.framework from the targetted platform to the Embedded Binaries section of the General tab of your target.

See MustacheDemoiOS for an example of such integration.


To fiddle with the library, open the Xcode/Mustache.xcworkspace workspace: it contains a Mustache-enabled Playground at the top of the files list.

External links:

Rendering templates:

Feeding templates:


Loading Templates

Templates may come from various sources:

  • Raw Swift strings:

    let template = try Template(string: "Hello {{name}}")
  • Bundle resources:

    // Loads the "document.mustache" resource of the main bundle:
    let template = try Template(named: "document")
  • Files and URLs:

    let template = try Template(path: "/path/to/document.mustache")
    let template = try Template(URL: templateURL)
  • Template Repositories:

    Template repositories represent a group of templates. They can be configured independently, and provide neat features like template caching. For example:

    // The repository of Bash templates, with extension ".sh":
    let repo = TemplateRepository(bundle: Bundle.main, templateExtension: "sh")
    // Disable HTML escaping for Bash scripts:
    repo.configuration.contentType = .text
    // Load the "script.sh" resource:
    let template = repo.template(named: "script")!

For more information, check:


Not funny, but they happen. Standard errors of domain NSCocoaErrorDomain, etc. may be thrown whenever the library needs to access the file system or other system resource. Mustache-specific errors are of type MustacheError:

do {
    let template = try Template(named: "Document")
    let rendering = try template.render(data)
} catch let error as MustacheError {
    // Parse error at line 2 of template /path/to/template.mustache:
    // Unclosed Mustache tag.

    // templateNotFound, parseError, or renderError

    // The eventual template at the source of the error. Can be a path, a URL,
    // a resource name, depending on the repository data source.

    // The eventual faulty line.

    // The eventual underlying error.

Mustache Tags Reference

Mustache is based on tags: {{name}}, {{#registered}}...{{/registered}}, {{>include}}, etc.

Each one of them performs its own little task:

Variable Tags

A Variable tag {{value}} renders the value associated with the key value, HTML-escaped. To avoid HTML-escaping, use triple mustache tags {{{value}}}:

let template = try Template(string: "{{value}} - {{{value}}}")

// Mario & Luigi - Mario & Luigi
let data = ["value": "Mario & Luigi"]
let rendering = try template.render(data)

Section Tags

A Section tag {{#value}}...{{/value}} is a common syntax for three different usages:

  • conditionally render a section.
  • loop over a collection.
  • dig inside an object.

Those behaviors are triggered by the value associated with value:

Falsey values

If the value is falsey, the section is not rendered. Falsey values are:

  • missing values
  • false boolean
  • zero numbers
  • empty strings
  • empty collections
  • NSNull

For example:

let template = try Template(string: "<{{#value}}Truthy{{/value}}>")

// "<Truthy>"
try template.render(["value": true])
// "<>"
try template.render([:])                  // missing value
try template.render(["value": false])     // false boolean


If the value is a collection (an array or a set), the section is rendered as many times as there are elements in the collection, and inner tags have direct access to the keys of elements:


{{# friends }}
- {{ name }}
{{/ friends }}


  "friends": [
    [ "name": "Hulk Hogan" ],
    [ "name": "Albert Einstein" ],
    [ "name": "Tom Selleck" ],


- Hulk Hogan
- Albert Einstein
- Tom Selleck

Other Values

If the value is not falsey, and not a collection, then the section is rendered once, and inner tags have direct access to the value's keys:


{{# user }}
- {{ name }}
- {{ score }}
{{/ user }}


  "user": [
    "name": "Mario"
    "score": 1500


- Mario
- 1500

Inverted Section Tags

An Inverted section tag {{^value}}...{{/value}} renders when a regular section {{#value}}...{{/value}} would not. You can think of it as the Mustache "else" or "unless".


{{# persons }}
- {{name}} is {{#alive}}alive{{/alive}}{{^alive}}dead{{/alive}}.
{{/ persons }}
{{^ persons }}
{{/ persons }}


  "persons": []




  "persons": [
    ["name": "Errol Flynn", "alive": false],
    ["name": "Sacha Baron Cohen", "alive": true]


- Errol Flynn is dead.
- Sacha Baron Cohen is alive.

Partial Tags

A Partial tag {{> partial }} includes another template, identified by its name. The included template has access to the currently available data:


{{# guests }}
  {{> person }}
{{/ guests }}


{{ name }}


  "guests": [
    ["name": "Frank Zappa"],
    ["name": "Lionel Richie"]


- Frank Zappa
- Lionel Richie

Recursive partials are supported, but your data should avoid infinite loops.

Partial lookup depends on the origin of the main template:

File System

Partial names are relative paths when the template comes from the file system (via paths or URLs):

// Load /path/document.mustache
let template = Template(path: "/path/document.mustache")

// {{> partial }} includes /path/partial.mustache.
// {{> shared/partial }} includes /path/shared/partial.mustache.

Partials have the same file extension as the main template.

// Loads /path/document.html
let template = Template(path: "/path/document.html")

// {{> partial }} includes /path/partial.html.

When your templates are stored in a hierarchy of directories, you can use absolute paths to partials, with a leading slash. For that, you need a template repository which will define the root of absolute partial paths:

let repository = TemplateRepository(directoryPath: "/path")
let template = repository.template(named: ...)

// {{> /shared/partial }} includes /path/shared/partial.mustache.

Bundle Resources

Partial names are interpreted as resource names when the template is a bundle resource:

// Load the document.mustache resource from the main bundle
let template = Template(named: "document")

// {{> partial }} includes the partial.mustache resource.

Partials have the same file extension as the main template.

// Load the document.html resource from the main bundle
let template = Template(named: "document", templateExtension: "html")

// {{> partial }} includes the partial.html resource.

General case

Generally speaking, partial names are always interpreted by a Template Repository:

  • Template(named:...) uses a bundle-based template repository: partial names are resource names.
  • Template(path:...) uses a file-based template repository: partial names are relative paths.
  • Template(URL:...) uses a URL-based template repository: partial names are relative URLs.
  • Template(string:...) uses a template repository that can’t load any partial.
  • templateRepository.template(named:...) uses the partial loading mechanism of the template repository.

Check TemplateRepository.swift for more information (read on cocoadocs.org).

Dynamic Partials

A tag {{> partial }} includes a template, the one that is named "partial". One can say it is statically determined, since that partial has already been loaded before the template is rendered:

let repo = TemplateRepository(bundle: Bundle.main)
let template = try repo.template(string: "{{#user}}{{>partial}}{{/user}}")

// Now the `partial.mustache` resource has been loaded. It will be used when
// the template is rendered. Nothing can change that.

You can also include dynamic partials. To do so, use a regular variable tag {{ partial }}, and provide the template of your choice for the key "partial" in your rendered data:

// A template that delegates the rendering of a user to a partial.
// No partial has been loaded yet.
let template = try Template(string: "{{#user}}{{partial}}{{/user}}")

// The user
let user = ["firstName": "Georges", "lastName": "Brassens", "occupation": "Singer"]

// Two different partials:
let partial1 = try Template(string: "{{firstName}} {{lastName}}")
let partial2 = try Template(string: "{{occupation}}")

// Two different renderings of the same template:
// "Georges Brassens"
try template.render(["user": user, "partial": partial1])
// "Singer"
try template.render(["user": user, "partial": partial2])

Partial Override Tags

GRMustache.swift supports Template Inheritance, like hogan.js, mustache.java and mustache.php.

A Partial Override Tag {{< layout }}...{{/ layout }} includes another template inside the rendered template, just like a regular partial tag {{> partial}}.

However, this time, the included template can contain blocks, and the rendered template can override them. Blocks look like sections, but use a dollar sign: {{$ overrideMe }}...{{/ overrideMe }}.

The included template layout.mustache below has title and content blocks that the rendered template can override:

    <title>{{$ title }}Default title{{/ title }}</title>
    <h1>{{$ title }}Default title{{/ title }}</h1>
    {{$ content }}
        Default content
    {{/ content }}}

The rendered template article.mustache:

{{< layout }}

    {{$ title }}{{ article.title }}{{/ title }}

    {{$ content }}
        {{{ article.html_body }}}
        <p>by {{ article.author }}</p>
    {{/ content }}

{{/ layout }}
let template = try Template(named: "article")
let data = [
    "article": [
        "title": "The 10 most amazing handlebars",
        "html_body": "<p>...</p>",
        "author": "John Doe"
let rendering = try template.render(data)

The rendering is a full HTML page:

    <title>The 10 most amazing handlebars</title>
    <h1>The 10 most amazing handlebars</h1>
    <p>by John Doe</p>

A few things to know:

  • A block {{$ title }}...{{/ title }} is always rendered, and rendered once. There is no boolean checks, no collection iteration. The "title" identifier is a name that allows other templates to override the block, not a key in your rendered data.

  • A template can contain several partial override tags.

  • A template can override a partial which itself overrides another one. Recursion is possible, but your data should avoid infinite loops.

  • Generally speaking, any part of a template can be refactored with partials and partial override tags, without requiring any modification anywhere else (in other templates that depend on it, or in your code).

Dynamic Partial Overrides

Like a regular partial tag, a partial override tag {{< layout }}...{{/ layout }} includes a statically determined template, the very one that is named "layout".

To override a dynamic partial, use a regular section tag {{# layout }}...{{/ layout }}, and provide the template of your choice for the key "layout" in your rendered data.

Set Delimiters Tags

Mustache tags are generally enclosed by "mustaches" {{ and }}. A Set Delimiters Tag can change that, right inside a template.

Default tags: {{ name }}
{{=<% %>=}}
ERB-styled tags: <% name %>
<%={{ }}=%>
Default tags again: {{ name }}

There are also APIs for setting those delimiters. Check Configuration.tagDelimiterPair in Configuration.swift (read on cocoadocs.org).

Comment Tags

{{! Comment tags }} are simply not rendered at all.

Pragma Tags

Several Mustache implementations use Pragma tags. They start with a percent % and are not rendered at all. Instead, they trigger implementation-specific features.

GRMustache.swift interprets two pragma tags that set the content type of the template:


HTML templates is the default. They HTML-escape values rendered by variable tags {{name}}.

In a text template, there is no HTML-escaping. Both {{name}} and {{{name}}} have the same rendering. Text templates are globally HTML-escaped when included in HTML templates.

For a more complete discussion, see the documentation of Configuration.contentType in Configuration.swift.

The Context Stack and Expressions

The Context Stack

Variable and section tags fetch values in the data you feed your templates with: {{name}} looks for the key "name" in your input data, or, more precisely, in the context stack.

That context stack grows as the rendering engine enters sections, and shrinks when it leaves. Its top value, pushed by the last entered section, is where a {{name}} tag starts looking for the "name" identifier. If this top value does not provide the key, the tag digs further down the stack, until it finds the name it looks for.

For example, given the template:

- {{firstName}} {{lastName}}


    "lastName": "Johnson",
    "family": [
        ["firstName": "Peter"],
        ["firstName": "Barbara"],
        ["firstName": "Emily", "lastName": "Scott"],

The rendering is:

- Peter Johnson
- Barbara Johnson
- Emily Scott

The context stack is usually initialized with the data you render your template with:

// The rendering starts with a context stack containing `data`

Precisely speaking, a template has a base context stack on top of which the rendered data is added. This base context is always available whatever the rendered data. For example:

// The base context contains `baseData`

// The rendering starts with a context stack containing `baseData` and `data`

The base context is usually a good place to register filters:

template.extendBaseContext(["each": StandardLibrary.each])

But you will generally register filters with the register(:forKey:) method, because it prevents the rendered data from overriding the name of the filter:

template.register(StandardLibrary.each, forKey: "each")

See Template for more information on the base context.


Variable and section tags contain Expressions. name is an expression, but also article.title, and format(article.modificationDate). When a tag renders, it evaluates its expression, and renders the result.

There are four kinds of expressions:

  • The dot . aka "Implicit Iterator" in the Mustache lingo:

    Implicit iterator evaluates to the top of the context stack, the value pushed by the last entered section.

    It lets you iterate over collection of strings, for example. {{#items}}<{{.}}>{{/items}} renders <1><2><3> when given [1,2,3].

  • Identifiers like name:

    Evaluation of identifiers like name goes through the context stack until a value provides the name key.

    Identifiers can not contain white space, dots, parentheses and commas. They can not start with any of those characters: {}&$#^/<>.

  • Compound expressions like article.title and generally <expression>.<identifier>:

    This time there is no going through the context stack: article.title evaluates to the title of the article, regardless of title keys defined by enclosing contexts.

    .title (with a leading dot) is a compound expression based on the implicit iterator: it looks for title at the top of the context stack.

    Compare these three templates:

    • ...{{# article }}{{ title }}{{/ article }}...
    • ...{{# article }}{{ .title }}{{/ article }}...
    • ...{{ article.title }}...

    The first will look for title anywhere in the context stack, starting with the article object.

    The two others are identical: they ensure the title key comes from the very article object.

  • Filter expressions like format(date) and generally <expression>(<expression>, ...):

    Filters are introduced below.


Templates render values:

template.render(["name": "Luigi"])
template.render(Person(name: "Luigi"))

You can feed templates with:

  • Values that adopt the MustacheBoxable protocol such as String, Int, NSObject and its subclasses (see Standard Swift Types Reference and Custom Types)

  • Arrays, sets, and dictionaries (Swift arrays, sets, dictionaries, and Foundation collections). This does not include other collections, such as Swift ranges.

  • A few function types such as filter functions, lambdas, and other functions involved in advanced boxes.

  • Goodies such as Foundation's formatters.

Standard Swift Types Reference

GRMustache.swift comes with built-in support for the following standard Swift types:


  • {{bool}} renders "0" or "1".
  • {{#bool}}...{{/bool}} renders if and only if bool is true.
  • {{^bool}}...{{/bool}} renders if and only if bool is false.

Numeric Types

GRMustache supports Int, UInt, Int64, UInt64, Float, Double and CGFloat:

  • {{number}} renders the standard Swift string interpolation of number.
  • {{#number}}...{{/number}} renders if and only if number is not 0 (zero).
  • {{^number}}...{{/number}} renders if and only if number is 0 (zero).

The Swift types Int8, UInt8, etc. have no built-in support: turn them into one of the three general types before injecting them into templates.

To format numbers, you can use NumberFormatter:

let percentFormatter = NumberFormatter()
percentFormatter.numberStyle = .percent

let template = try Template(string: "{{ percent(x) }}")
template.register(percentFormatter, forKey: "percent")

// Rendering: 50%
let data = ["x": 0.5]
let rendering = try template.render(data)

More info on Formatter.


  • {{string}} renders string, HTML-escaped.
  • {{{string}}} renders string, not HTML-escaped.
  • {{#string}}...{{/string}} renders if and only if string is not empty.
  • {{^string}}...{{/string}} renders if and only if string is empty.

Exposed keys:

  • string.length: the length of the string.


  • {{set}} renders the concatenation of the renderings of set elements.
  • {{#set}}...{{/set}} renders as many times as there are elements in the set, pushing them on top of the context stack.
  • {{^set}}...{{/set}} renders if and only if the set is empty.

Exposed keys:

  • set.first: the first element.
  • set.count: the number of elements in the set.


  • {{array}} renders the concatenation of the renderings of array elements.
  • {{#array}}...{{/array}} renders as many times as there are elements in the array, pushing them on top of the context stack.
  • {{^array}}...{{/array}} renders if and only if the array is empty.

Exposed keys:

  • array.first: the first element.
  • array.last: the last element.
  • array.count: the number of elements in the array.

In order to render array indexes, or vary the rendering according to the position of elements in the array, use the each filter from the Standard Library:


Users with their positions:
{{# each(users) }}
- {{ @indexPlusOne }}: {{ name }}

Comma-separated user names:
{{# each(users) }}{{ name }}{{^ @last }}, {{/}}{{/}}.
let template = try! Template(named: "document")

// Register StandardLibrary.each for the key "each":
template.register(StandardLibrary.each, forKey: "each")

// Users with their positions:
// - 1: Alice
// - 2: Bob
// - 3: Craig
// Comma-separated user names: Alice, Bob, Craig.
let users = [["name": "Alice"], ["name": "Bob"], ["name": "Craig"]]
let rendering = try! template.render(["users": users])


  • {{dictionary}} renders the standard Swift string interpolation of dictionary (not very useful).
  • {{#dictionary}}...{{/dictionary}} renders once, pushing the dictionary on top of the context stack.
  • {{^dictionary}}...{{/dictionary}} does not render.

In order to iterate over the key/value pairs of a dictionary, use the each filter from the Standard Library:


{{# each(dictionary) }}
    key: {{ @key }}, value: {{.}}
let template = try! Template(named: "document")

// Register StandardLibrary.each for the key "each":
template.register(StandardLibrary.each, forKey: "each")

// Renders "key: name, value: Freddy Mercury"
let dictionary = ["name": "Freddy Mercury"]
let rendering = try! template.render(["dictionary": dictionary])


The rendering of NSObject depends on the actual class:

  • NSFastEnumeration

    When an object conforms to the NSFastEnumeration protocol, like NSArray, it renders just like Swift Array. NSSet is an exception, rendered as a Swift Set. NSDictionary, the other exception, renders as a Swift Dictionary.

  • NSNumber is rendered as a Swift Bool, Int, UInt, Int64, UInt64, Float or Double, depending on its value.

  • NSString is rendered as String

  • NSNull renders as:

    • {{null}} does not render.
    • {{#null}}...{{/null}} does not render.
    • {{^null}}...{{/null}} renders.
  • For other NSObject, those default rules apply:

    • {{object}} renders the description method, HTML-escaped.
    • {{{object}}} renders the description method, not HTML-escaped.
    • {{#object}}...{{/object}} renders once, pushing the object on top of the context stack.
    • {{^object}}...{{/object}} does not render.

    With support for Objective-C runtime, templates can render object properties: {{ user.name }}.

    Subclasses can alter this behavior by overriding the mustacheBox method of the MustacheBoxable protocol. For more information, check the rendering of Custom Types below.

Custom Types

NSObject subclasses

NSObject subclasses can trivially feed your templates:

// An NSObject subclass
class Person : NSObject {
    let name: String

    init(name: String) {
        self.name = name

// Charlie Chaplin has a mustache.
let person = Person(name: "Charlie Chaplin")
let template = try Template(string: "{{name}} has a mustache.")
let rendering = try template.render(person)

When extracting values from your NSObject subclasses, GRMustache.swift uses the Key-Value Coding method valueForKey:, as long as the key is "safe" (safe keys are the names of declared properties, including NSManagedObject attributes).

Subclasses can alter this default behavior by overriding the mustacheBox method of the MustacheBoxable protocol, described below:

Pure Swift Values and MustacheBoxable

Key-Value Coding is not available for Swift enums, structs and classes, regardless of eventual @objc or dynamic modifiers. Swift values can still feed templates, though, with a little help.

// Define a pure Swift object:
struct Person {
    let name: String

To let Mustache templates extract the name key out of a person so that they can render {{ name }} tags, we need to explicitly help the Mustache engine by conforming to the MustacheBoxable protocol:

extension Person : MustacheBoxable {

    // Feed templates with a dictionary:
    var mustacheBox: MustacheBox {
        return Box(["name": self.name])

Your mustacheBox implementation will generally call the Box function on a regular value that itself adopts the MustacheBoxable protocol (such as String or Int), or an array, a set, or a dictionary.

Now we can render persons, arrays of persons, dictionaries of persons, etc:

// Freddy Mercury has a mustache.
let person = Person(name: "Freddy Mercury")
let template = try Template(string: "{{name}} has a mustache.")
let rendering = try template.render(person)

Boxing a dictionary is an easy way to build a box. However there are many kinds of boxes: check the rest of this documentation.


Mustache lambdas are functions that let you perform custom rendering. There are two kinds of lambdas: those that process section tags, and those that render variable tags.

// `{{fullName}}` renders just as `{{firstName}} {{lastName}}.`
let fullName = Lambda { "{{firstName}} {{lastName}}" }

// `{{#wrapped}}...{{/wrapped}}` renders the content of the section, wrapped in
// a <b> HTML tag.
let wrapped = Lambda { (string) in "<b>\(string)</b>" }

// <b>Frank Zappa is awesome.</b>
let templateString = "{{#wrapped}}{{fullName}} is awesome.{{/wrapped}}"
let template = try Template(string: templateString)
let data: [String: Any] = [
    "firstName": "Frank",
    "lastName": "Zappa",
    "fullName": fullName,
    "wrapped": wrapped]
let rendering = try template.render(data)

Lambdas are a special case of custom rendering functions. The raw RenderFunction type gives you extra flexibility when you need to perform custom rendering. See CoreFunctions.swift (read on cocoadocs.org).

☝️ Note: Mustache lambdas slightly overlap with dynamic partials. Lambdas are required by the Mustache specification. Dynamic partials are more efficient because they avoid parsing lambda strings over and over.


Filters apply like functions, with parentheses: {{ uppercase(name) }}.

Generally speaking, using filters is a three-step process:

// 1. Define the filter using the `Filter()` function:
let uppercase = Filter(...)

// 2. Assign a name to your filter, and register it in a template:
template.register(uppercase, forKey: "uppercase")

// 3. Render

It helps thinking about four kinds of filters:

Value Filters

Value filters transform any type of input. They can return anything as well.

For example, here is a square filter which squares integers:

// Define the `square` filter.
// square(n) evaluates to the square of the provided integer.
let square = Filter { (n: Int?) in
    guard let n = n else {
        // No value, or not an integer: return nil.
        // We could throw an error as well.
        return nil

    // Return the result
    return n * n

// Register the square filter in our template:
let template = try Template(string: "{{n}} × {{n}} = {{square(n)}}")
template.register(square, forKey:"square")

// 10 × 10 = 100
let rendering = try template.render(["n": 10])

Filters can accept a precisely typed argument as above. You may prefer managing the value type yourself:

// Define the `abs` filter.
// abs(x) evaluates to the absolute value of x (Int or Double):
let absFilter = Filter { (box: MustacheBox) in
    switch box.value {
    case let int as Int:
        return abs(int)
    case let double as Double:
        return abs(double)
        return nil

You can process collections and dictionaries as well, and return new ones:

// Define the `oneEveryTwoItems` filter.
// oneEveryTwoItems(collection) returns the array of even items in the input
// collection.
let oneEveryTwoItems = Filter { (box: MustacheBox) in
    // `box.arrayValue` returns a `[MustacheBox]` for all boxed collections
    // (Array, Set, NSArray, etc.).
    guard let boxes = box.arrayValue else {
        // No value, or not a collection: return the empty box
        return nil

    // Rebuild another array with even indexes:
    var result: [MustacheBox] = []
    for (index, box) in boxes.enumerated() where index % 2 == 0 {

    return result

// A template where the filter is used in a section, so that the items in the
// filtered array are iterated:
let templateString = "{{# oneEveryTwoItems(items) }}<{{.}}>{{/ oneEveryTwoItems(items) }}"
let template = try Template(string: templateString)

// Register the oneEveryTwoItems filter in our template:
template.register(oneEveryTwoItems, forKey: "oneEveryTwoItems")

// <1><3><5><7><9>
let rendering = try template.render(["items": Array(1..<10)])

Multi-arguments filters are OK as well. but you use the VariadicFilter() function, this time:

// Define the `sum` filter.
// sum(x, ...) evaluates to the sum of provided integers
let sum = VariadicFilter { (boxes: [MustacheBox]) in
    var sum = 0
    for box in boxes {
        sum += (box.value as? Int) ?? 0
    return sum

// Register the sum filter in our template:
let template = try Template(string: "{{a}} + {{b}} + {{c}} = {{ sum(a,b,c) }}")
template.register(sum, forKey: "sum")

// 1 + 2 + 3 = 6
let rendering = try template.render(["a": 1, "b": 2, "c": 3])

Filters can chain and generally be part of more complex expressions:

Circle area is {{ format(product(PI, circle.radius, circle.radius)) }} cm².

When you want to format values, just use NumberFormatter, DateFormatter, or generally any Foundation's Formatter. They are ready-made filters:

let percentFormatter = NumberFormatter()
percentFormatter.numberStyle = .percent

let template = try Template(string: "{{ percent(x) }}")
template.register(percentFormatter, forKey: "percent")

// Rendering: 50%
let data = ["x": 0.5]
let rendering = try template.render(data)

More info on formatters.

Pre-Rendering Filters

Value filters as seen above process input values, which may be of any type (bools, ints, collections, etc.). Pre-rendering filters always process strings, whatever the input value. They have the opportunity to alter those strings before they get actually included in the final template rendering.

You can, for example, reverse a rendering:

// Define the `reverse` filter.
// reverse(x) renders the reversed rendering of its argument:
let reverse = Filter { (rendering: Rendering) in
    let reversedString = String(rendering.string.characters.reversed())
    return Rendering(reversedString, rendering.contentType)

// Register the reverse filter in our template:
let template = try Template(string: "{{reverse(value)}}")
template.register(reverse, forKey: "reverse")

// ohcuorG
try template.render(["value": "Groucho"])

// 321
try template.render(["value": 123])

Such filter does not quite process a raw string, as you have seen. It processes a Rendering, which is a flavored string, a string with its contentType (text or HTML).

This rendering will usually be text: simple values (ints, strings, etc.) render as text. Our reversing filter preserves this content-type, and does not mangle HTML entities:

// &gt;lmth&lt;
try template.render(["value": "<html>"])

Custom Rendering Filters

An example will show how they can be used:

// Define the `pluralize` filter.
// {{# pluralize(count) }}...{{/ }} renders the plural form of the
// section content if the `count` argument is greater than 1.
let pluralize = Filter { (count: Int?, info: RenderingInfo) in

    // The inner content of the section tag:
    var string = info.tag.innerTemplateString

    // Pluralize if needed:
    if let count = count, count > 1 {
        string += "s"  // naive

    return Rendering(string)

// Register the pluralize filter in our template:
let templateString = "I have {{ cats.count }} {{# pluralize(cats.count) }}cat{{/ }}."
let template = try Template(string: templateString)
template.register(pluralize, forKey: "pluralize")

// I have 3 cats.
let data = ["cats": ["Kitty", "Pussy", "Melba"]]
let rendering = try template.render(data)

As those filters perform custom rendering, they are based on RenderFunction, just like lambdas. Check the RenderFunction type in CoreFunctions.swift for more information about the RenderingInfo and Rendering types (read on cocoadocs.org).

Advanced Filters

All the filters seen above are particular cases of FilterFunction. "Value filters", "Pre-rendering filters" and "Custom rendering filters" are common use cases that are granted with specific APIs.

Yet the library ships with a few built-in filters that don't quite fit any of those categories. Go check their documentation. And since they are all written with public GRMustache.swift APIs, check also their source code, for inspiration. The general FilterFunction itself is detailed in CoreFunctions.swift (read on cocoadocs.org).

Advanced Boxes

Values that feed templates are able of many different behaviors. Let's review some of them:

  • Bool can trigger or prevent the rendering of sections:

    {{# isVerified }}VERIFIED{{/ isVerified }}
    {{^ isVerified }}NOT VERIFIED{{/ isVerified }}
  • Arrays render sections multiple times, and expose the count, first, and last keys:

    You see {{ objects.count }} objects:
    {{# objects }}
    - {{ name }}
    {{/ objects }}
  • Dictionaries expose all their keys:

    {{# user }}
    - {{ name }}
    - {{ age }}
    {{/ user }}
  • NSObject exposes all its properties:

    {{# user }}
    - {{ name }}
    - {{ age }}
    {{/ user }}
  • Foundation's Formatter is able to format values (more information):

    {{ format(date) }}
  • StandardLibrary.each is a filter that defines some extra keys when iterating an array (more information):

    {{# each(items) }}
    - {{ @indexPlusOne }}: {{ name }}

This variety of behaviors is made possible by the MustacheBox type. Whenever a value, array, filter, etc. feeds a template, it is turned into a box that interact with the rendering engine.

Let's describe in detail the rendering of the {{ F(A) }} tag, and shed some light on the available customizations:

  1. The A and F expressions are evaluated: the rendering engine looks in the context stack for boxes that return a non-empty box for the keys "A" and "F". The key-extraction service is provided by a customizable KeyedSubscriptFunction.

    This is how NSObject exposes its properties, and Dictionary, its keys.

  2. The customizable FilterFunction of the F box is evaluated with the A box as an argument.

    The Result box may well depend on the customizable value of the A box, but all other facets of the A box may be involved. This is why there are various types of filters.

  3. The rendering engine then looks in the context stack for all boxes that have a customized WillRenderFunction. Those functions have an opportunity to process the Result box, and eventually return another one.

    This is how, for example, a boxed DateFormatter can format all dates in a section: its WillRenderFunction formats dates into strings.

  4. The resulting box is ready to be rendered. For regular and inverted section tags, the rendering engine queries the customizable boolean value of the box, so that {{# F(A) }}...{{/}} and {{^ F(A) }}...{{/}} can't be both rendered.

    The Bool type obviously has a boolean value, but so does String, so that empty strings are considered falsey.

  5. The resulting box gets eventually rendered: its customizable RenderFunction is executed. Its Rendering result is HTML-escaped, depending on its content type, and appended to the final template rendering.

    Lambdas use such a RenderFunction, so do pre-rendering filters and custom rendering filters.

  6. Finally the rendering engine looks in the context stack for all boxes that have a customized DidRenderFunction.

    This one is used by Localizer and Logger goodies.

All those customizable properties are exposed in the low-level MustacheBox initializer:

// MustacheBox initializer
    value value: Any? = nil,
    boolValue: Bool? = nil,
    keyedSubscript: KeyedSubscriptFunction? = nil,
    filter: FilterFunction? = nil,
    render: RenderFunction? = nil,
    willRender: WillRenderFunction? = nil,
    didRender: DidRenderFunction? = nil)

We'll below describe each of them individually, even though you can provide several ones at the same time:

  • value

    The optional value parameter gives the boxed value. The value is used when the box is rendered (unless you provide a custom RenderFunction). It is also returned by the value property of MustacheBox.

    let aBox = MustacheBox(value: 1)
    // Renders "1"
    let template = try Template(string: "{{a}}")
    try template.render(["a": aBox])
  • boolValue

    The optional boolValue parameter tells whether the Box should trigger or prevent the rendering of regular {{#section}}...{{/}} and inverted {{^section}}...{{/}} tags. The default value is true.

    // Render "true", "false"
    let template = try Template(string:"{{#.}}true{{/.}}{{^.}}false{{/.}}")
    try template.render(MustacheBox(boolValue: true))
    try template.render(MustacheBox(boolValue: false))
  • keyedSubscript

    The optional keyedSubscript parameter is a KeyedSubscriptFunction that lets the Mustache engine extract keys out of the box. For example, the {{a}} tag would call the subscript function with "a" as an argument, and render the returned box.

    The default value is nil, which means that no key can be extracted.

    Check the KeyedSubscriptFunction type in CoreFunctions.swift for more information (read on cocoadocs.org).

    let box = MustacheBox(keyedSubscript: { (key: String) in
        return Box("key:\(key)")
    // Renders "key:a"
    let template = try Template(string:"{{a}}")
    try template.render(box)
  • filter

    The optional filter parameter is a FilterFunction that lets the Mustache engine evaluate filtered expression that involve the box. The default value is nil, which means that the box can not be used as a filter.

    Check the FilterFunction type in CoreFunctions.swift for more information (read on cocoadocs.org).

    let box = MustacheBox(filter: Filter { (x: Int?) in
        return x! * x!
    // Renders "100"
    let template = try Template(string:"{{square(x)}}")
    try template.render(["square": box, "x": Box(10)])
  • render

    The optional render parameter is a RenderFunction that is evaluated when the Box is rendered.

    The default value is nil, which makes the box perform default Mustache rendering:

    • {{box}} renders the built-in Swift String Interpolation of the value, HTML-escaped.
    • {{{box}}} renders the built-in Swift String Interpolation of the value, not HTML-escaped.
    • {{#box}}...{{/box}} pushes the box on the top of the context stack, and renders the section once.

    Check the RenderFunction type in CoreFunctions.swift for more information (read on cocoadocs.org).

    let box = MustacheBox(render: { (info: RenderingInfo) in
        return Rendering("foo")
    // Renders "foo"
    let template = try Template(string:"{{.}}")
    try template.render(box)
  • willRender & didRender

    The optional willRender and didRender parameters are a WillRenderFunction and DidRenderFunction that are evaluated for all tags as long as the box is in the context stack.

    Check the WillRenderFunction and DidRenderFunction type in CoreFunctions.swift for more information (read on cocoadocs.org).

    let box = MustacheBox(willRender: { (tag: Tag, box: MustacheBox) in
        return "baz"
    // Renders "baz baz"
    let template = try Template(string:"{{#.}}{{foo}} {{bar}}{{/.}}")
    try template.render(box)

By mixing all those parameters, you can finely tune the behavior of a box.

Built-in goodies

The library ships with built-in goodies that will help you render your templates: format values, render array indexes, localize templates, etc.