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stefankroes
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Description

Scribble is a customer facing template language similar to Liquid build in Ruby

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Scribble

Scribble is a customer facing template language similar to Liquid. Scribble was written in Ruby and can be used in any Ruby or Ruby on Rails project. It takes a template file, consisting of text and Scribble tags and transforms it into text. Scribble can be used to transform any plain text format like HTML, Markdown, JSON, XML or plain text. Customer facing means that it is safe to use Scribble to run/evaluate user provided templates.

Scribble is only compatible with Ruby 2.0 and Ruby 2.1 because it uses keyword arguments.

Scribble was developed by Stefan Kroes at Lab 01 (Dutch website, available for hire ;-) as a part of Sitebox.io (Service that lets you create/edit a website using files in your Dropbox).

How do I use it?

The following example shows how to load a Scribble template and evaluate it.

Scribble::Template.new("I'm a template! {{1 + 1}}").render

The template constructor and and render method both take options. The following example shows the same call but with all options. We will describe the specifics of formats, loaders, converters and registers later.

template_options = {
  format: :markdown,             # The template itself is in Markdown format
  loader: my_partial_loader,     # Object handling the loading of partials and layouts
  converters: [html2md, md2html] # Two converters to convert between HTML and Markdown
}

render_options = { variables: {a: 1, b: 'Foo'}, # Expose some data to the template registers: {c: 2, d: 'Bar'}, # For internal use by Scribble method implementations format: :html # Request output conversion to HTML format }

Scribble::Template.new(source, template_options).render render_options

What does it look like?

The following example is a Scribble template repeating some text, folowed by an

if
statement.
{{ 3.times }}Hello World!{{ end }}

{{ if foo = 3 | bar = 'baz' }} Either foo equals the number 3 or bar equals the string 'baz' {{ elsif 3 * 3 != foo & bar }} Foo does not equal 9 and bar is either true, a non-zero number or a non-empty string {{ else }} None of the above {{ end }}

In the example above:

  • Everything between {{ and }} are Scribble tags, the rest is text.
  • times
    and
    if
    are block methods that can be used to manipulate the associated block (up until the next
    end
    ).
  • foo
    and
    bar
    are variables that can be inserted by passing them into the
    render
    method from Ruby.
  • =
    ,
    |
    ,
    &
    ,
    *
    ,
    !=
    are operators. Operators are invoked on their left hand side with their right hand side as an argument.
  • elsif
    and
    else
    are methods that are defined only in the context of an
    if
    block to split up the block.

Why Scribble instead of Liquid?

While Liquid is fine for many use-cases, we decided to replace it on the Sitebox.io project for the following reasons:

  • Liquid uses regular expressions for parsing and tends to ignore most runtime errors, we want to be a little more strict and present the user with helpful error messages that include line and column numbers
  • We needed a template language that's able to convert between different formats when using partials (in particular between Markdown and HTML)
  • While the Liquid syntax can be friendly to non-programmers, an object oriented expression based language is more powerful and expressive
  • Liquid uses different syntax for blocks and inlining, we wanted a single syntax for both

Installation

Add this line to your application's Gemfile:

gem 'scribble'

And then execute:

$ bundle

Or install it yourself as:

$ gem install scribble

Project status and features

Scribble currently has solid architecture and features and it is already used in production in Sitebox.io. The actual scripting API is still somewhat sparse regarding supported types and methods but will be extended in future minor releases and can also easily be extended on a per application basis. Pull requests enriching the API are very welcome. Scribble currently supports:

  • A proper grammar based parser, generating user-friendly syntax errors that unclude line and column numbers
  • Excellent runtime error reporting (
    Wrong number of arguments (0 for 1-2) for 'partial' at line 1 column 4
    )
  • Simple and consistent tag syntax using {{ ... }}
  • Unary and binary operators with proper precedence rules and parentheses
  • Method invocation, method chaining, block methods and command style method invocation (without parentheses)
  • Nested execution scopes (blocks) and rescursive resolving of methods and variables
  • Boolean, integer, string and nil scripting types (Arrays or hashes are not currently supported)
  • Rich object oriented API for scripting with these types
  • Pluggable architecture for additional (application specific) types
  • If/elsif/else, layout and partial methods
  • Pluggable loader architecture for working with partials and layouts
  • Ability to convert between formats when inlining partials and layouts with mixed formats (for example: Markdown and HTML)

Template and rendering options

Scribble supports a few options when initializing and rendering templates. This section describes those options.

Supplying a partial loader

When you supply Scribble with a partial loader you can use the

partial
and
layout
methods in order to inline other Scribble templates. A loader is an objects that implements a single instance method named
load
which takes the partial name as a single string argument. The method should return a Scribble::Partial or nil if the partial cannot be found. Initializing the partial with a format is optional.
class MyLoader
  def load name

# ...

unless partial_source.nil?
  Scribble::Partial.new partial_source, format: partial_format
end

end end

Scribble::Template.new(template_source, loader: MyLoader.new).render

Formats and converters

Scribble is able to convert between different formats when inlining partials and rendering templates. This functionality is completely optional, just ignore all format and converter options to disable it. If you want to use this functionality you should tell Scribble the format of your templates and partials when initializing them using symbols. Additionally you should tell Scribble in what format to render and supply the format convertors that will be needed to do so.

md2html = Scribble.converter :markdown => :html do |markdown|
  Kramdown::Document.new(markdown).to_html
end

template = Scribble::Template.new template_source, format: :markdown, converters: [md2html]

template.render :html

Variables and registers

Variables are the way to pass data from your template into your Scribble template. In addition to variables, you can pas registers. Registers will not be available as Scribble variables but they will be available to the Ruby implementations of your methods.

template = Scribble::Template.new template_source

template.render variables: {a: 1}, registers: {some_private_resource: 2}

Template language

The Scribble template language takes a lot of cues from Ruby:

  • Everything is an object
  • Objects only support methods, no attributes
  • Methods can take a block of code, delimited by the keyword
    end
  • Operators are just method calls, invoked on their left hand side
  • Parentheses are optional when passing arguments into a method
  • Multiple methods can be chained using dot
    .
    notation

Some key differences are:

  • Scribble only implements a very small subset of the Ruby syntax.
  • Blocks of code don't need to be opened. The interpreter knows which methods take a block.
  • Constructs like an
    if
    statement are also implemented as block methods, using so called split methods for constructs like
    elsif
    and
    else
    .
  • Meanings of operators have been simplified because Scribble only needs to support certain operations.

Syntax

The Scribble grammar is defined using Parslet in parser.rb.

A Scribble template consists of text and tags. Tags are everything between {{ and }}. Tags containing only the keyword

end
are special tags that close a block.
3 times 6 is: {{ 3 * 6 }}

{{ 3.times }} Hello World! {{ end }}

Scribble supports integers, strings, booleans and nil. Nil doesn't have a literal, it can only be returned. Other objects do have literals.

{{ 'This is string' }}

This is a number: {{ 3 }}

{{ true }} and {{ false }} are booleans

Methods are called globally or on objects with the dot

.
notation. Parentheses are optional when invoking a method and are not allowed when calling a variable. Methods can be chained and used as arguments to other methods.
{{ some_method }}
{{ some_method() }}
{{ some_method(1, true, 'test') }}
{{ some_method 1, true, 'test' }}
{{ some_variable }}
{{ -3.abs }}
{{ 'Scribble'.replace('Scr', 'Dr').upcase }}
{{ true.or 1 }}
{{ 3.multiply(5.subtract(2)) }}
{{ some_method another_method }}
This will cause an error: {{ some_variable() }}

Unary and binary operators are supported. As are parentheses.

{{ -2 * (3 + 5) }}
{{ 'Scribble' - 'Scr' }}
{{ 3 * 'Repeat me! ' }}

Operators and precedence

The table below shows all supported operators in order of precedence. Operators are invoked as methods on their left hand side (or on their only operand in case of a unary operator). The table also shows the method name that is associated with each operator for this purpose. This essentially enables operator overloading since you just define the associated method.

Operator

Precedence Description Associated method

!
| 1 | Unary not |
not
-
| 2 | Unary minus |
negative
*
| 3 | Multiplication |
multiply
/
| 3 | Division |
divide
%
| 3 | Remainder |
remainder
+
| 4 | Addition |
add
-
| 4 | Subtraction |
subtract
>
| 5 | Greater |
greater
<
| 5 | Less |
less
>=
| 5 | Greater or equal |
greater_or_equal
<=
| 5 | Less or equal |
less_or_equal
=
| 6 | Equality |
equals
!=
| 6 | Inequality |
differs
&
| 7 | Logical and |
and
| | 8 | Logical or |
or

Standard types and methods

Scribble provides an extensible set of types and methods as a base API. This section describes Scribble functionality that is always available.

String

Strings support a literal notation using single quotes. Single quotes and backslashes can be escaped within a string using a backslash (

\'
and
\\
). Strings are considered false when they are empty and true when they are not. Strings support many methods and many operators (through methods), an overview can be found in objects/string.rb.
{{ 'I\'m a string!' }}

{{ if foo }} Text in the string foo: {{ foo }} {{ else }} The string foo is empty! {{ end }}

{{ 'foo' * 3 }} {{ 'foo'.repeat 3 }} {{ 'foo' + 'bar' }} {{ 'foo' - 'oo' }} {{ 'foo'.replace 'foo', 'bar' }} {{ 'foo'.upcase.downcase.capitalize }} {{ 'Hello world!'.truncate 5, '...' }} {{ 'Hello world!'.truncate_words 1 }}

Integer

Integers support a literal notation using one or more digits. Integers are considered false when they are zero and true when they are not. Integers support many methods and many operators (through methods), an overview can be found in objects/fixnum.rb. Additionally, integers support the times method for repeating a block of content multiple times. Times needs to be implemented as its own class because it takes a block.

{{ if foo }}
  Integer foo: {{ foo }}
{{ else }}
  The integer foo is zero!
{{ end }}

{{ (-5 + 3) * 5 / 3 }} {{ 5.negative }} {{ -3.abs }} {{ 4.even }} {{ 4.odd }} {{ 4.times }}Hi! {{ end }}

Boolean

Booleans support a literal notation using either the keyword true or the keyword false. Booleans support a subset of operators and associated methods, an overview can be found in objects/boolean.rb.

{{ true | false & 'foo' }}
{{ !true }}
{{ true != 3 }}

Nil

Nil does not support a literal notation but it can be returned by methods. It is always considered to be false. It implements the same methods as false, an overview can be found in objects/nil.rb.

Global methods

Scribble supports the global

if
,
partial
, and
layout
methods. The if method was shown in several previous examples. The
partial
and
layout
methods are similar in that they load a partial through the loader and render it (converting between any format disparities). The difference is that layout takes a block of content to which the partial can yield using the
content
method.
# Partial template (loaded by loader as 'foo')

Hello from the partial!

Layout template (loaded by loader as 'bar')

{{ content }}

Template

{{ partial 'foo' }}

{{ layout 'foo' }}I will be in red!{{ end }}

Extending the Scribble language

The Scribble language can be extended by inserting new methods into the registry. The registry keeps track of methods and their properties:

  • The method name
  • The receiver class/type
  • The argument signature (number of arguments and their types)

Any Ruby class can also be a Scribble type as long as methods are defined on it.

Introducing a new type

In order to add everything to the registry that will make a Ruby class well behaved as a Scribble type, use Scribble::Registry.for and define its name, cast methods and regular methods as shown below. After that you can just pass the Ruby object along to your template using variables.

Scribble::Registry.for User do
  name 'user'

to_boolean { true } to_string { "user: #{name}" }

Logical operators

method :or, Object, cast: 'to_boolean' method :and, Object, cast: 'to_boolean'

Equality

method :equals, User, as: '==' method :differs, User, as: '!=' method :equals, Object, returns: false method :differs, Object, returns: true

Attributes

method :name method :email

method :first_name, to: -> { self.name.split(/\s/).first } end

In the example above the call to

name
describes how the class
User
should be called in error messages and such. The next two lines describe how it should be cast to either a boolean (when used in an
if
statement for example) or a string (when it is rendered to the template). Then a bunch of methods are defined using different keywords.
  • The
    cast
    keyword tells Scribble to cast this
    User
    to a boolean first, then call the same method (
    or
    or
    and
    ) on that boolean.
  • The
    as
    keyword means the method is delegated to the Ruby implementation under a different name.
  • The
    returns
    keyword specifies that the method should just always return the same value.
  • The
    to
    keyword specifies a block to implement the method.
  • When
    method
    is called without any keywords, it is delegated to the Ruby method with the same name.

In this case

equals
and
differs
are defined twice with a different signature. The first method with a matching signature will be called so you can use pattern matching when implementing methods. In this case
User
can be equal to another user (delegated to Ruby equality) but can never be equal to another object (always returns false).

Introducing a complex method

Global methods and any methods taking a block should be implemented as a Ruby class that subclasses either Scribble::Method or Scribble::Block. The

if
method is shown below as an example. Some more examples can be found in the methods folder of the project.
class If < Scribble::Block
  register :if, Object

def if object @paths = [] send :elsif, object

render(nodes: @paths.map { |condition, nodes| nodes if condition }.compact.first || [])

end

method :elsif, Object, split: true

def elsif object @paths.unshift [Registry.to_boolean(object), split_nodes] end

method :else, split: true

def else @paths.unshift [true, nodes] end end

The first line of the class implementation registers this method with the registry. The two calls to the

method
method register methods (
elsif
and
else
) that will be available within the block passed to the
if
call. These are split methods, meaning they will split up the block when
split_nodes
is called. Lets examine what happens when the following code is evaluated.
{{ if false }}
  Some text
{{ elsif true }}
  Some more text
{{ else }}
  Last bit of text
{{ end }}
  1. if
    is a block method so the block up to the
    end
    is assigned to it and the
    elsif
    and
    else
    methods are considered regular method calls by the parser.
  2. if
    is called in Scribble which is delegated to the
    if
    method in Ruby
  3. The
    if
    method initializes the path array and calls
    elsif
    (using
    send
    because
    elsif
    is a keyword)
  4. split_nodes
    is called by
    elsif
    which skips over
    Some text
    and evaluates the Scribble
    elsif
    (the next split method)
  5. split_nodes
    is called by
    elsif
    which skips over
    Some more text
    and evaluates the Scribble
    else
    (the next split method)
  6. The implementation of
    else
    takes the rest of the nodes (
    Last bit of text
    ) and inserts it into the array together with a
    true
    value.
  7. The last call to
    split_nodes
    returns
    Some more text
    which is added to the beginning of the array, together with the argument to
    elsif
    cast to a boolean (
    true
    ).
  8. The first call to
    split_nodes
    returns
    Some text
    which is added to the beginning of the array, together with the argument to
    if
    cast to a boolean (
    false
    ).
  9. The
    send :elsif
    returns and it renders the first list of nodes with a positive condition (
    Some more text
    )

This might be hard to wrap your head around but once you do it is a clean and easy way to extend the language with new

if
-like constructs without touching the parser. For examples of this, check out Sitebox.io forms and Sitebox.io columns. Implementations of block methods that don't use split methods are much easier, lake a look at the implementation of the
times
method.

Method implementation API

This sections describes the API you can use when implementing methods as classes (like the

if
method). Please read the previous section first to gain some context.

Method or instance variable

Description

@receiver
| The object receiving the method call (object before the method invocation dot or left hand side of an operation)
@call
| The parse tree node of the method call
@context
| The execution context (block) of the method call
@context.variables
| The variables that are defined in that context (doesn't include variables of containing contexts)
@context.set_variable name, value
| Set a variable in the context (and nested contexts)
@context.registers
| The registers passed to the render method
@context.template
| The template or partial being rendered
@context.format
| The format of the text nodes in the context (before conversion)
@context.render_format
| The target rendering format (after conversion)
render
| (
Scribble::Block
only) Render the contents of the block (or specific nodes using the
nodes
keyword) to a string
split_nodes
| (
Scribble::Block
only) Evaluate the next split method and return nodes that came before it
nodes
| (
Scribble::Block
only) Get all nodes not skipped over by calling
split_nodes

All methods available on

@context
are also available directly when the method is a
Scribble::Block
as it is also a context.
@context
holds the context containing the block in case of a
Scribble::Block
.

Method signatures

In previous sections you have seen some method signatures. Both the

method
and
register
methods seen above take zero or more arguments after the method name. These arguments are the method signature. Any options/named arguments passed (
cast
,
as
, etc.) are not part of the method signature. A method matches when arguments of the same classes (or subclasses) as those in the signature are passed. Signatures also support optional arguments and multiple arguments as explained below.
Scribble::Registry.for String do
  # method1 takes an integer and a required string
  method :method1, Fixnum, String

method2 takes an integer and zero or more strings

method :method2, Fixnum, [String]

method3 takes an integer and up to one string (an optional string)

method :method3, Fixnum, [String, 1]

method4 takes an integer, one to four strings and after that zero or more integers

method :method4, Fixnum, String, [String, 3], [Fixnum] end

Future work

  • Extend language with arrays and possibly hashes
  • Extend language with more built-in methods like loops
  • Setup continuous integration and coverage reports
  • Write more documentation, in particular on error reporting

Contributing

  1. Fork it ( https://github.com/stefankroes/scribble/fork )
  2. Create your feature branch (
    git checkout -b my-new-feature
    )
  3. Commit your changes (
    git commit -am 'Add some feature'
    )
  4. Push to the branch (
    git push origin my-new-feature
    )
  5. Create a new Pull Request

License and copyright

Released under the MIT license

Copyright © 2014 Stefan Kroes

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