module ECR

Embedded Crystal (ECR) is a template language for embedding Crystal code into other text, that includes but is not limited to HTML. The template is read and transformed at compile time and then embedded into the binary.

There are <%= %> and <% %> syntax. The former will render returned values. The latter will not, but instead serve to control the structure as we do in Crystal.

Using a dash inside <...> either eliminates previous indentation or removes the next newline:

• <%- ... %>: removes previous indentation
• <% ... -%>: removes next newline
• <%-= ... %>: removes previous indentation
• <%= ... -%>: removes next newline

A comment can be created the same as normal code: <% # hello %> or by the special tag: <%# hello %>. An ECR tag can be inserted directly (i.e. the tag itself may be escaped) by using a second % like so: <%% a = b %> or <%%= foo %>.

Quick Example:

Create a simple ECR file named greeter.ecr:

Greetings, <%= @name %>!

and then use it like so with the #def_to_s macro:

require "ecr"

class Greeter
  def initialize(@name : String)
  end

  ECR.def_to_s "greeter.ecr"
end

Greeter.new("John").to_s # => "Greetings, John!\n"

Using logical statements:

# greeter.ecr
<%- if @name -%>
Greetings, <%= @name %>!
<%- else -%>
Greetings!
<%- end -%>

require "ecr"

class Greeter
  def initialize(@name : String | Nil)
  end

  ECR.def_to_s "greeter.ecr"
end

Greeter.new(nil).to_s    # => "Greetings!\n"
Greeter.new("Jill").to_s # => "Greetings, Jill!\n"

Using loops:

# greeter.ecr
<%- @names.each do |name| -%>
Hi, <%= name %>!
<%- end -%>

require "ecr"

class Greeter
  @names : Array(String)

  def initialize(*names)
    @names = names.to_a
  end

  ECR.def_to_s "greeter.ecr"
end

Greeter.new("John", "Zoe", "Ben").to_s # => "Hi, John!\nHi, Zoe!\nHi, Ben!\n"

Comments and Escapes:

# demo.ecr
<%# Demonstrate use of ECR tag -%>
A valid ECR tag looks like this: <%%= foo %>

require "ecr"
foo = 2
ECR.render("demo.ecr") # => "A valid ECR tag looks like this: <%= foo %>\n"

Likewise, other Crystal logic can be implemented in ECR text.

Macros

def_to_s(filename)

Defines a to_s(io) method whose body is the ECR contained in filename, translated to Crystal code.

# greeting.ecr
Hello <%= @name %>!

require "ecr/macros"

class Greeting
  def initialize(@name : String)
  end

  ECR.def_to_s "greeting.ecr"
end

Greeting.new("World").to_s # => "Hello World!"

The macro basically translates the text inside the given file to Crystal code that appends to the IO:

class Greeting
  def to_s(io)
    io << "Hello "
    io << @name
    io << '!'
  end
end

View source

embed(filename, io_name)

Embeds an ECR file filename into the program and appends the content to an IO in the variable io_name.

The generated code is the result of translating the contents of the ECR file to Crystal, a program that appends to an IO.

# greeting.ecr
Hello <%= name %>!

require "ecr/macros"

name = "World"

io = IO::Memory.new
ECR.embed "greeting.ecr", io
io.to_s # => "Hello World!"

The ECR.embed line basically generates this Crystal code:

io << "Hello "
io << name
io << '!'

View source

render(filename)

Embeds an ECR file filename into the program and renders it to a string.

The generated code is the result of translating the contents of the ECR file to Crystal, a program that appends to an IO and returns a string.

# greeting.ecr
Hello <%= name %>!

require "ecr/macros"

name = "World"

rendered = ECR.render "greeting.ecr"
rendered # => "Hello World!"

The ECR.render basically generates this Crystal code:

String.build do |io|
  io << "Hello "
  io << name
  io << '!'
end

View source