module ECR

Overview

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 %>. Dashes may also be present in those escaped tags and have no effect on the surrounding text.

NOTE To use ECR, you must explicitly import it with require "ecr"

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.

Defined in:

Macro Summary

• def_to_s(filename)

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

• embed(filename, io_name)

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

• render(filename)

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

Macro Detail

# 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!"

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

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

require "ecr/macros"

name = "World"

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

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

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

require "ecr/macros"

name = "World"

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

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