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

Quick Example:

require "ecr"

class Greeting
  def initialize(@name : String)
  end

  ECR.def_to_s "greeting.ecr"
end

# greeting.ecr
Greeting, <%= @name %>!

Greeting.new("John").to_s #=> Greeting, John!

Using logical statements:

# greeting.ecr
<%- if @name -%>
Greeting, <%= @name %>!
<%- else -%>
Greeting!
<%- end -%>

Greeting.new(nil).to_s #=> Greeting!

Using loops:

require "ecr"

class Greeting
  @names : Array(String)

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

  ECR.def_to_s "greeting.ecr"
end

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

Greeting.new("John", "Zoe", "Ben").to_s
#=> Hi, John!
#=> Hi, Zoe!
#=> Hi, Ben!

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 contained in filename into the program.

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 << "!"