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

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 << '!'

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