Class: ERB

Inherits:
Object
  • Object
show all
Defined in:
lib/erb.rb,
lib/erb/version.rb,
ext/erb/escape/escape.c

Overview

ERB – Ruby Templating

Introduction

ERB provides an easy to use but powerful templating system for Ruby. Using ERB, actual Ruby code can be added to any plain text document for the purposes of generating document information details and/or flow control.

A very simple example is this:

require 'erb'

x = 42
template = ERB.new <<-EOF
  The value of x is: <%= x %>
EOF
puts template.result(binding)

Prints: The value of x is: 42

More complex examples are given below.

Recognized Tags

ERB recognizes certain tags in the provided template and converts them based on the rules below:

<% Ruby code -- inline with output %>
<%= Ruby expression -- replace with result %>
<%# comment -- ignored -- useful in testing %> (`<% #` doesn't work. Don't use Ruby comments.)
% a line of Ruby code -- treated as <% line %> (optional -- see ERB.new)
%% replaced with % if first thing on a line and % processing is used
<%% or %%> -- replace with <% or %> respectively

All other text is passed through ERB filtering unchanged.

Options

There are several settings you can change when you use ERB:

  • the nature of the tags that are recognized;

  • the binding used to resolve local variables in the template.

See the ERB.new and ERB#result methods for more detail.

Character encodings

ERB (or Ruby code generated by ERB) returns a string in the same character encoding as the input string. When the input string has a magic comment, however, it returns a string in the encoding specified by the magic comment.

# -*- coding: utf-8 -*-
require 'erb'

template = ERB.new <<EOF
<%#-*- coding: Big5 -*-%>
  \_\_ENCODING\_\_ is <%= \_\_ENCODING\_\_ %>.
EOF
puts template.result

Prints: _ENCODING_ is Big5.

Examples

Plain Text

ERB is useful for any generic templating situation. Note that in this example, we use the convenient “% at start of line” tag, and we quote the template literally with %q{...} to avoid trouble with the backslash.

require "erb"

# Create template.
template = %q{
  From:  James Edward Gray II <[email protected]>
  To:  <%= to %>
  Subject:  Addressing Needs

  <%= to[/\w+/] %>:

  Just wanted to send a quick note assuring that your needs are being
  addressed.

  I want you to know that my team will keep working on the issues,
  especially:

  <%# ignore numerous minor requests -- focus on priorities %>
  % priorities.each do |priority|
    * <%= priority %>
  % end

  Thanks for your patience.

  James Edward Gray II
}.gsub(/^  /, '')

message = ERB.new(template, trim_mode: "%<>")

# Set up template data.
to = "Community Spokesman <spokesman@ruby_community.org>"
priorities = [ "Run Ruby Quiz",
               "Document Modules",
               "Answer Questions on Ruby Talk" ]

# Produce result.
email = message.result
puts email

Generates:

From:  James Edward Gray II <[email protected]>
To:  Community Spokesman <spokesman@ruby_community.org>
Subject:  Addressing Needs

Community:

Just wanted to send a quick note assuring that your needs are being addressed.

I want you to know that my team will keep working on the issues, especially:

    * Run Ruby Quiz
    * Document Modules
    * Answer Questions on Ruby Talk

Thanks for your patience.

James Edward Gray II

Ruby in HTML

ERB is often used in .rhtml files (HTML with embedded Ruby). Notice the need in this example to provide a special binding when the template is run, so that the instance variables in the Product object can be resolved.

require "erb"

# Build template data class.
class Product
  def initialize( code, name, desc, cost )
    @code = code
    @name = name
    @desc = desc
    @cost = cost

    @features = [ ]
  end

  def add_feature( feature )
    @features << feature
  end

  # Support templating of member data.
  def get_binding
    binding
  end

  # ...
end

# Create template.
template = %{
  <html>
    <head><title>Ruby Toys -- <%= @name %></title></head>
    <body>

      <h1><%= @name %> (<%= @code %>)</h1>
      <p><%= @desc %></p>

      <ul>
        <% @features.each do |f| %>
          <li><b><%= f %></b></li>
        <% end %>
      </ul>

      <p>
        <% if @cost < 10 %>
          <b>Only <%= @cost %>!!!</b>
        <% else %>
           Call for a price, today!
        <% end %>
      </p>

    </body>
  </html>
}.gsub(/^  /, '')

rhtml = ERB.new(template)

# Set up template data.
toy = Product.new( "TZ-1002",
                   "Rubysapien",
                   "Geek's Best Friend!  Responds to Ruby commands...",
                   999.95 )
toy.add_feature("Listens for verbal commands in the Ruby language!")
toy.add_feature("Ignores Perl, Java, and all C variants.")
toy.add_feature("Karate-Chop Action!!!")
toy.add_feature("Matz signature on left leg.")
toy.add_feature("Gem studded eyes... Rubies, of course!")

# Produce result.
rhtml.run(toy.get_binding)

Generates (some blank lines removed):

<html>
  <head><title>Ruby Toys -- Rubysapien</title></head>
  <body>

    <h1>Rubysapien (TZ-1002)</h1>
    <p>Geek's Best Friend!  Responds to Ruby commands...</p>

    <ul>
        <li><b>Listens for verbal commands in the Ruby language!</b></li>
        <li><b>Ignores Perl, Java, and all C variants.</b></li>
        <li><b>Karate-Chop Action!!!</b></li>
        <li><b>Matz signature on left leg.</b></li>
        <li><b>Gem studded eyes... Rubies, of course!</b></li>
    </ul>

    <p>
         Call for a price, today!
    </p>

  </body>
</html>

Notes

There are a variety of templating solutions available in various Ruby projects. For example, RDoc, distributed with Ruby, uses its own template engine, which can be reused elsewhere.

Other popular engines could be found in the corresponding Category of The Ruby Toolbox.

Defined Under Namespace

Modules: DefMethod, Escape, Util Classes: Compiler

Constant Summary collapse

Revision =

:nodoc: #‘

'$Date::                           $'

Instance Attribute Summary collapse

Class Method Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(str, safe_level = NOT_GIVEN, legacy_trim_mode = NOT_GIVEN, legacy_eoutvar = NOT_GIVEN, trim_mode: nil, eoutvar: '_erbout') ⇒ ERB

Constructs a new ERB object with the template specified in str.

An ERB object works by building a chunk of Ruby code that will output the completed template when run.

If trim_mode is passed a String containing one or more of the following modifiers, ERB will adjust its code generation as listed:

%  enables Ruby code processing for lines beginning with %
<> omit newline for lines starting with <% and ending in %>
>  omit newline for lines ending in %>
-  omit blank lines ending in -%>

eoutvar can be used to set the name of the variable ERB will build up its output in. This is useful when you need to run multiple ERB templates through the same binding and/or when you want to control where output ends up. Pass the name of the variable to be used inside a String.

Example

require "erb"

# build data class
class Listings
  PRODUCT = { :name => "Chicken Fried Steak",
              :desc => "A well messages pattie, breaded and fried.",
              :cost => 9.95 }

  attr_reader :product, :price

  def initialize( product = "", price = "" )
    @product = product
    @price = price
  end

  def build
    b = binding
    # create and run templates, filling member data variables
    ERB.new(<<~'END_PRODUCT', trim_mode: "", eoutvar: "@product").result b
      <%= PRODUCT[:name] %>
      <%= PRODUCT[:desc] %>
    END_PRODUCT
    ERB.new(<<~'END_PRICE', trim_mode: "", eoutvar: "@price").result b
      <%= PRODUCT[:name] %> -- <%= PRODUCT[:cost] %>
      <%= PRODUCT[:desc] %>
    END_PRICE
  end
end

# setup template data
listings = Listings.new
listings.build

puts listings.product + "\n" + listings.price

Generates

Chicken Fried Steak
A well messages pattie, breaded and fried.

Chicken Fried Steak -- 9.95
A well messages pattie, breaded and fried.



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# File 'lib/erb.rb', line 335

def initialize(str, safe_level=NOT_GIVEN, legacy_trim_mode=NOT_GIVEN, legacy_eoutvar=NOT_GIVEN, trim_mode: nil, eoutvar: '_erbout')
  # Complex initializer for $SAFE deprecation at [Feature #14256]. Use keyword arguments to pass trim_mode or eoutvar.
  if safe_level != NOT_GIVEN
    warn 'Passing safe_level with the 2nd argument of ERB.new is deprecated. Do not use it, and specify other arguments as keyword arguments.', uplevel: 1
  end
  if legacy_trim_mode != NOT_GIVEN
    warn 'Passing trim_mode with the 3rd argument of ERB.new is deprecated. Use keyword argument like ERB.new(str, trim_mode: ...) instead.', uplevel: 1
    trim_mode = legacy_trim_mode
  end
  if legacy_eoutvar != NOT_GIVEN
    warn 'Passing eoutvar with the 4th argument of ERB.new is deprecated. Use keyword argument like ERB.new(str, eoutvar: ...) instead.', uplevel: 1
    eoutvar = legacy_eoutvar
  end

  compiler = make_compiler(trim_mode)
  set_eoutvar(compiler, eoutvar)
  @src, @encoding, @frozen_string = *compiler.compile(str)
  @filename = nil
  @lineno = 0
  @_init = self.class.singleton_class
end

Instance Attribute Details

#encoding ⇒ Object (readonly)

The encoding to eval



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# File 'lib/erb.rb', line 370

def encoding
  @encoding
end

#filename ⇒ Object

The optional filename argument passed to Kernel#eval when the ERB code is run



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# File 'lib/erb.rb', line 374

def filename
  @filename
end

#lineno ⇒ Object

The optional lineno argument passed to Kernel#eval when the ERB code is run



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# File 'lib/erb.rb', line 378

def lineno
  @lineno
end

#src ⇒ Object (readonly)

The Ruby code generated by ERB



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# File 'lib/erb.rb', line 367

def src
  @src
end

Class Method Details

.version ⇒ Object

Returns revision information for the erb.rb module.



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# File 'lib/erb.rb', line 267

def self.version
  VERSION
end

Instance Method Details

#def_class(superklass = Object, methodname = 'result') ⇒ Object

Define unnamed class which has methodname as instance method, and return it.

example:

class MyClass_
  def initialize(arg1, arg2)
    @arg1 = arg1;  @arg2 = arg2
  end
end
filename = 'example.rhtml'  # @arg1 and @arg2 are used in example.rhtml
erb = ERB.new(File.read(filename))
erb.filename = filename
MyClass = erb.def_class(MyClass_, 'render()')
print MyClass.new('foo', 123).render()



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# File 'lib/erb.rb', line 501

def def_class(superklass=Object, methodname='result')
  cls = Class.new(superklass)
  def_method(cls, methodname, @filename || '(ERB)')
  cls
end

#def_method(mod, methodname, fname = '(ERB)') ⇒ Object

Define methodname as instance method of mod from compiled Ruby source.

example:

filename = 'example.rhtml'   # 'arg1' and 'arg2' are used in example.rhtml
erb = ERB.new(File.read(filename))
erb.def_method(MyClass, 'render(arg1, arg2)', filename)
print MyClass.new.render('foo', 123)



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# File 'lib/erb.rb', line 465

def def_method(mod, methodname, fname='(ERB)')
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)
  end
end

#def_module(methodname = 'erb') ⇒ Object

Create unnamed module, define methodname as instance method of it, and return it.

example:

filename = 'example.rhtml'   # 'arg1' and 'arg2' are used in example.rhtml
erb = ERB.new(File.read(filename))
erb.filename = filename
MyModule = erb.def_module('render(arg1, arg2)')
class MyClass
  include MyModule
end



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# File 'lib/erb.rb', line 482

def def_module(methodname='erb')
  mod = Module.new
  def_method(mod, methodname, @filename || '(ERB)')
  mod
end

#location=(filename, lineno) ⇒ Object

Sets optional filename and line number that will be used in ERB code evaluation and error reporting. See also #filename= and #lineno=

erb = ERB.new('<%= some_x %>')
erb.render
# undefined local variable or method `some_x'
#   from (erb):1

erb.location = ['file.erb', 3]
# All subsequent error reporting would use new location
erb.render
# undefined local variable or method `some_x'
#   from file.erb:4



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# File 'lib/erb.rb', line 395

def location=((filename, lineno))
  @filename = filename
  @lineno = lineno if lineno
end

#make_compiler(trim_mode) ⇒ Object

Creates a new compiler for ERB. See ERB::Compiler.new for details



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# File 'lib/erb.rb', line 362

def make_compiler(trim_mode)
  ERB::Compiler.new(trim_mode)
end

#result(b = new_toplevel) ⇒ Object

Executes the generated ERB code to produce a completed template, returning the results of that code. (See ERB::new for details on how this process can be affected by safe_level.)

b accepts a Binding object which is used to set the context of code evaluation.



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# File 'lib/erb.rb', line 425

def result(b=new_toplevel)
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  eval(@src, b, (@filename || '(erb)'), @lineno)
end

#result_with_hash(hash) ⇒ Object

Render a template on a new toplevel binding with local variables specified by a Hash object.



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# File 'lib/erb.rb', line 434

def result_with_hash(hash)
  b = new_toplevel(hash.keys)
  hash.each_pair do |key, value|
    b.local_variable_set(key, value)
  end
  result(b)
end

#run(b = new_toplevel) ⇒ Object

Generate results and print them. (see ERB#result)



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# File 'lib/erb.rb', line 413

def run(b=new_toplevel)
  print self.result(b)
end

#set_eoutvar(compiler, eoutvar = '_erbout') ⇒ Object

Can be used to set eoutvar as described in ERB::new. It’s probably easier to just use the constructor though, since calling this method requires the setup of an ERB compiler object.



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# File 'lib/erb.rb', line 405

def set_eoutvar(compiler, eoutvar = '_erbout')
  compiler.put_cmd = "#{eoutvar}.<<"
  compiler.insert_cmd = "#{eoutvar}.<<"
  compiler.pre_cmd = ["#{eoutvar} = +''"]
  compiler.post_cmd = [eoutvar]
end