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Code Comments

Ruby has two types of comments: inline and block.

Inline comments start with the # character and continue until the end of the line:

# On a separate line
class Foo # or at the end of the line
  # can be indented
  def bar
  end
end

Block comments start with =begin and end with =end. Each should start on a separate line.

=begin
This is
commented out
=end

class Foo
end

=begin some_tag
this works, too
=end

=begin and =end can not be indented, so this is a syntax error:

class Foo
  =begin
  Will not work
  =end
end

Magic Comments

While comments are typically ignored by Ruby, special “magic comments” contain directives that affect how the code is interpreted.

Top-level magic comments must start on the first line, or on the second line if the first line looks like #! shebang line.

NOTE: Magic comments affect only the file in which they appear; other files are unaffected.

# frozen_string_literal: true

var = 'hello'
var.frozen? # => true

Alternative syntax

Magic comments may consist of a single directive (as in the example above). Alternatively, multiple directives may appear on the same line if separated by “;” and wrapped between “-*-” (see Emacs' file variables).

# emacs-compatible; -*- coding: big5; mode: ruby -*-

p 'hello'.frozen? # => true
p 'hello'.encoding # => #<Encoding:Big5>

encoding Directive

Indicates which string encoding should be used for string literals, regexp literals and __ENCODING__:

# encoding: big5

''.encoding # => #<Encoding:Big5>

Default encoding is UTF-8.

It must appear in the first comment section of a file.

The word “coding” may be used instead of “encoding”.

frozen_string_literal Directive

Indicates that string literals should be allocated once at parse time and frozen.

# frozen_string_literal: true

3.times do
  p 'hello'.object_id # => prints same number
end
p 'world'.frozen? # => true

The default is false; this can be changed with --enable=frozen-string-literal. Without the directive, or with # frozen_string_literal: false, the example above would print 3 different numbers and “false”.

Starting in Ruby 3.0, string literals that are dynamic are not frozen nor reused:

# frozen_string_literal: true

p "Addition: #{2 + 2}".frozen? # => false

It must appear in the first comment section of a file.

warn_indent Directive

This directive can turn on detection of bad indentation for statements that follow it:

def foo
  end # => no warning

# warn_indent: true
def bar
  end # => warning: mismatched indentations at 'end' with 'def' at 6

Another way to get these warnings to show is by running Ruby with warnings (ruby -w). Using a directive to set this false will prevent these warnings to show.

shareable_constant_value Directive

Note: This directive is experimental in Ruby 3.0 and may change in future releases.

This special directive helps to create constants that hold only immutable objects, or Ractor-shareable constants.

The directive can specify special treatment for values assigned to constants:

Mode none (default)

No special treatment in this mode (as in Ruby 2.x): no automatic freezing and no checks.

It has always been a good idea to deep-freeze constants; Ractor makes this an even better idea as only the main ractor can access non-shareable constants:

# shareable_constant_value: none
A = {foo: []}
A.frozen? # => false
Ractor.new { puts A } # => can not access non-shareable objects by non-main Ractor.

Mode literal

In “literal” mode, constants assigned to literals will be deeply-frozen:

# shareable_constant_value: literal
X = [{foo: []}] # => same as [{foo: [].freeze}.freeze].freeze

Other values must be shareable:

# shareable_constant_value: literal
X = Object.new # => cannot assign unshareable object to X

Note that only literals directly assigned to constants, or recursively held in such literals will be frozen:

# shareable_constant_value: literal
var = [{foo: []}]
var.frozen? # => false (assignment was made to local variable)
X = var # => cannot assign unshareable object to X

X = Set[1, 2, {foo: []}].freeze # => cannot assign unshareable object to X
                                # (`Set[...]` is not a literal and
                                # `{foo: []}` is an argument to `Set.[]`)

The method Module#const_set is not affected.

Mode experimental_everything

In this mode, all values assigned to constants are made shareable.

# shareable_constant_value: experimental_everything
FOO = Set[1, 2, {foo: []}]
# same as FOO = Ractor.make_sharable(...)
# OR same as `FOO = Set[1, 2, {foo: [].freeze}.freeze].freeze`

var = [{foo: []}]
var.frozen? # => false (assignment was made to local variable)
X = var # => calls `Ractor.make_shareable(var)`
var.frozen? # => true

This mode is “experimental”, because it might be error prone, for example by deep-freezing the constants of an external resource which could cause errors:

# shareable_constant_value: experimental_everything
FOO = SomeGem::Something::FOO
# => deep freezes the gem's constant!

This will be revisited before Ruby 3.1 to either allow `everything` or to instead remove this mode.

The method Module#const_set is not affected.

Mode experimental_copy

In this mode, all values assigned to constants are deeply copied and made shareable. It is safer mode than experimental_everything.

# shareable_constant_value: experimental_everything
var = [{foo: []}]
var.frozen? # => false (assignment was made to local variable)
X = var # => calls `Ractor.make_shareable(var, copy: true)`
var.frozen? # => false
Ractor.shareable?(X) #=> true
var.object_id == X.object_id #=> false

This mode is “experimental” and has not been discussed thoroughly. This will be revisited before Ruby 3.1 to either allow `copy` or to instead remove this mode.

The method Module#const_set is not affected.

Scope

This directive can be used multiple times in the same file:

# shareable_constant_value: none
A = {foo: []}
A.frozen? # => false
Ractor.new { puts A } # => can not access non-shareable objects by non-main Ractor.

# shareable_constant_value: literal
B = {foo: []}
B.frozen? # => true
B[:foo].frozen? # => true

C = [Object.new] # => cannot assign unshareable object to C (Ractor::IsolationError)

D = [Object.new.freeze]
D.frozen? # => true

# shareable_constant_value: experimental_everything
E = Set[1, 2, Object.new]
E.frozen? # => true
E.all(&:frozen?) # => true

The directive affects only subsequent constants and only for the current scope:

module Mod
  # shareable_constant_value: literal
  A = [1, 2, 3]
  module Sub
    B = [4, 5]
  end
end

C = [4, 5]

module Mod
  D = [6]
end
p Mod::A.frozen?, Mod::Sub::B.frozen? # => true, true
p C.frozen?, Mod::D.frozen? # => false, false