# 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 appear in the first comment section of a file.

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; frozen_string_literal: true -*-

    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.

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

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:

- `none`: (default)

- `literal`: literals are implicitly frozen, others must be Ractor-shareable

- `experimental_everything`: all made shareable

- `experimental_copy`: copy deeply and make it shareable

#### 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_copy
    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