[ruby-core:70880] [Ruby trunk - Feature #8976] file-scope freeze_string directive

Issue #8976 has been updated by Yukihiro Matsumoto.

Assignee set to Akira Tanaka

It seems (for me) frozen string literals are good trade-off between current Ruby (every string is mutable) and immutable strings.
If all strings are immutable, it introduces huge incompatibility. With frozen string literals, we may be able to find more errors earlier.
Or maybe some Ruby programs run faster.

But first we have to prove if this is a good idea. So my plan is to introduce frozen-string-literal magic comment (pragma) for 2.3, and see whether it works well or not. If it works well, I'd love to make string literals frozen by default in Ruby3. But if not, frozen-string-literal ends as an experiment.

Matz.


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Feature #8976: file-scope freeze_string directive
https://bugs.ruby-lang.org/issues/8976#change-54256

* Author: Akira Tanaka
* Status: Open
* Priority: Normal
* Assignee: Akira Tanaka
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Yesterday, we had a face-to-face developer meeting.
https://bugs.ruby-lang.org/projects/ruby/wiki/DevelopersMeeting20131001Japan
Several committers attended.
matz didn't attended, though.  (This means this issue is not concluded.)

We believe we found a better way to freeze static string literals for
less GC pressure.
"static string literal" is a string literal without dynamic expression.

Currently, f-suffix, "..."f, is used to freeze a string literal to avoid
String object allocation.

There are several problems for f-suffix:

* The notation is ugly.
* Syntax error on Ruby 2.0.
  We cannot use the feature in version independent libraries.
  So, it is difficult to deploy.
* Need to modify for each string literal.
  This is cumbersome.

The new way we found is a file-scope directive as follows

  # freeze_string: true

The above comment at top of a file changes semantics of
static string literals in the file.
The static string literals will be frozen and always returns same object.
(The semantics of dynamic string literals is not changed.)

This way has following benefits:

* No ugly f-suffix.
* No syntax error on older Ruby.
* We need only a line for each file.

We can write version independent library using frozen static string literals as follows.

* Use the directive at top of the file: # freeze_string: true
  Older Ruby ignore this as a comment.
* Use "...".dup for strings to be modified.
  Older Ruby has small disadvantage: useless dup is called.

Note that the directive effects all static string literals regardless of
single quotes, double quotes, %q-string, %qq-string and here documents.
The reason that the directive is effective not only single quotes is
we want to use escape sequences such as \n in frozen string literals.

Also note that similar directive is already exist:

  % ruby -w -e '
  def m
    end
  '
  -e:3: warning: mismatched indentations at 'end' with 'def' at 2
  % ruby -w -e '# -*- warn_indent: false -*-
  def m
    end
  '

The directive, warn_indent: false, disables "mismatched indentations" warning.

nobu implemented this feature in the meeting.
Please attach the patch, nobu.




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