From: akr@...
Date: 2015-11-04T11:12:03+00:00
Subject: [ruby-core:71333] [Ruby trunk - Feature #8976] file-scope	freeze_string directive

Issue #8976 has been updated by Akira Tanaka.


Shugo Maeda wrote:

> It's not just for compatibility.
> 
> The original problem was that a new String object has to be allocated by a string literal
> for each evaluation.  So I don't understand the reason why a dynamic string literal should
> be frozen in spite of the fact freezing dynamic strings can't reduce object allocation.

It is not a big problem.
We can reduce extra object allocation with "foo#{exp}bar".dup using an optimization similar for "foo".freeze.

I think the pragma and option name should explain the behavior.
The name is "frozen-string-literal".
So, basically, all string literal should return a frozen object.

We need an explanation if we introduce an exception.




----------------------------------------
Feature #8976: file-scope freeze_string directive
https://bugs.ruby-lang.org/issues/8976#change-54706

* Author: Akira Tanaka
* Status: Open
* Priority: Normal
* Assignee: Yukihiro Matsumoto
----------------------------------------
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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