From: "headius (Charles Nutter)" <headius@...>
Date: 2013-10-10T00:44:00+09:00
Subject: [ruby-core:57781] [ruby-trunk - Feature #8976] file-scope	freeze_string directive


Issue #8976 has been updated by headius (Charles Nutter).


boris_stitnicky (Boris Stitnicky) wrote:
> "..."f might be mildly ugly, but is hard to beat.
> 5 minutes of my thinking did not yield any better idea.
> I share mame's feeling abou the file-scope directive.

I still don't like file-scope directive since it changes *behavior* rather than just *content*. In other words, a file that gains a frozen string directive suddenly creates strings that are only half functional.

See also #8992 that might address all issues by simply making the compiler and #freeze methods smarter.

* Compiler would see through "literal".freeze and do what "literal"f" does now.
* String#freeze could be adapted to use the fstring cache internally, so all frozen strings would be interned (in the Java sense).
* No new backward-incompatible syntax.
* Easy expansion to other literal syntaxes like arrays and hashes.

I think we need to kill off the "literal"f syntax and a file-scope directive is not the right way to do it.
----------------------------------------
Feature #8976: file-scope freeze_string directive
https://bugs.ruby-lang.org/issues/8976#change-42388

Author: akr (Akira Tanaka)
Status: Open
Priority: Normal
Assignee: 
Category: 
Target version: current: 2.1.0


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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