Issue #8992 has been updated by bughit (bug hit).


I recently had to remind myself what kind of magic is hiding behind 'str'.freeze and would like to provide some feedback.

While tactically this may have been justified, strategically this is a wrong direction. This approach moves the language towards a collection of magical spells. You end up with code that can't be understood through decomposition into primitives.

'str'.method1 - means construct a string object with a value 'str' and call method 'method1' on it.

The notion that a method call should have time travel powers and be able to influence how its already constructed receiver is allocated is absurd. It makes mockery of such fundamental concepts as expression evaluation order and method invocation. Perhaps this is already being done elsewhere, not sure, but that's not a justification, doubling down on nonsense does not make it sense.

Whereas special syntax (whatever it is) does not have this flaw. You get to define what new syntax means without corrupting existing concepts, and in the long run this is more valuable than helping gem authors transition to immutable strings.

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Feature #8992: Use String#freeze and compiler tricks to replace "str"f suffix
https://bugs.ruby-lang.org/issues/8992#change-77994

* Author: headius (Charles Nutter)
* Status: Closed
* Priority: Normal
* Assignee: matz (Yukihiro Matsumoto)
* Target version: 2.1.0
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BACKGROUND:

In https://bugs.ruby-lang.org/issues/8579 @charliesome introduced the "f" suffix for creating already-frozen strings. A string like "str"f would have the following characteristics:

* It would be frozen before the expression returned
* It would be the same object everywhere, pulling from a global "fstring" table

To avoid memory leaks, these pooled strings would remove themselves from the "fstring" table on GC.

However, there are problems with this new syntax:

* It will never parse in Ruby 2.0 and earlier.
* It's not particularly attractive, though this is a subjective matter.
* It does not lend itself well to use in other scenarios, such as for arrays and hashes (http://bugs.ruby-lang.org/issues/8909 )

PROPOSAL:

I propose that we eliminate the new "f" suffix and just make the compiler smart enough to see literal strings with .frozen the same way.

So this code:

str = "mystring".freeze

Would be equivalent in the compiler to this code:

str = "mystring"f

And the fstring table would still be used to return pooled instances.

IMPLEMENTATION NOTES:

The fstring table already exists on master and would be used for these pooled strings. An open question is whether the compiler should forever optimize "str".frozen to return the pooled version or whether it should check (inline-cache style) whether String#freeze has been replaced. I am ok with either, but the best potential comes from ignoring String#freeze redefinitions...or making it impossible to redefine String#freeze.

BONUS BIKESHEDDING:

If we do not want to overload the existing .freeze method in this way, we could follow suggestions in http://bugs.ruby-lang.org/issues/8977 to add a new "frozen" method (or some other name) that the compiler would understand.

If it were "frozen", the following two lines would be equivalent:

str = "mystring".frozen
str = "mystring"f

In addition, using .frozen on any string would put it in the fstring table and return that pooled version.

I also propose one alternative method name: the unary ~ operator.

There is no ~ on String right now, and it has no meaning for strings that we'd be overriding. So the following two lines would be equivalent:

str = ~"mystring"
str = "mystring"f

JUSTIFICATION:

Making the compiler aware of normal method-based String freezing has the following advantages:

* It will parse in all versions of Ruby.
* It will be equivalent in all versions of Ruby other than the fstring pooling.
* It extends neatly to Array and Hash; the compiler can see Array or Hash with literal elements and return the same object.
* It does not require a pragma (http://bugs.ruby-lang.org/issues/8976 )
* It looks like Ruby.



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