[ruby-core:53347] [ruby-trunk - Bug #8078] Once nil is frozen, an unfrozen instance cannot be reached

Issue #8078 has been updated by marcandre (Marc-Andre Lafortune).


kosaki (Motohiro KOSAKI) wrote:
>  I don't think nil is mutable logically. Why can't we make nil is frozen at beginning?

I can't see a really good use for it, but `nil` can have attributes and is mutable.

Up to Ruby 2.0, the same was true for Fixnum, Bignum and Floats. But flonum optimization made it uncertain if a float was an immediate or not (e.g. 1.0 is an immediate in Ruby 2.0.0 on 64 bit platforms, but not if 32 bit). So it was decided to freeze them all, to avoid platform dependence. Since the same reasoning applied to Fixnum vs Bignum (e.g. 1<<42 is a Bignum on 32 bit platforms, so not an immediate, but is a Fixnum and thus an immediate on 64 bits), it was judged safer to freeze fixnums & bignums too.

On the other hand, `nil` is always the same immediate, so there is no danger in leaving it unfrozen.
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Bug #8078: Once nil is frozen, an unfrozen instance cannot be reached
https://bugs.ruby-lang.org/issues/8078#change-37544

Author: mhuggins (Matt Huggins)
Status: Rejected
Priority: Normal
Assignee: 
Category: core
Target version: 
ruby -v: ruby 1.9.3p327 (2012-11-10) [i386-mingw32]


=begin
This is a rather peculiar bug I stumbled across.  The basic issue is that once `nil` has been frozen, there is no way to access an unfrozen instance of nil.

 nil.frozen?    => false
 nil.freeze
 nil.frozen?    => true

I say this is a peculiar bug because generally it doesn't matter if `nil` is frozen or not (since it is basically frozen even if not explicitly defined as such).  However, since Ruby is based upon duck typing, you might run into a scenario like the following:

 def get(cache_key)
   content = Rails.cache.read(cache_key)
   content.frozen? ? content.dup : content
 end

If I call the above method and the cache key exists, `content` is a frozen string, which I then `dup` to get an unfrozen copy.  If I call the above method and the cache key does NOT exist, `content` is a frozen `nil`, which it then attempts to `dup` unsuccessfully since a NilClass instance cannot be duped.
=end



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