From: Charlie <peckcharlie@...>
Date: 2006-04-30T23:44:46+09:00
Subject: Re: Constant in Ruby.

Mike Fletcher wrote:
 $ cat spork
 #!/usr/local/bin/ruby

 A_NUMBER = 10
 puts "A_NUMBER is #{A_NUMBER}"
 A_NUMBER = 20
 puts "A_NUMBER is #{A_NUMBER}"

 A_STRING = "foo"
 puts "A_STRING is #{A_STRING}"
 A_STRING.sub!( %r/^f/, "b" )
 puts "A_STRING is #{A_STRING}"
 A_STRING.freeze

 begin
   A_STRING.sub!( %r/^.*$/, "this won't work" )
 rescue TypeError => e
   warn "sub! on A_STRING failed: #{e}"
 end

 A_STRING = "this will work but causes a warning"
 print "A_STRING is #{A_STRING}"

 $ ruby spork
 A_NUMBER is 10
 spork:5: warning: already initialized constant A_NUMBER
 A_NUMBER is 20
 A_STRING is foo
 A_STRING is boo
 sub! on A_STRING failed: can't modify frozen string
 spork:20: warning: already initialized constant A_STRING
 A_STRING is this will work but causes a warning

> Reassigning to a constant after the initial assignment causes a warning, 
> but it works.
> 
> The String instance in A_STRING has methods which will change the 
> contents of the instance.  The constant name "A_STRING" still points to 
> the same String instance (so I didn't get the warning that I was 
> reassigning to an already initialized constant), but its internal 
> state/contents can still be modified in place.  Using freeze makes it so 
> that that internal state can't be changed and the second in-place 
> substitution call throws an exception.  Since I placed begin / rescue / 
> end around that call I can recovered, but A_STRING would still be "boo".
> 
> Lastly, even though I've frozen the particular instance A_STRING points 
> to I can still reassign a new String instance to A_STRING.  However this 
> produces the warning that I'm altering an already initialized constant 
> just as the first examples with A_NUMBER did.  And A_STRING is once 
> again mutable since the new instance hasn't been frozen.

Thanks Mike for clearing things up. That helps.

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