From: Mike Fletcher <lemurific+rforum@...>
Date: 2006-04-30T21:02:36+09:00
Subject: Re: Constant in Ruby.

Charlie wrote:
[...]
>> 
>> It's constant in that you can't reassign a different instance to the 
>> same name (try setting X = 10 after you've set it to 20 and Ruby will 
>> gripe).  The instances themselves may still be mutable (see 
>> Object#freeze if you want to change that).
> 
> okay, I just try this:
> 
> X = 20
> X = 10
> puts(X)
> 10
> X.freeze
> X = 30
> puts(X)
> 30
> 
>    Sorry, but I find it difficult to understand as I'm new to it. Would 
> be helpful if you can enlighten me with some examples. Thanks.

$ 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.

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