From: "Thomas B." <tpreal@...>
Date: 2008-09-20T02:27:44+09:00
Subject: Re: That was unexpected -- strange scoping behavior

Patrick Doyle wrote:
> Consider the following piece of code:
> #!/usr/bin/env ruby
> 
> def wow(idx, donetest)
>   until donetest.call(idx)
>     puts idx
>     idx = idx + 1
>   end #until
> end #def wow
> 
> idx = 2
> puts "before wow, idx = #{idx}\n"
> wow(3, lambda {|x| x > 5})
> puts "after wow, idx = #{idx}\n"
> 
> # Now, suppose I used a different dummy variable for the lambda function
> 
> puts "before wow, idx = #{idx}\n"
> wow(3, lambda {|idx| idx > 5})
> puts "after wow, idx = #{idx}\n"
> 
> ---------------------------
> When I just happened to use the name "idx" for my dummy variable in my
> lambda function, I was quite surprised to learn that it changed the
> value of "idx" in the enclosing scope.  I suppose that makes sense
> (given Ruby's scoping rules), but it sure was unexpected!
> 
> --wpd

Yes, it is a small gotcha. In Ruby 1.9 this is going to work like 
expected - lambda (or block) arguments will be separate from the lambda 
defining scope.

There will also be an option to declare additional local variables in 
lambdas. Ruby 1.8:

a=5
[3].each{|x| a=x;puts a}
a #=> 3
The a in the block refers to the outside a, and nothing can be done 
about this. This might sometimes cause errors, difficult to track down. 
If you don't want the a in the block to refer to the a outside, you have 
to change the variable name.

Ruby 1.9:
a=5
[3].each{|x;a| a=x;puts a}
a #=> 5

After semicolon, you can define variables that will be local in their 
block or lambda, even despite name clash.

TPR.
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