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Blocks and scoping question

Mischa Fierer

9/11/2008 8:12:00 AM

Hello,

The following makes sense to me:

lambda {|x| puts x; [1,2].collect{|x| x+1} }.call("aaa")
aaa
=> [2, 3]

The following does not so much:

lambda {|x| puts x; [1,2].collect{|x| x+1}; puts x }.call("aaa")
aaa
2
=> nil

Why is the final puts x not return a?
--
Posted via http://www.ruby-....
5 Answers

Stefano Crocco

9/11/2008 8:17:00 AM

0

On Thursday 11 September 2008, Mischa Fierer wrote:
> Hello,
>
> The following makes sense to me:
>
> lambda {|x| puts x; [1,2].collect{|x| x+1} }.call("aaa")
> aaa
> => [2, 3]
>
> The following does not so much:
>
> lambda {|x| puts x; [1,2].collect{|x| x+1}; puts x }.call("aaa")
> aaa
> 2
> => nil
>
> Why is the final puts x not return a?

Because puts always returns nil:

ri IO#puts

---------------------------------------------------------------- IO#puts
ios.puts(obj, ...) => nil
------------------------------------------------------------------------

Stefano

Mischa Fierer

9/11/2008 8:18:00 AM

0

A friend informs me that this is changed in 1.9

For this who are interested:

http://groups.google.com/group/sbonrails/browse_thread/thread/cfcfda...

Mischa Fierer wrote:
> Hello,
>
> The following makes sense to me:
>
> lambda {|x| puts x; [1,2].collect{|x| x+1} }.call("aaa")
> aaa
> => [2, 3]
>
> The following does not so much:
>
> lambda {|x| puts x; [1,2].collect{|x| x+1}; puts x }.call("aaa")
> aaa
> 2
> => nil
>
> Why is the final puts x not return a?

--
Posted via http://www.ruby-....

Mischa Fierer

9/11/2008 8:20:00 AM

0

Stefano Crocco wrote:
> On Thursday 11 September 2008, Mischa Fierer wrote:
>> lambda {|x| puts x; [1,2].collect{|x| x+1}; puts x }.call("aaa")
>> aaa
>> 2
>> => nil
>>
>> Why is the final puts x not return a?
>
> Because puts always returns nil:
>
> ri IO#puts
>
> ---------------------------------------------------------------- IO#puts
> ios.puts(obj, ...) => nil
> ------------------------------------------------------------------------
>
> Stefano

Sorry, phrased this wrong, i wasn't looking at the return value, but the
2 above it.
--
Posted via http://www.ruby-....

Peña, Botp

9/11/2008 8:32:00 AM

0

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IHgNCmFhYQ0KYWFhDQoNCg==

TPReal

9/11/2008 8:34:00 AM

0

Mischa Fierer wrote:
> Hello,
>
> The following makes sense to me:
>
> lambda {|x| puts x; [1,2].collect{|x| x+1} }.call("aaa")
> aaa
> => [2, 3]
>
> The following does not so much:
>
> lambda {|x| puts x; [1,2].collect{|x| x+1}; puts x }.call("aaa")
> aaa
> 2
> => nil
>
> Why is the final puts x not return a?

It's because the x in collect{|x| ...} in fact uses the variable x that
previously contained your "aaa", because it is already defined in this
scope. If x wasn't defined at the point of calling collect, then x
inside collect's block would be a separate variable each iteration.

This is going to change in Ruby 1.9, and block variables will always be
separate from variables visible in the scope.

TPR.
--
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