From: Josh Cheek <josh.cheek@...>
Date: 2010-05-05T16:35:33+09:00
Subject: Re: for-in vs. map closures

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On Wed, May 5, 2010 at 1:35 AM, Mike Austin <mike_ekim@yahoo.com> wrote:

> I was experimenting with closures and JavaScript's and Ruby's
> behavior.  Ruby seems to close over its arguments automatically if
> using map:
>
> [1, 2, 3].map { |n| lambda { n * n } }.map { |b| b.call() }  # [1, 4,
> 9]
>
> But using "for" iteration, returns a different result:
>
> a = []
> for n in [1, 2, 3] do a << lambda { n * n } end
> a.map { |b| b.call() }  # [9, 9, 9]
>
> This is also the behavior in JavaScript, which frequently catches
> developers off guard when attaching event handlers to elements.
>
> Can anyone explain the difference?  Is map creating a new anonymous
> function every iteration?
>
> Mike
>
>
I don't know, it seems like very peculiar behaviour. It does create new
lambdas every time, but strangely, the objects created within the block are
shared.

a = Array.new
for s in %w(a b c)
  s2 = s
  a << lambda { [ s2 , s , s2.object_id ] }
end

a.map { |b| b.call() }  # => [["c", "c", 2148175880], ["c", "c",
2148175880], ["c", "c", 2148175880]]
a[0].object_id       # => 2148175860
a[1].object_id       # => 2148175840
a[2].object_id       # => 2148175820

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