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I saw the code from here

Post.published.collect(&:views_count)

I guess it equals to 

.collect { |p| p.views_count }

But I never saw this usage before, does this have a name? Where can I find more information about it?

Andrew Grimm
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Cheng
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2 Answers2

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This is actually a rather clever hack made it into ruby 1.9.

Basically, & in front of a variable in ruby coerces it into a proc. It does that by calling to_proc. Some clever fellow (first time I saw this was in _whys code, but I won't credit him cause I don't know if he came up with it) added a to_proc method to Symbol, that is essentially {|obj| obj.send self}.

There aren't many coercians in ruby, but it seems like all of them are mostly used to do hacks like this (like !! to coerce any type into a boolean)

Matt Briggs
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    Since the code snippet in question is using rails, it should be pointed out that `Symbol#to_proc` was in Active Support long before ruby 1.9, so you can use it in rails code without worrying about compatibility with ruby version. – sepp2k Oct 08 '10 at 06:11
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It's a use of Symbol#to_proc. The & operator turns a Proc object into a block, and because Ruby 1.8.7 and newer implement Symbol#to_proc, it can be used with a symbol like :views_count. And yes, it's equivalent to {|p| p.views_count}.

Chuck
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