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Understanding Ruby Blocks, Procs and Lambdas

50 points by robertsosinski 18y ago

Learn the subtle differences between Blocks, Procs and Lambdas with Ruby and how to use each effectively in your methods.

aaronblohowiak 18y ago
Don't forget the changes in 1.9 http://railspikes.com/2008/9/8/lambda-in-ruby-1-9 http://railspikes.com/2008/9/8/lambda-in-ruby-1-9 And the much cooler Fibers, which you can think of as a reentrant block, kind of.
tlrobinson 18y ago
What's the reason for blocks not being real objects (i.e. Procs) themselves (like in JavaScript or Lisp)? Is it simply to allow for the magic syntax of passing a block to a method so it can be called with yield?
- raganwald 18y ago
  Most of the problem is the `return` keyword. In a proc or lambda, it returns from the proc or lambda. In a block, it returns from whatever is enclosing the block, like a proc, lambda, or method. If you had a block that was an actual object, it could outlive its enclosing environment. Calling return within it would reincarnate its enclosing environment. Calling return twice would fork time. In other words, if a block was an object, the return keyword would create a continuation. This is a very expensive, very heavyweight feature in Ruby, and the whole point of blocks is to provide something cheap and lightweight. Note that a proc or lambda has no such problem, since a return within it returns to whatever invokes the #call method of the proc or lambda. Imagine how much simpler things would be if Matz had worked this all out and decided that the return keyword was more trouble than it was worth?
  - aaronblohowiak 18y ago
    I respectfully disagree that the return keyword is more trouble than it is worth. "return false if simple_question?" ... rest of the body is a very useful thing to do. granted, you might say it is "programming by GOTO", but it lowers the indentation level, and so keeps me happy. To the GP: we can also .to_proc on blocks.
    - raganwald 18y ago
      Sorry I didn't see this when you posted it. The problem is not `return :foo if bar()`, it's something like `pizzles.each { |p| return :foo if p.bar() }`. In Ruby, that returns from the surrounding method, not from the block where it's defined. You can decide for yourself if this behaviour of return more trouble than it's worth. I didn't say it was, I said that this definition is what forces a block to be something different from a proc, which makes the language more complicated.
- Andys 18y ago
  Yup, in your method signature you simply have one last parameter called &bl and then bl is set to the Proc object. You can no longer call yield(...) but instead you have to use bl.call(...) You can indeed save the Proc object and use it later.
charlesju 18y ago
Great post, I had the longest time trying to figure out what blocks were. They aren't used that much in PHP and Java and stuff so it was a totally new concept for me. I still don't understand Procs and Lambdas but my head is too full today, will finish reading your post tomorrow. As a side note, please enable comments, I'd love to leave you some good feedback on your actual site.
davidmathers 18y ago
"But I have yet to find a good, definitive guide out there." Really? I thought this was the definitive guide: http://innig.net/software/ruby/closures-in-ruby.rb http://innig.net/software/ruby/closures-in-ruby.rb It seems like a good guide to me.

Learn the subtle differences between Blocks, Procs and Lambdas with Ruby and how to use each effectively in your methods.

aaronblohowiak 18y ago

Don't forget the changes in 1.9 http://railspikes.com/2008/9/8/lambda-in-ruby-1-9 http://railspikes.com/2008/9/8/lambda-in-ruby-1-9 And the much cooler Fibers, which you can think of as a reentrant block, kind of.

tlrobinson 18y ago

What's the reason for blocks not being real objects (i.e. Procs) themselves (like in JavaScript or Lisp)? Is it simply to allow for the magic syntax of passing a block to a method so it can be called with yield?

raganwald 18y ago
Most of the problem is the `return` keyword. In a proc or lambda, it returns from the proc or lambda. In a block, it returns from whatever is enclosing the block, like a proc, lambda, or method. If you had a block that was an actual object, it could outlive its enclosing environment. Calling return within it would reincarnate its enclosing environment. Calling return twice would fork time. In other words, if a block was an object, the return keyword would create a continuation. This is a very expensive, very heavyweight feature in Ruby, and the whole point of blocks is to provide something cheap and lightweight. Note that a proc or lambda has no such problem, since a return within it returns to whatever invokes the #call method of the proc or lambda. Imagine how much simpler things would be if Matz had worked this all out and decided that the return keyword was more trouble than it was worth?
- aaronblohowiak 18y ago
  I respectfully disagree that the return keyword is more trouble than it is worth. "return false if simple_question?" ... rest of the body is a very useful thing to do. granted, you might say it is "programming by GOTO", but it lowers the indentation level, and so keeps me happy. To the GP: we can also .to_proc on blocks.
  - raganwald 18y ago
    Sorry I didn't see this when you posted it. The problem is not `return :foo if bar()`, it's something like `pizzles.each { |p| return :foo if p.bar() }`. In Ruby, that returns from the surrounding method, not from the block where it's defined. You can decide for yourself if this behaviour of return more trouble than it's worth. I didn't say it was, I said that this definition is what forces a block to be something different from a proc, which makes the language more complicated.
Andys 18y ago
Yup, in your method signature you simply have one last parameter called &bl and then bl is set to the Proc object. You can no longer call yield(...) but instead you have to use bl.call(...) You can indeed save the Proc object and use it later.

charlesju 18y ago

Great post, I had the longest time trying to figure out what blocks were. They aren't used that much in PHP and Java and stuff so it was a totally new concept for me. I still don't understand Procs and Lambdas but my head is too full today, will finish reading your post tomorrow. As a side note, please enable comments, I'd love to leave you some good feedback on your actual site.

davidmathers 18y ago

"But I have yet to find a good, definitive guide out there." Really? I thought this was the definitive guide: http://innig.net/software/ruby/closures-in-ruby.rb http://innig.net/software/ruby/closures-in-ruby.rb It seems like a good guide to me.