Using 'return' in a Ruby block

Question

I'm trying to use Ruby 1.9.1 for an embedded scripting language, so that "end-user" code gets written in a Ruby block. One issue with this is that I'd like the users to be able to use the 'return' keyword in the blocks, so they don't need to worry about implicit return values. With this in mind, this is the kind of thing I'd like to be able to do:

def thing(*args, &block)   value = block.call   puts "value=#{value}" end  thing {   return 6 * 7 } 

If I use 'return' in the above example, I get a LocalJumpError. I'm aware that this is because the block in question is a Proc and not a lambda. The code works if I remove 'return', but I'd really prefer to be able to use 'return' in this scenario. Is this possible? I've tried converting the block to a lambda, but the result is the same.

Answer 1

Simply use next in this context:

$ irb irb(main):001:0> def thing(*args, &block) irb(main):002:1>   value = block.call irb(main):003:1>   puts "value=#{value}" irb(main):004:1> end => nil irb(main):005:0> irb(main):006:0* thing { irb(main):007:1*   return 6 * 7 irb(main):008:1> } LocalJumpError: unexpected return         from (irb):7:in `block in irb_binding'         from (irb):2:in `call'         from (irb):2:in `thing'         from (irb):6         from /home/mirko/.rvm/rubies/ruby-1.9.1-p378/bin/irb:15:in `<main>' irb(main):009:0> thing { break 6 * 7 } => 42 irb(main):011:0> thing { next 6 * 7 } value=42 => nil 

• return always returns from method, but if you test this snippet in irb you don't have method, that's why you have LocalJumpError
• break returns value from block and ends its call. If your block was called by yield or .call, then break breaks from this iterator too
• next returns value from block and ends its call. If your block was called by yield or .call, then next returns value to line where yield was called

Answer 2

You cannot do that in Ruby.

The return keyword always returns from the method or lambda in the current context. In blocks, it will return from the method in which the closure was defined. It cannot be made to return from the calling method or lambda.

The Rubyspec demonstrates that this is indeed the correct behaviour for Ruby (admittedly not a real implementation, but aims full compatibility with C Ruby):

describe "The return keyword" do # ... describe "within a block" do # ... it "causes the method that lexically encloses the block to return" do # ... it "returns from the lexically enclosing method even in case of chained calls" do # ...