How to `break` a outside loop in closure (Proc, lambda)?

Question

loop { break } can work fine, but

block = Proc.new { break }
# or
# block = lambda { break }
loop(&block) # => LocalJumpError: break from proc-closure

Is it possible to break in a block variable ?

Update:

A example to explain more:

def odd_loop
    i = 1
    loop do
        yield i
        i += 2
    end
end

def even_loop
    i = 2
    loop do
        yield i
        i += 2
    end
end

# This work
odd_loop do |i|
    puts i
    break if i > 10
end

# This doesn't work
break_greater_10 = Proc.new do |i|
    puts i
    break if i > 10
end

odd_loop(&break_greater_10) # break from proc-closure (LocalJumpError)
even_loop(&break_greater_10) # break from proc-closure (LocalJumpError)

As my comprehension, Proc.new should work same as block (it can return a function from block), but I don't understand why can't break a loop.

P.S. Sorry for my bad english >~<

99% related: http://stackoverflow.com/questions/626/when-to-use-lambda-when-to-use-proc-new — tokland, Mar 15 '13 at 10:43
@Sergio See update, @tokland I know the different in `Proc.new` and `lambda`, I am asking `break` in `closure` — Steely Wing, Mar 15 '13 at 10:55

score 7 · Answer 1 · answered Feb 05 '15 at 15:54

7

To solve this problem you could

raise StopIteration

this worked for me.

answered Feb 05 '15 at 15:54

Jazz

1,190
14
11

1

+1 Not only does it work, it's actually used in an [example in the ruby docs](http://ruby-doc.org/core-2.5.3/StopIteration.html) to halt a `loop`. – Kelvin Dec 21 '18 at 21:22

score 3 · Answer 2 · answered Mar 15 '13 at 10:14

3

To return from a block you can use the next keyword.

def foo
  f = Proc.new {next ; p 1}
  f.call
  return 'hello'
end

puts foo     # => 'hello' , without 1

answered Mar 15 '13 at 10:14

halfelf

8,964
13
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61

`break` a outside loop is different return from block – Steely Wing Mar 15 '13 at 10:56
2

In your case, you can use `lambda` and use `return`, I think `next` is not good for this (although it works) – Steely Wing Mar 15 '13 at 11:19
This doesn't actually work to halt a `loop` as the OP asked. – Kelvin Dec 26 '18 at 21:58

How to `break` a outside loop in closure (Proc, lambda)?

2 Answers2