Feature #20444
closedKernel#loop: returning the "result" value of StopIteration doesn't work when raised directly
Description
There was a https://bugs.ruby-lang.org/issues/11498 a while ago which was merged in, but I was surprised to find out that raising StopIteration
in a loop like
loop { raise StopIteration.new(3) }
returns nil and not 3.
Updated by nobu (Nobuyoshi Nakada) 7 months ago
- Status changed from Open to Closed
StopIteration.new(3)
does not set result
, and no way to set it in Ruby level.
$ ruby -e 'e = StopIteration.new(3); p e.message, e.result'
"3"
nil
Updated by esad (Esad Hajdarevic) 7 months ago
nobu (Nobuyoshi Nakada) wrote in #note-1:
StopIteration.new(3)
does not setresult
, and no way to set it in Ruby level.$ ruby -e 'e = StopIteration.new(3); p e.message, e.result' "3" nil
Thanks for the hint. It seems that subclassing StopIteration to provide result works:
class MyException < StopIteration
def result = 5
end
loop { raise MyException } # => 5
Updated by nobu (Nobuyoshi Nakada) 6 months ago
I'm curious what your use case is.
Although I don't know the reason why StopIteration#initialize
does not have the argument for result
, it would be difficult to change it now because of the backward compatibility.
Updated by nobu (Nobuyoshi Nakada) 6 months ago
- Status changed from Closed to Feedback
Updated by nobu (Nobuyoshi Nakada) 6 months ago
- Tracker changed from Bug to Feature
- ruby -v deleted (
ruby 3.3.0 (2023-12-25 revision 5124f9ac75) [arm64-darwin20]) - Backport deleted (
3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN)
Updated by esad (Esad Hajdarevic) 6 months ago
nobu (Nobuyoshi Nakada) wrote in #note-3:
I'm curious what your use case is.
Although I don't know the reason why
StopIteration#initialize
does not have the argument forresult
, it would be difficult to change it now because of the backward compatibility.
I think my use case is a bit of an edge case - I am passing a block into a Ractor where it runs in a loop. This way I can control exit from the loop, but obviously this can be also refactored into a normal result value of the block and "manual" looping control depending on the result.
Updated by ufuk (Ufuk Kayserilioglu) 6 months ago
@esad (Esad Hajdarevic) If you just want to return a result from the loop
, you can use break <value>
to do that:
$ ruby -e "puts loop { break 3 }"
3
You shouldn't have to deal with anything low level like StopIteration
to do that.
Updated by esad (Esad Hajdarevic) 6 months ago ยท Edited
ufuk (Ufuk Kayserilioglu) wrote in #note-7:
@esad (Esad Hajdarevic) If you just want to return a result from the
loop
, you can usebreak <value>
to do that:$ ruby -e "puts loop { break 3 }" 3
Calling break from a block passed to a ractor will raise an exception. I think some sample code about my example will be helpful:
Let's make a ractor that just calls all blocks passed to it in a loop:
r = Ractor.new do
block = Ractor.receive
loop { block.call() }
end
Now let's send it a block that raises StopIteration:
block = true.instance_eval { proc { raise StopIteration, 3 } } # instance_eval in true gives us a "shareable" proc
r.send Ractor.make_shareable(block)
r.take # => nil
Let's try with a subclass (this works)
class MyStop < ::StopIteration
attr_reader :result
def initialize(result)
@result = result
end
end
block = true.instance_eval { proc { raise MyStop, 3 } }
r.send Ractor.make_shareable(block)
r.take # => 3
Updated by nobu (Nobuyoshi Nakada) 6 months ago
That example does not need Ractor
.
class MyStop < ::StopIteration
attr_reader :result
def initialize(result)
@result = result
end
end
block = proc {raise StopIteration, 3}
p Thread.start {loop {block.call}}.value #=> nil
block = proc { raise MyStop, 3 }
p Thread.start {loop {block.call}}.value #=> 3
Updated by esad (Esad Hajdarevic) 6 months ago
nobu (Nobuyoshi Nakada) wrote in #note-9:
That example does not need
Ractor
.
Yes, you are right, it actually doesn't need Thread either, and is simply about calling a block in a loop and how to break the loop from the called block:
proc { break 3}.then { |p| loop { p.call() }}
raises exception, and proc { raise StopIteration, 3 }.then { |p| loop { p.call() }}
returns nil
, etc.