From: Alex Gutteridge <alexg@...>
Date: 2007-12-04T14:29:26+09:00
Subject: Re: Everyone's favorite flow control: retry

On 4 Dec 2007, at 00:06, Gregory Seidman wrote:

> On Mon, Dec 03, 2007 at 02:54:32PM +0900, Charles Oliver Nutter wrote:
>> Today I was thinking about retry support in JRuby, and figured we've
>> reached a decision point.
> [...]
>> The problem I've run into is that it's really cumbersome to  
>> implement this
>> correctly in the compiler, cumbersome enough I'm debating whether  
>> we'll
>> support it. There's a few reasons for this:
>>
>> - retry is handled using exception; so every method call that takes  
>> a block
>> would have to be wrapped with exception-handling for retry to work.
>> Exception-handling is expensive, even if no exceptions are actually  
>> raised.
>> It would also add a significant amount of code.
>> - there's no way to know that a method will be reevaluated by  
>> looking at
>> it, which makes it both dangerous and impossible to predict.
>> - nobody understands retry, and nobody uses it.
>
> Okay, I disagree with that last point. Retry is very handy when
> appropriate, and I have, indeed, used it.

I'm pretty sure Charles meant that nobody uses or understands retry  
*outside a rescue* there. Clearly it's very useful in the context of a  
rescue.

>> This also affects retry within a block
>>
>> def foo(a); puts 3; yield; end
>>
>> (print 1; self).foo((print 2; nil)) { retry }
>>
>> This also prints "123" repeatedly.

This works for me in irb:

irb(main):004:0> def foo(a); puts 3; yield; end
=> nil
irb(main):005:0> (print 1; self).foo((print 2; nil)) { retry }
123
123
123
123
123
123
......Ad infinitum

But not in 1.8.6:

[alexg@powerbook]/Users/alexg/Desktop(27): cat retry2.rb
def foo(a); puts 3; yield; end

(print 1; self).foo((print 2; nil)) { retry }
[alexg@powerbook]/Users/alexg/Desktop(28): ruby retry2.rb
retry2.rb:3: private method `foo' called for main:Object (NoMethodError)

Though this does:

[alexg@powerbook]/Users/alexg/Desktop(31): cat retry.rb
class Foo
   def initialize
     print 1
   end
   def foo(a); puts 3; yield; end
end

Foo.new.foo((print 2; nil)) { retry }
[alexg@powerbook]/Users/alexg/Desktop(32): ruby retry.rb
123
123
123
123
123
......Ad infinitum

Color me confused, and one extra vote for removing retry outside of  
rescue.

Alex Gutteridge

Bioinformatics Center
Kyoto University