Incorrect handling of begin/end while

[wmnnd]

Opal seems to evaluate the condition in a begin/end while loop before executing the scope while MRI runs it once before checking the condition.

The following code will print "konichiwa" in MRI but not in Opal.:

begin
  puts "konichiwa"
end while false

#Prints "konichiwa" in MRI, but not in Opal

Regular while loops work fine and have their condition checked both in MRI and Opal:

while false
    puts "hello"
end

#Will never print anything

[Ajedi32]

You know, now that I think about it that behavior from MRI is kind of strange:

puts "hi" while false # Prints nothing

begin
  puts "konichiwa"
end while false
# Prints "konichiwa"

puts "hi" if false # Prints nothing

begin
  puts "konichiwa"
end if false
# Prints nothing

😜

[vendethiel]

The first one is a do-while (special form). The second one is a postfix if

[Ajedi32]

@Nami-Doc Yes, I realize it's there to allow do-while. I just think it's funny that postfixing while results in the condition being checked after block execution, whereas postfixing if does not. It feels inconsistent, even though I know it's like that for a good reason.

[sergei-kucher]

@Ajedi32 My opinion is nothing strange here.

if is keyword for execute expression or not, but while is keyword for loop expression or not. So then we use if false, expression doesn't execute. And in case of while false, expression executes but loop doesn't performs.

So that behavior is very logical.

[Ajedi32]

@sergey-kucher Except that Ruby's behavior is inconsistent with that interpretation in this case:

puts "hi" while false # Prints nothing

😝

Obviously I'm not saying Opal should implement this differently, I'm just saying the existing behavior feels kind of strange (even though I understand why it was done that way).

[elia]

I think in the we'll just look at rubyspec. I know for sure that some stuff has been "marked" as bug by Matz himself.

Anyway I don't think I'll ever use this "feature" :P

I also consider this rather low-priority :)

[Ajedi32]

Yeah, for more on the usage of this feature: http://stackoverflow.com/questions/136793/is-there-a-do-while-loop-in-ruby

Also, apparently Matz agrees with me: http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-core/6745

[sergei-kucher]

@Ajedi32 puts "hi" while false # Prints nothing - that's really strange ))

[elia]

Will keep this open and will find out who will come first, if the MRI that will remove this feature eventually or us implementing it 😄

---

Here's the response from matz, just for reference
http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-core/6741

Subject: Re: semenatics of if/unless/while statement modifiers
From: Yukihiro Matsumoto <matz ruby-lang.org>
Date: Wed, 23 Nov 2005 10:54:34 +0900
In-reply-to: 6719
Hi,

In message "Re: semenatics of if/unless/while statement modifiers"
    on Wed, 23 Nov 2005 01:11:39 +0900, ts <decoux / moulon.inra.fr> writes:

| What is a post-test loop ? This
|
|moulon% ruby -e 'i = 0; begin puts i; end while i > 0'
|0

Don't use it please.  I'm regretting this feature, and I'd like to
remove it in the future if it's possible.

                            matz.