From: David Alan Black <dblack@...>
Date: 2002-07-10T21:58:56+09:00
Subject: Re: for ... else ... end

Hi --

On Wed, 10 Jul 2002, Dirk Detering wrote:

>
> I've got the feeling that this thread is
> drifting from the original problem (detect
> successful looping) to the discussion about
> some related but different problems.

One problem is the definition of "success" in looping.  Personally I
don't think there's any such thing.  This:

  [].each {|x| puts x}

is completely successful, as far as I can see.  Python apparently
considers success to be indicated by 'break', which I think is a
style/taste position which I would not like to see given
language-level blessing.

[...]

> Problem 1:  Identify bypassing (e.g. empty array)
> and declare coding that shall run in this case

if array.empty? ...  No, seriously :-)  This includes testing results:

  res = array.find_all { <test> }
  if res.empty? ...

> Problem 2:  Identify interruption (by break or throw)
> and declare coding that shall run in this case

I would say: if you want to branch this way, then use catch/throw with
your break (rather than trying to enhance 'break' so that it more or
less duplicates what catch/throw already does).

> Problem 3:  Identify full passing (no break)
> and declare coding that shall run in this case.

This goes hand-in-hand with #2.

> David Alan Black schrieb:
> >   if array.empty?
> >     puts "Empty array"
> >   else
> >     array.each {|e| puts e}
> >   end
>
> Ok, this solves problem 1 in some way (although it
> does not 'identify' the bypassing itself, but checks
> explicitly a precondition). So in relation to loops
> you would check a condition twice:
>
> if a > b
>    while a > b
>     ...
>    end
> else
>    ...
> end

I see this as a case of this:

  if x
    while y
    ...

where x and y happen to be the same test.  I would say: it's easier to
live with the occasional coincidence like this, than to introduce
language-level constructs to handle it.

[...]

> So the complete example looks something like:
>
> if array.empty?
>   print "Loop bypassed \n"
> else
>   catch (:done) do
>        for i in array
>            if #<some condition>
>                print "Interruption branch\n"
>                throw :done
>            end
>            print "Working in loop \n"
>        end
>        print "Loop terminated without break\n"
>    end
> end

I would rewrite that like this:

  if array.empty?
    puts "Array is empty -- no test"
  else
    if array.detect {|e| puts "Working in loop"; <some condition>}
      puts "Condition met"
    else
      puts "Condition not met"
    end
  end

> Hum, the discussed construct with different
> for-branches would simplify things ... don't you
> think too?

I don't know... it seems like a lot of what's in this thread is very
complex, at least to my eye, compared to the simplicity Ruby offers.


David

-- 
David Alan Black
home: dblack@candle.superlink.net
work: blackdav@shu.edu
Web:  http://pirate.shu.edu/~blackdav