From: Eric Christopherson <echristopherson@...>
Date: 2013-03-06T04:14:16+09:00
Subject: Re: How `next` works in ruby with `unless` ?

On Tue, Mar 5, 2013 at 12:53 PM, Robert Klemme
<shortcutter@googlemail.com> wrote:
> On Tue, Mar 5, 2013 at 7:38 PM, Hans Mackowiak <lists@ruby-forum.com> wrote:
>> Robert Klemme wrote in post #1100233:
>>> On Tue, Mar 5, 2013 at 7:23 PM,  <sto.mar@web.de> wrote:
>>>>> executed. Why does next not skip this statement ?
>>> Probably because arguments are always evaluated before the "method"
>>> they are handed over to.
>>>
>>>> Nobody writes code like this, so why care.
>>>
>>> I wasn't even aware that next accepts an argument - funnily it won't
>>> accept two (syntax error).
>
>> hm it does:
>>
>> [1,2,3,4].map {|d| next d,4,5;p "is not printed" }
>> #=> [[1, 4, 5], [2, 4, 5], [3, 4, 5], [4, 4, 5]]
>
> Oh, I tested only with brackets.  Here's the whole story
>
> $ ruby -ce 'f { next }'
> Syntax OK
> $ ruby -ce 'f { next 1 }'
> Syntax OK
> $ ruby -ce 'f { next 1, 2 }'
> Syntax OK
> $ ruby -ce 'f { next 1, 2, 3 }'
> Syntax OK
> $ ruby -ce 'f { next(1) }'
> Syntax OK
> $ ruby -ce 'f { next(1, 2) }'
> -e:1: syntax error, unexpected ',', expecting ')'
> f { next(1, 2) }
>            ^
>
> It's getting weirder - especially since the value(s) passed to next
> are not used for anything as far as I can see.  And it wouldn't make
> sense, would it?  I mean, next is different from break which can be
> used to return a value from an iteration.

`next val` causes an exit from a block, returning val. In a block
passed to Kernel#loop, the value is discarded; so Kumar's `next puts
'hi'` just causes 'hi' to be printed but the nil return value of puts
is discarded.

`break val` also exits from a block, returning val, but in some cases
also exits from the method calling the block. I wrote a test case for
this on my work computer, but I don't have access to it here; but I
seem to recall that break in a lambda simply exits the block, while
break in a proc exits the method. (If you create the proc by passing a
block to Proc.new, and then pass that proc to another method, which
calls it, break will raise a LocalJumpError, since the original stack
frame of Proc.new no longer exists.)