From: shevegen@...
Date: 2017-11-06T05:50:52+00:00
Subject: [ruby-core:83684] [Ruby trunk Feature#14084] Introduce	Enumerator#next?

Issue #14084 has been updated by shevegen (Robert A. Heiler).


I guess we have to wait how matz feels about it, that is, .next? in
this context. I personally have no problem with it - I wanted to 
say something to this, though:

> I am aware that we can currently figure out if there is a next
> value by using rescue (as in the code snippet above), but it
> is ugly since it covers many lines and uses exceptions for
> control flow.

I think you were not the first one here to want to avoid using
exceptions for control flow; there is also some other issue request
about worker/queue, which is a bit similar to what you suggested
here, if I understood it correctly so, which also avoids begin/rescue.

The ability to  "look into the future" and ask ruby whether this or
that is ok, without depending on begin/rescue control flow, in regards
to methods. I think the one who suggested it was asking so in anticipation
as to whether an ArgumentError will be triggered, if this or that input
parameter is used.

I found it interesting because it is a bit of a mix between 
runtime-introspection, and also somewhat similar to begin/rescue,
just without ... using begin/rescue. :)

But I think it's best to see how matz feels it may interconnect
with other ruby features (your proposal is of course simpler and
different than what I wrote just now; I just wanted to provide
a slightly different context, since I found it similar - some 
ruby people preferring to avoid begin/rescue and I think that 
is understandable).

----------------------------------------
Feature #14084: Introduce Enumerator#next?
https://bugs.ruby-lang.org/issues/14084#change-67711

* Author: tessi (Philipp Tessenow)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
I'd like to propose a new method for Enumerator which returns whether there will be a next value in the enumerator when calling `Enumerator#next`.

It should work like this ruby demo:

~~~ ruby
class Enumerator
  def next?
    begin
      peek
    rescue StopIteration
      return false
    end
    true
  end
end

a = [1,2,3]
e = a.to_enum
p e.next?   #=> true
p e.next    #=> 1
p e.next?   #=> true
p e.next    #=> 2
p e.next?   #=> true
p e.next    #=> 3
p e.next?   #=> false
p e.next    #raises StopIteration
~~~

I propose the method to be called `next?` as it returns either `true` or `false`.
I am aware that we can currently figure out if there is a next value by using `rescue` (as in the code snippet above), but it is ugly since it covers many lines and uses exceptions for control flow.
Introducing the `next?` method makes enumerators a little nicer to work with.


A patch with an example implementation for ruby trunk is attached (in git-diff format, any feedback welcome).
I agree that my patch will be licensed under the Ruby License.

---Files--------------------------------
enumerator_next.diff (2.26 KB)


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