Databases
StackOverflow2013
Postsdbo
POHow to stop iteration in a Enumerator::Lazy method?
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POHow to stop iteration in a Enumerator::Lazy method?
    primarykey
    data
    text
    <p>I am trying to implement a <code>take_until</code> method for Ruby 2's Enumerator::Lazy class. It should work similar to <code>take_while</code> but instead stop iteration when the yielded block returns true. The result should include the item where the yielded block matches.</p> <p>My question is how do I signal that the end of the iteration is reached? When using regular Enumerators you can raise the StopIteration error in an each method to signal the end of the iterator. But that doesn't seem to work for lazy enum's:</p> <pre><code>class Enumerator::Lazy def take_until Lazy.new(self) do |yielder, *values| yielder &lt;&lt; values raise StopIteration if yield *values end end end (1..Float::INFINITY).lazy.take_until{ |i| i == 5 }.force </code></pre> <p>I also tried to break out of the block to no effect. <a href="http://ruby-doc.org/core-2.0.0/Enumerator/Lazy.html" rel="nofollow">The documentation for Enumerator::Lazy</a> doesn't seem to help either.</p> <p><strong>Why using <code>take_while</code> is not a valid option.</strong></p> <p>The main problem with take_while is that by its nature it will attempt to evaluate one more item than you need. In my application the Enumerator doesn't yield numbers, but messages fetched over the network. Trying to evaluate a message that is not there (yet?) is a blocking action which is highly undesirable. This is illustrated by the following contrived example:</p> <pre><code>enum = Enumerator.new do |y| 5.times do |i| y &lt;&lt; i end sleep end enum.lazy.take_while{ |i| i &lt; 5 }.force </code></pre> <p>To receive the first five items from this enumerator you will need to evaluate the sixth result. This is not as lazy as it could be. In my use case this is undesirable since the process would block.</p> <p><strong>Providing a pure Ruby implementation of <code>take</code> for Enumerator::Lazy</strong></p> <p>The standard library includes a <code>take</code> method that does something similar to what I want. It doesn't use a block as a condition but a number, but it does break out of the the iteration once that number is reached instead of evaluating one more item. Following on from the example above:</p> <pre><code>enum.lazy.take(5).force </code></pre> <p>This does not get to the 6th item and so does not block. Problem is the version in the standard library is implemented in C and I can't seem to figure out how this could be implemented in pure Ruby. A ruby implementation of that method would be an acceptable answer.</p> <p>Thanks in advance!</p>
    singulars
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PTQuestion
    1. USDirk Geurs
    1. USDirk Geurs
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    2. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. POHow to stop iteration in a Enumerator::Lazy method?
      1. This table or related slice is empty.
      1. VTUpMod
    1. COI hate to ask, but couldn't you just use take_while and amend the condition as needed?
      singulars
      1. POHow to stop iteration in a Enumerator::Lazy method?
      1. USDenis de Bernardy
    2. COThat's a valid question but I think the answer is: no I can't. My use case involves a stream of responses where I specifically want to delimit the sequence once I encounter certain condition. Take_while wouldn't include the matched item itself but instead returns the sequence up to the first 'miss'. Hope that makes sense.
      singulars
      1. POHow to stop iteration in a Enumerator::Lazy method?
      1. USDirk Geurs
    3. COSo `take_until` would be a negated `take_while` which does one additional `yield next`?
      singulars
      1. POHow to stop iteration in a Enumerator::Lazy method?
      1. USsteenslag
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload