Databases
StackOverflow2013
Postsdbo
POHow to make Ruby var= return value assigned, not value passed in?
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POHow to make Ruby var= return value assigned, not value passed in?
    primarykey
    data
    text
    <p>There's a nice idiom for adding to lists stored in a hash table:</p> <pre><code>(hash[key] ||= []) &lt;&lt; new_value </code></pre> <p>Now, suppose I write a derivative hash class, like the ones found in Hashie, which does a deep-convert of any hash I store in it. Then what I store will not be the same object I passed to the = operator; Hash may be converted to Mash or Clash, and arrays may be copied.</p> <p>Here's the problem. Ruby apparently returns, from the var= method, the value passed in, not the value that's stored. It doesn't matter what the var= method returns. The code below demonstrates this:</p> <pre><code>class C attr_reader :foo def foo=(value) @foo = (value.is_a? Array) ? (value.clone) : value end end c=C.new puts "assignment: #{(c.foo ||= []) &lt;&lt; 5}" puts "c.foo is #{c.foo}" puts "assignment: #{(c.foo ||= []) &lt;&lt; 6}" puts "c.foo is #{c.foo}" </code></pre> <p>output is</p> <pre><code>assignment: [5] c.foo is [] assignment: [6] c.foo is [6] </code></pre> <p>When I posted this as a bug to Hashie, Danielle Sucher explained what was happening and pointed out that "foo.send :bar=, 1" returns the value returned by the bar= method. (Hat tip for the research!) So I guess I could do:</p> <pre><code>c=C.new puts "clunky assignment: #{(c.foo || c.send(:foo=, [])) &lt;&lt; 5}" puts "c.foo is #{c.foo}" puts "assignment: #{(c.foo || c.send(:foo=, [])) &lt;&lt; 6}" puts "c.foo is #{c.foo}" </code></pre> <p>which prints</p> <pre><code>clunky assignment: [5] c.foo is [5] assignment: [5, 6] c.foo is [5, 6] </code></pre> <p>Is there any more elegant way to do this?</p>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. This table or related slice is empty.
    1. USChrisPhoenix
    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
    3. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. POHow to make Ruby var= return value assigned, not value passed in?
      1. This table or related slice is empty.
      1. VTUpMod
    1. COI don't think you can override the return value of an assignment.
      singulars
      1. POHow to make Ruby var= return value assigned, not value passed in?
      1. USJohn Dvorak
    2. COI think using a `default_proc` is a much nicer idiom than that mess of syntax at the top of your question. I also think that your `foo=` method is an abuse of the expected behavior of assignment in Ruby, the caller should do the cloning.
      singulars
      1. POHow to make Ruby var= return value assigned, not value passed in?
      1. USmu is too short
    3. COMu, how would you write a Hashie::Mash, without modifying what is stored? I agree that normally assignment should store what's given. But with a hash, you're more adding to a data structure than assigning a value to a variable.
      singulars
      1. POHow to make Ruby var= return value assigned, not value passed in?
      1. USChrisPhoenix
 

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