TypeError: can't convert into Array

Dan Rathbun

I got a custom class inherited that include Enumerable. The class also has a #to_a method.
...
Granted it's not an Array, but it says it cannot convert the object - which makes me wonder why since it has an #to_a method.

The #to_a method can come from more than one ancestor.

For instance... the basic source of the #to_a method, comes from Object which states:
@unknownuser said:

Returns an array representation of obj. For objects of class Object and others that don’t explicitly override the method, the return value is an array containing self. However, this latter behavior will soon be obsolete.
This implies that custom classes need to override the #to_a method, so Ruby can know how to do the conversion.

OR... the #to_a method can come from Array, if the custom class, is some descendant level subclass of Array. Which, referring to the docs:
@unknownuser said:

Returns self. If called on a subclass of Array, converts the receiver to an Array object.
Note: That Array itself, has Enumerable mixed in, but it appears (looking at the C source,) that the "mixing" is done first.. then the Array class definition overrides the method inherited from Enumerable, with it's own edition of the method.

thomthom

@jim said:

I think Enumerable requires that your class implement a .each method. Some Enumerable methods also need a <=> method (comparison) defined.

I do have an .each method implemented. But in any case would that matter for converting to an Array?

I manually added an #to_a method to my that returns an array. So I don't understand why it raises this error...

Jim

You need to yield in your .each iterator.

Something like this (not tested.)

@entities.each{|e| yield(e)}

See also: block_given?

Here's a related article about the performance of passing blocks: http://mudge.github.com/2011/01/26/passing-blocks-in-ruby-without-block.html

Dan Rathbun

@thomthom said:

I manually added an #to_a method to my [class] that returns an array. So I don't understand why it raises this error...

Is the superclass one of those API C++ collection classes ? Most of these give us problems when we try to subclass them.

thomthom

@jim said:

You need to yield in your .each iterator.

It does that.

But I fail to see why .each got to do with converting to an array when I have a .to_a method ... ? I've been searching for info on it, but I cannot find anything.

Jim

@thomthom said:

It does that.

Sorry, I was looking at your latest bitbucket repo, which must not be your latest version.

thomthom

No - it's still on my computer. A larger rewrite. The one at the repo was just an early framework WIP.

thomthom

A simplified example:

<span class="syntaxdefault"></span><span class="syntaxkeyword">class&nbsp;</span><span class="syntaxdefault">Foo</span><span class="syntaxkeyword">;&nbsp;</span><span class="syntaxdefault">def&nbsp;to_a</span><span class="syntaxkeyword">;&nbsp;[</span><span class="syntaxdefault">1</span><span class="syntaxkeyword">,</span><span class="syntaxdefault">2</span><span class="syntaxkeyword">,</span><span class="syntaxdefault">3</span><span class="syntaxkeyword">,</span><span class="syntaxdefault">4</span><span class="syntaxkeyword">,</span><span class="syntaxdefault">5</span><span class="syntaxkeyword">];&nbsp;</span><span class="syntaxdefault">end</span><span class="syntaxkeyword">;&nbsp;</span><span class="syntaxdefault">end</span><span class="syntaxkeyword">;<br /></span><span class="syntaxdefault">nil<br /><br />f</span><span class="syntaxkeyword">=</span><span class="syntaxdefault">Foo</span><span class="syntaxkeyword">.new<br /></span><span class="syntaxcomment">#<Foo;0x15259760><br /><br /></span><span class="syntaxdefault">x</span><span class="syntaxkeyword">=[</span><span class="syntaxdefault">3</span><span class="syntaxkeyword">,</span><span class="syntaxdefault">4</span><span class="syntaxkeyword">,</span><span class="syntaxdefault">5</span><span class="syntaxkeyword">,</span><span class="syntaxdefault">6</span><span class="syntaxkeyword">]<br />[</span><span class="syntaxdefault">3</span><span class="syntaxkeyword">,&nbsp;</span><span class="syntaxdefault">4</span><span class="syntaxkeyword">,&nbsp;</span><span class="syntaxdefault">5</span><span class="syntaxkeyword">,&nbsp;</span><span class="syntaxdefault">6</span><span class="syntaxkeyword">]<br /><br /></span><span class="syntaxdefault">x&nbsp;</span><span class="syntaxkeyword">|&nbsp;</span><span class="syntaxdefault">f<br />Error</span><span class="syntaxkeyword">;&nbsp;</span><span class="syntaxcomment">#<TypeError;&nbsp;(eval);155;in&nbsp;`|';&nbsp;can't&nbsp;convert&nbsp;Foo&nbsp;into&nbsp;Array><br /></span><span class="syntaxkeyword">(eval);</span><span class="syntaxdefault">155<br /></span><span class="syntaxkeyword">(eval);</span><span class="syntaxdefault">155</span>

Off course, x | f.to_a works, but since the error message indicate some conversion was tried, I wonder why the .to_a method wasn't used.

Jim

use .to_ary although I don't have a good explanation yet.

class Foo
  def to_ary
    [1,2,3,4,5]
  end
end

This is as close to an explanation as I could find: http://stackoverflow.com/questions/766712/make-an-object-behave-like-an-array-for-parallel-assignment-in-ruby/768201#768201

thomthom

Thanks!

Class: Array (Ruby 1.8.6)

Class : Array - Ruby 1.8.6

(www.ruby-doc.org)

@unknownuser said:

Returns a new array. In the first form, the new array is empty. In the second it is created with size copies of obj (that is, size references to the same obj). The third form creates a copy of the array passed as a parameter (the array is generated by calling to_ary on the parameter). In the last form, an array of the given size is created. Each element in this array is calculated by passing the element’s index to the given block and storing the return value

So it appears what happens is not that f.to_a is called, but instead Array.new( f ) - and Array.new uses .to_ary .

Dan Rathbun

@thomthom said:

So it appears what happens is not that f.to_a is called, but instead Array.new( f ) - and Array.new uses .to_ary .

Actually (if you use the CHM Ruby refdoc,) the source shows that the #| method explicitly calls:
%(#0000BF)[arg2 = to_ary(arg2);]
ie, it makes a copy of the second arg to use locally (within its body.)