From: Markus <markus@...>
Date: 2005-05-24T21:28:29+09:00
Subject: Re: Array.=== in Rails Bug

On Mon, 2005-05-23 at 21:47, Jim Weirich wrote:

> FileList in Rake used to inherit from Array to get array-like behavior.  I 
> recently changed it to delegate to an internally held array.  I found that 
> the delegation allowed a FileList to be used in more places where an Array 
> was expected ... mainly because some methods check for the to_ary method to 
> see if an object should be treated like an array.

This is a much more thorough wrapping (see earlier on this thread).  It
fools everything (x.class, x.is_a?, etc.) but does not fool Class#===,
which produces incorrect behaviour.  

If you are going to produce something that walks like a duck, by all
means use a delegater; if you want something to _be_ a duck, you need to
make sure that Duck#=== accepts it as one.

> Given the following code:
> 
>  [FileList['a', 'b'], FileList['c', 'd']].flatten
> 
> Rake 0.5.0 which inherits from Array will return: []
> Rake 0.5.3 which delegates to Array will return: ['a', 'b', 'c', 'd']
> 
> That's because flatten accesses the array elements directly.  Since FileList 
> is lazy loaded, bypassing the methods means that the elements are never 
> loaded.  By switching to a delegation method, flatten checks for :to_ary, 
> which can be used to trigger loading and make everyone happy.

In this case. _all_ the methods are delegated, so it should work
correctly.

> Short version:  Inheriting from Array is not always the most compatible.
> 
> Another moral to the story:  If you want to check for something that is array 
> like (but doesn't have to be an array), check to see if it responds to 
> to_ary.

Agreed.  Here though, you could do all the checking you wanted and still
get caught out.  If the _only_ way to distinguish a surrogate from it's
intended class is a bug in the impersonation, it is a bug.  This is not
to say that they might have gotten away with something Array like, just
that if they are going to return true for x.is_a?(Array), and return
Array for x.class, then Array === x should return true as well.

--MarkusQ