From: Rick DeNatale <rick.denatale@...>
Date: 2008-07-02T00:28:53+09:00
Subject: Re: Hash#select returns an array but Hash#reject returns a hash...

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On Tue, Jul 1, 2008 at 10:18 AM, David A. Black <dblack@rubypal.com> wrote:

> Hi --
>
>
> On Tue, 1 Jul 2008, Rick DeNatale wrote:
>
>
>> I'm having a hard time making the connection between this analogy and the
>> methods.
>>
>> In the first case one could say that we end up with two lines, the
>> 'original' one and the line of rejects.
>>
>> But first of all, x.reject leaves x alone so the original 'line' is
>> unchanged.
>>
>> And why can't we see your select example exactly the same way, except with
>> the resultant line of 'selects' just closer to you.
>>
>>
> It's specifically not about the exact objects, though. As I said,
> "same line" in this model doesn't map to "same object". Rather, the
> analogy has to do with types of structure or container.
>
> Here's another, perhaps better analogy. You've got a shelf of books,
> and you've got an empty box. You select some books from the shelf, and
> put them in the box. Now you've got two structures: a shelf, and a
> box.
>
> If you were describing this operation, you'd say: "I selected some
> books and put them in a box." The box of books is the result of the
> operation, and is structurally different from the shelf. The point is
> not that this is the only way it can be done, but that it *can* be
> done this way. It's reasonable to select from a collection into
> something that is not only a different collection, but a different
> container type.
>
> Now, if you do it reject-wise, you take the books you don't want and
> throw them out the window. The box never enters into it. You're not
> selecting, so you don't care about observing or measuring the books
> that you're removing from the shelf.
>

I still have the same problem with this analogy.

reject and select are not about the books.  They don't move books from one
collection to another.  They BOTH return a new collection which references
some of the objects referenced by the receiver.

select returns a collection of all of the objects in the receiver which meet
the criterion specified by the block, reject returns a collection of all the
objects in the receiver which DON'T meet the criterion specified by the
block.

The fact that Ruby <1.9 tends to return arrays rather than something of the
same species as the receiver on such methods was something I had to get used
to after my Smalltalk experience.

And, compatibility aside, I think that 1.9 is going in the right direction
here.

-- 
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/

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