On Fri, 20 Oct 2000, Dan Winkler wrote:

> Thank you.
> 
> That's interesting that collect: returns different classes of
> collections based on the class of the source collection.  When would
> you want that behavior?

Er..all the time? #collect, #select, etc. return collection that is
"like" the receiver. Think of them as being like filter and map in Scheme.
 
> I see in this particular method, the next line allocates and Array
> explicitly.  Is there a reason we want to let collect: decide the
> class of collection in the first line but use an Array in particular
> in the second line?

Dunno, what method are you looking at? Offhand, it's a little hard to
tell. I don't see any *harm* in using #collect: to initialize the Array in
the first case. As I wrote, maybe you'll want to modify it in the future.

You can't use Array class>>#new:withAll: because you'd end up with only
one OrderedCollectoin, which is clearly not what you want. You could have
a #new:withAllValuesFrom: or something, that took a block as it's second
arg, but I'm not seeing a huge gain here.
 
> 	allText _ pages collect: [:pg | OrderedCollection new].
> 	allTextUrls _ Array new: pages size.
> 
> Would you say we should change the second line to:
> 
>         allTextUrls _ pages collect: [:pg | nil].

No that's silly, since all slots are niled by default. I *might* say

	 allTextUrls _ pages species new: pages size

*if* I thought that keeping all the colletions the same type were
important. For all I know, there's a reason to have allTextUrls be an
Array.

> As an old C programmer I'd be tempted to go the other way and force them both
> to be Arrays for compactness since I know they're not going to grow.

Do you? Is the compactness worth it?

Even if so, I'm still inclined to prefer the #collect: to any more verbose
version. I mean, what's the *clarity* gain?


	 allText _ pages species new: pages size.
	"or, allText _ Array new: pages size."
	"Gah! I imediately want to do a collect :))"
	1 to: pages size do:  [:i | allText at: i do: OrderedCollection new].


I *never* see this. So, I would *definitely say it's a Smalltalk
idiom. Frankly, I don't see a bit of clarity gained, and much lost.

After all, it's not like one is *confused* by the collect into thinking
it's somehow attending to the pages. It's perfectly *clear* what it's
doing and why (to initialize allText).

In the end, when it gets down to it, my rewrite is basically the
implementation of #collect:!

	| newCollection |
	newCollection _ self species new: self size.
	1 to: self size do:
		[:index |
		newCollection at: index put: (aBlock value: (self 
			at: index))].
	^ newCollection

Why not reuse it? (Follow the once and only once rule ;))

Cheers,
Bijan Parsia.