I've put some of the proposed composition features into

SInce it's Squeak based, it uses <=>, but never mind, that's the same functions.

Stephane: the original comments were from Travis Griggs.

It's more a tutorial for using the SortFunction than a detailed explanation of implementation.

The original idea is that oSortFunction are composable.

For example, it's possible to chain sorting on a first criterion, then resort on a second criterion if objects have same rank with first.

For this, we need a to distinguish when objects have same rank (=) and cannot use a binary result (Boolean) like legacy sortBlock,

So we rather need a ternary comparison (<=>).

This thread is about extending composition, and generalizing implementation by composition (like Xtreams)

- for sorting nil first (or last)

- for reversing the order

- for sorting properties with any collation order, and not just default <=>

2017-11-05 17:52 GMT+01:00 Stephane Ducasse <stepharo.self@gmail.com>:

Hi guys

Do you have some nice comments somewhere? because I do not understand
anything about this thread.
I check the code and the comments are well... unclear and confusing.

Stef

On Sun, Nov 5, 2017 at 4:15 PM, Nicolas Cellier
<nicolas.cellier.aka.nice@gmail.com> wrote:
>
>
> 2017-11-05 16:06 GMT+01:00 Denis Kudriashov <dionisiydk@gmail.com>:
>>
>>
>> 2017-11-05 11:33 GMT+01:00 Nicolas Cellier
>> <nicolas.cellier.aka.nice@gmail.com>:
>>>
>>>
>>> Ah, I messed up, UndefinedSorter must not be chained, it must be a
>>> wrapper!
>>> Otherwise comparing to nil will resort to sorting by properties and
>>> fail...
>>>
>>>�� SortFunction>>undefinedFirst
>>>�� ^UndefinedSorter descending wrap: self
>>>
>>>�� UndefinedSorter>> collate: value1 with: value2
>>>�� "sort all nil according to the direction (first if -1, last if
>>> +1), then"
>>>�� value1 ifNil: [value2 ifNil: [^0] ifNotNil: [^direction]].
>>>�� value2 ifNil: [^direction negated].
>>>�� ^sorterForNonNil collate: value1 with: value2
>>>
>>> It's important to have the UndefinedSorter :
>>>�� - decoupled from property sort, because it can be generally usefull
>>>�� - collating 2 nil as 0, so that another property can be chained
>>
>>
>> I like your idea.
>> It also forced me to think that direction itself should be implemented as
>> wrapper. I would name it InvertedSortFunction:
>>
>> InvertedSortFunction>>collate: value1 with: value2
>>�� ^(actualSortFunction collate: value1 with: value2) * -1
>>
>>
>> If we will do it then direction will be not part of SortFunction. And all
>> current functions will be in fact ascending.
>> And to explicitly reflect this fact I would introduce
>> AscendingSortFunction as their superclass.
>> InvertedSortFunction and ChainedSortFunction will stay subclasses of
>> SortFunction.
>>
>> So what you think?
>
>
> Yes, I was thinking the same.
> On another hand, direction makes thing symmetric and has its elegance too.
> What I don't like with it is that it forces the library to have two
> different messages for the same thing:
> - collate:with: in base class accounting for direction
> - threeWayCompare:with: in every subclass
>
> The fact to hardcode the direction in base class could be seen as an
> optimization too.
> I'm not sure what Sista optimization could bring in this case, because the
> selectors may get megamorphic...
>
>
>>
>>>
>>>
>>> In
>>>
>>>�� people sortBy: #name ascending undefinedFirst , #age descending
>>>
>>> we could then have people with name nil still sorted by age, what is not
>>> possible with current implementation
>>>
>>
>