Re: [Scheme-reports] Seeking review of sets and hash tables proposals

[Per Bothner <per@x>]

On 05/25/2013 02:06 PM, Vassil Nikolov wrote:
>
>
> Per Bothner <per@x> wrote:
>
>> On 05/25/2013 01:50 PM, Vassil Nikolov wrote:
>>>
>>> Per Bothner <per@x> wrote:
>>>
>>>> ...
>>>> a bag of T is just
>>>> a minor optimization of a map (hash-table) from T to integers.
>>>
>>>     Except when equality only depends on
>>>     parts of the elements.
>>
>> I don't understand this comment.
>
>    If a bag's notion of element equality only
>    takes into account a part of each element
>    and ignores the rest of the element,
>    keeping a count instead of the elements
>    themselves would lose information.

I don't the think the bag API supports this.  I.e. using
a bag will also lose information, unless the bag API is
clearly specified to support this use case.  And I don't
believe it does.

Both bad and maps may support element equality which only
takes into account a part of each element and ignores the
rest of the element,  But it's not clear to me the bag
API has a way to iterate over all of the "equivalent but
distinct" API.  I guess it depends on what bag-for-each does.

To support this use-case you could use a map that maps
T to list-of-T.  So there is still no strong need for a
separate bag API.
-- 
	--Per Bothner
per@x   http://per.bothner.com/

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