Re: [Ocaml-lib-devel] Map with set operations over the keys

[fva <fv...@ts...>]

Hello


Christophe Raffalli wrote:

>Nicolas Cannasse wrote:
>  
>
>>>>After a second though I think it's little overweighted to make the user
>>>>specify a merge function. Why not simply specify that in such cases, the
>>>>item of the first map is selected ? That seems more reasonable.
>>>>        
>>>>
I like the idea of the merge function parameter... This makes sense when 
you do not rely on the identity to provide for equality of 
representations in the base set.

>>>No, you can for instance implement multiset as map to natural ... and
>>>the merge function is then addition.
>>>
>>>Your merge function should even raise the exception Remove when you want
>>>to say that the result should be removed from the map ... (For insance,
>>>if you have Map to integer and want to say that 0 should be removed from
>>>the map).
>>>      
>>>
>>I would say it's bad API design to use a "merge" function in order to remove
>>elements.
>>A more specific function should be provided.
>>    
>>
>
>I do not think so. When your merge gives accidentally 0, and the value
>should be removed, you do not want to scan the map once more to remove
>the zero ...
>  
>

In my opinion, this depends on the representation (i.e. eventually the 
semantics):

1) If you are representing the *support* i.e. those elements *not* 
mapping to the null element (whatever that means) you should remove 
those elements the merge operationg of which map to null

2) If you are representing the whole map/set/many-valued set whatever in 
its entirety, you should keep the merged value, whatever the result.

But again, this is just an opinion...

Regards,

    F. Valverde