[pro] why :key arguments?

[Stas Boukarev]

Tamas Papp <tkpapp at gmail.com> writes:

> On Mon, 04 Jul 2011 11:39:39 +0200, Hans Hübner wrote:
>
>> On Mon, Jul 4, 2011 at 11:31 AM, Tamas Papp
>> <tkpapp at gmail.com> wrote:
>>> Why do some CL library functions have :key arguments?
>> [...]
>>> but it is a bit cumbersome.  I can make my code simpler by relying on
>>> calls like
>>>
>>> (quantiles (map 'vector key vector) quantiles)
>> 
>> This not only conses "a bit more", it also duplicates traversal efforts
>> - The original list must be traversed, and the consed-up list of key
>> values as well.  I think it is prudent that the CL library functions
>> offer ways to reduce consing for cases where "a bit" is too much (and "a
>> bit" can become a lot if a program operates on long lists).
>
> I understand this.  My main question is: why not do this with compiler 
> macros?  Is there any reason for this, other than historical?
Because it's not easy to do with compiler macros.

> Note that I am not complaining about the standard, I just want to learn 
> the reason for this design choice so that I can take it into account when 
> writing my own libraries.
I find (foo sequence :key #'key) much nicer than
(foo (map 'sequence #'key sequence))

-- 
With best regards, Stas.