[pro] why :key arguments?

[Tamas Papp]

On Mon, 04 Jul 2011 14:20:32 +0400, Stas Boukarev wrote:

> 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.

Can you (or someone) please elaborate on that?  I have just started 
reading up on compiler macros, and I don't understand why.

Thanks,

Tamas