[pro] why :key arguments?

[Marco Antoniotti]

On Jul 5, 2011, at 11:51 , Tamas Papp wrote:

> I am very happy to learn about these things.  Currently I am working
> on the algorithms and my main concern is to ensure correctness; speed
> is secondary at this point, but even though I am not optimizing, I
> want to keep my code optimizable later on.
> 
> My problem with the key argument is that it complicates the interface.  I 
> would like to use the same interface for sample statistics and random 
> variables, eg currently in CL-NUM-UTILS and CL-RANDOM I have
> 
> (mean #(1d0 2d0 3d0)) ; => 2, a sample mean
> (mean (r-normal 2 1)) ; => 2d0, mean of a univariate normal distribution
> 
> If I had a :KEY argument, I would have to check that it is EQ to
> #'identity or not provided in methods for random variables.

But this is exactly where compiler macros can help.  With the &key argument you keep a consistent (and useful) interface.  The check whether to do away with a possible IDENTITY can be done in an appropriate compiler-macro.

> APPLY is not a major concern for me at the moment, all of these
> functions have a fixed number of arguments (usually one or two).  So
> compiler macros still look attractive: I guess I could just write them
> for the function I define (eg MAP1), with the understanding that if
> the user wants speed, he should stick to mapping with this function.
> 
> I also thought of the following possibility using runtime dispatch: 
> 
> (defstruct (w/key (:constructor w/key (key object)))
>  key object)
> 
> (defgeneric mean (object)
>  (:method ((obj w/key))
>    (mean-w/key (w/key-object obj) (w/key-key obj)))
>  (:method ((obj sequence))
>    (/ (reduce #'+ obj) (length obj))))
> 
> (defmethod mean-w/key ((obj sequence) key)
>  (/ (reduce #'+ obj :key key) (length obj)))
> 
> (mean #(1 2 3))                ; => 2
> (mean (w/key #'1+ #(1 2 3)))   ; => 3

You can have your cake and eat it too.  Why limit yourself?

Cheers
--
Marco