Re: Using CL in Gnus

[sigurd@12move.de (Karl Pflästerer)]

On 27 Nov 2003, Reiner Steib <- 4.uce.03.r.s@nurfuerspam.de wrote:

> On Wed, Nov 26 2003, Karl Pflästerer wrote:

>> No they differ  more.  `mapc' can take an arbitrary number of sequences
>> as arguments `mapcar' only one.

> I don't doubt your explanations, but I don't understand it from the
> doc-strings (GNU Emacs 21.3):

> ,----[ C-h f mapc RET ]
> | mapc is a built-in function.
> | (mapc FUNCTION SEQUENCE)
> | 
> | Apply FUNCTION to each element of SEQUENCE for side effects only.
> | Unlike `mapcar', don't accumulate the results.  Return SEQUENCE.
> | SEQUENCE may be a list, a vector, a bool-vector, or a string.
> `----

> ,----[ C-h f mapcar RET ]
> | mapcar is a built-in function.
> | (mapcar FUNCTION SEQUENCE)
> | 
> | Apply FUNCTION to each element of SEQUENCE, and make a list of the results.
> | The result is a list just as long as SEQUENCE.
> | SEQUENCE may be a list, a vector, a bool-vector, or a string.
> `----

I tried it now with an Emacs I have here (only for testing it's a
precompiled version and does not really fit in my cygwin environment as
I couldn't compile an Emacs under Cygwin).

And here is another extremly silly difference between Emacs and XEmacs.
The XEmacs `mapc' can take an arbitrary number of lists as arguments as
the original CL `mapc'; the Emacs `mapc' can take only one list as
argument.  I can't understand why the function was written that way.
Someone who knows `mapc' from CL will know that it can work on multiple
lists (and in XEmacs it can).  I can't iagine why soeone would want to
create such a silly incompatibility.

>> Furthermore IMO if you read `mapc' you know a function is used only
>> for the side effects it has; on the other hand `mapcar' shouldn't be
>> used for side effects IMO.

> Thanks for your explanations.  Is this the case in the examples of
> mapc from gnus.el and gnus-agent.el?

Speaking og `gnus-method-to-server' in gnus.el: IMO instead of mapc or
mapcar (which would give the completely wrong intention here for a
reader since the loops are only for the side effects) `dolist' could be
used here.  That's in cl-macs.el so could be used with eval-when-compile
and for a reader is easier to see what happens (IMO).  The same is true
for gnus-agent.el

That leads me to another point of mine.  If we start No Gnus we should
look for a lot of places in the code where mapping over a list is
written like that:
        (let ((lst inputlist))
          (while lst
            (do-something-with-list lst)
            .
            .
            .
            (setq lst (cdr lst))))
To cdr-down a list is not uncommon (above code would be nicer as
recursive version) but `dolist' is IMO a much better alternative at such
places.

>> Better check for the Emacs version and provide a macro.  People have to
>> read code and the meaning of mapc is different from the one of mapcar.

>> [...]
>>> To make clear that the code in Gnus 5.10 _should_ not use mapc (beside

>> I don't understand your point here. 

> Well, if programmers see mapc at many places in Gnus code, they might
> be tempted to use it without thinking about possible alternatives

That might be true; but a programmer who doesn't think why he uses mapc
instead of mapcar or dolist at a special place should not provide code
for Gnus.

> (alternatives w/o using CL functions).  But -as we are in feature

CL functions are not evil.

> freeze- we will not have a significant amount of _new_ code.  Thus
> _if_ there is no disadvantage in using mapcar in gnus.el and
> gnus-agent.el (only 4 occurrences in total, see above) we _may_
> replace them instead of providing a "compatibility hack".

Think about dolist.  Also if syntactilly you may use mapcar at these
places, semantically it would be completely wrong.
Here a famous quote I like to make my point clear:
,----
| "Programs must be written for people to read, and only incidentally
| for machines to execute."
|                 -- Abelson & Sussman, SICP (preface to the first edition)
`----



   KP

-- 
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