Re: Using CL in Gnus

[Reiner Steib <4.uce.03.r.s@nurfuerspam.de>]

On Wed, Nov 26 2003, Karl Pflästerer wrote:

> On 26 Nov 2003, Reiner Steib <- 4.uce.03.r.s@nurfuerspam.de wrote:
>
>> Do I understand the docs correctly, that mapc and mapcar _only_ differ
>> in the result they give, but not in the side effects?
>
> 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.
`----

> 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?

>> check if we could use mapcar instead of mapc in gnus.el and
>> gnus-agent?  IMHO we should replace them, if there's no disadvantage.
>
> 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
(alternatives w/o using CL functions).  But -as we are in feature
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".

Maybe it's better if I cease this thread, because I'm sure you and
Katsumi know much more about this (not being ironic at all here!) and
he did the right thing.

Bye, Reiner.
-- 
       ,,,
      (o o)
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