From mboxrd@z Thu Jan  1 00:00:00 1970
From: ludo@gnu.org (Ludovic =?utf-8?Q?Court=C3=A8s?=)
Subject: Re: [PATCH] gnu-maintenance: Improve 'official-gnu-packages';
	add the related procedures.
Date: Tue, 26 Mar 2013 21:50:57 +0100
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To: Nikita Karetnikov <nikita@karetnikov.org>
Cc: bug-guix@gnu.org

Nikita Karetnikov <nikita@karetnikov.org> skribis:

>> =E2=80=9CEval is evil=E2=80=9D, as lispers love to say.
>
> Yeah, I've heard this one.
>
>> It should never be used, unless there=E2=80=99s a very good reason to do=
 so.
>
> I've heard this too.  But people always fail to explain the "a very good
> reason" part.

There are many reasons, including: it=E2=80=99s hard to reason about code
generated at run time, it=E2=80=99s easy to generate invalid code, generated
code cannot be compiled & optimized, and evaluating code is much slower
than running compiled code.

>> In my unfinished binary substitute where a similar situation arises,
>> I=E2=80=99ve done this:
>
> [...]
>
>> And then, it is used like this:
>
>>          (alist->record properties
>>                         (cut %make-cache url <...>)
>>                         '("StoreDir" "WantMassQuery"))
>
> So, keys and values are strings here, right?

Keys are strings, and values can be anything.

> But I want to store setters as values.

The suggestion I made was in favor of using a single
=E2=80=98make-gnu-record-descriptor=E2=80=99 call with all the field values=
 (as opposed
to creating the record with all fields set to #f, and then using
=E2=80=98setters=E2=80=99 to change them to their actual value.)

In the example above, the end result is a single call to the
constructor, equivalent to:

  (%make-cache url
               (assoc-ref properties "StoreDir")
               (assoc-ref properties "WantMassQuery"))

HTH,
Ludo=E2=80=99.