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From: Andy Wingo <wingo@igalia.com>
Subject: Re: [PATCH] gnu: Add Kerberos client service.
Date: Wed, 30 Nov 2016 14:52:37 +0100
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In-Reply-To: <874m2pktc2.fsf@gnu.org> ("Ludovic =?utf-8?Q?Court=C3=A8s=22'?=
	=?utf-8?Q?s?= message of "Wed, 30 Nov 2016 14:09:17 +0100")
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To: Ludovic =?utf-8?Q?Court=C3=A8s?= <ludo@gnu.org>
Cc: guix-devel@gnu.org, John Darrington <jmd@gnu.org>

On Wed 30 Nov 2016 14:09, ludo@gnu.org (Ludovic Court=C3=A8s) writes:

>>  (define (validate-configuration config fields)
>>    (for-each (lambda (field)
>>                (let ((val ((configuration-field-getter field) config)))
>> -                (unless ((configuration-field-predicate field) val)
>> +                (unless (or (not val) ((configuration-field-predicate f=
ield) val))
>>                    (configuration-field-error
>>                     (configuration-field-name field) val))))
>
> Here you=E2=80=99re assuming that when VAL is #f, it=E2=80=99s necessary =
invalid, an
> assumption that=E2=80=99s questionable and wasn=E2=80=99t made until now.
>
> Can you instead change your own field predicate to do that?

Agreed; the usual way to do this is to define the default value as a
sentinel value that your field predicate rejects.  E.g.

  (define unset-field (list 'unset-field))

You'd make the default value be `unset-field' (by reference).  Then
assuming you defined a field of type "foo" then assuming you have an
associated predicate `foo?', you can do

  (define (predicate/not-unset pred)
    (lambda (x) (and (not (eq? x unset-field)) (pred x))))

  (define foo? (predicate/not-unset foo?))

Andy