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Re: guix-package --roll-back

From: Ludovic Courtès
Subject: Re: guix-package --roll-back
Date: Fri, 11 Jan 2013 14:39:59 +0100
User-agent: Gnus/5.130005 (Ma Gnus v0.5) Emacs/24.2 (gnu/linux)


Nikita Karetnikov <address@hidden> skribis:

> This one doesn't work at all:
> (option '("foo") #f #t
>         (lambda (opt name arg result)
>           (alist-cons 'foo arg result)))

It actually does, but it has no side effect.

‘args-fold’ uses a common functional programming pattern, whereby the
functions called when an option is encountered just /contribute/ to the
resulting value.

Here’s an example:

--8<---------------cut here---------------start------------->8---
scheme@(guile-user)> (use-modules(srfi srfi-37))
scheme@(guile-user)> (define options
                       (list (option '("foo") #f #t
                               (lambda (opt name arg result)
                                  (if arg
                                      (cons arg result)
scheme@(guile-user)> (args-fold '("--foo") options (const #f) (const #f) 
$19 = nothing
scheme@(guile-user)> (args-fold '("--foo=42") options (const #f) (const #f) 
$20 = ("42" something)
scheme@(guile-user)> (args-fold '("--foo=42" "--foo=3") options (const #f) 
(const #f) '(something))
$21 = ("3" "42" something)
scheme@(guile-user)> (args-fold '("--foo=42" "--foo=3" "--foo") options (const 
#f) (const #f) '(something))
$22 = nothing
--8<---------------cut here---------------end--------------->8---

In guix-package, ‘parse-options’ returns an alist (list of pairs) that
indicates the actions to be performed, etc.  IOW, ‘args-fold’ is a tool
to build “translators” from command-line options to an internal
representation of those options.

> Actually, the above helped me to understand that we want '--roll-back'
> to behave differently than '--foo'.  '--roll-back' shouldn't accept any
> options at all, but it should somehow get the argument of '--profile'.
> How can I do it?

When ‘--profile’ is passed, the result of ‘parse-options’ contains a
pair whose key is ‘profile’.  The (assoc-ref opts 'profile) calls that
you see retrieve the argument given to ‘--profile’.


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