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Re: [PATCH] gnu-maintenance: Improve 'official-gnu-packages'; add the re

From: Ludovic Courtès
Subject: Re: [PATCH] gnu-maintenance: Improve 'official-gnu-packages'; add the related procedures.
Date: Fri, 22 Mar 2013 13:19:25 +0100
User-agent: Gnus/5.130005 (Ma Gnus v0.5) Emacs/24.2 (gnu/linux)

Nikita Karetnikov <address@hidden> skribis:

> But it doesn't work.  These lines rise an error:
> +                 (cons (gnu-package-descriptor
> +                        (inherit (first state))
> +                        ((eval (match-field str)
> +                               (interaction-environment)) str))
> (There may be other problems.  For instance, it should remove fields'
> names from 'str' before creating a record.)
> What do you think about the 'eval' idea?  It's used to avoid unnecessary
> repetition.

“Eval is evil”, as lispers love to say.  It should never be used, unless
there’s a very good reason to do so.

In my unfinished binary substitute where a similar situation arises,
I’ve done this:

  (define (fields->alist port)
    "Read recutils-style record from PORT and return them as a list of key/value
    (define field-rx
      (make-regexp "^([[:graph:]]+): (.*)$"))

    (let loop ((line   (read-line port))
               (result '()))
      (cond ((eof-object? line)
             (reverse result))
            ((regexp-exec field-rx line)
             (lambda (match)
               (cons (match:substring match 1)
                     (match:substring match 2))))
             (error "unmatched line" line)))))

  (define (alist->record alist make keys)
    "Apply MAKE to the values associated with KEYS in ALIST."
    (let ((args (map (cut assoc-ref alist <>) keys)))
      (apply make args)))

And then, it is used like this:

         (alist->record properties
                        (cut %make-cache url <...>)
                        '("StoreDir" "WantMassQuery"))

To summarize, the input port containing a list of key/value pairs (like
yours) is first read by ‘fields->alist’, which returns a list of
key/value pairs.

Then ‘alist->record’ converts that alist into a record.  In this
example, it calls ‘%make-cache’ in a way equivalent to:

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

Here ‘%make-cache’ is the ‘normal’ SRFI-9 constructor, which is called
‘make-gnu-package-descriptor’ in your code.

Let me know if you need more details.


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