Nikita Karetnikov <nik...@karetnikov.org> 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
pairs."
(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))))
(else
(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.
Thanks,
Ludo’.
