Re: How to lower a record to the build code ?

[Edouard Klein]

Dear Christopher,

I can't thank you enough, your idea of "ungexp[ing] the record access
bits" is what got me out of the tarpit.

You can see the resulting code here, which will hopefully clear the
misunderstandings generated by my oversimplification of the minimal
nonworking example:
https://issues.guix.gnu.org/issue/48277

Thanks again !

Cheers,

Edouard.


Christopher Baines writes:

> edk@beaver-labs.com writes:
>
>> I've been stuck for a few days on the following:
>> Let's say I have a record type:
>>
>> (define-record-type* <my-record> my-record make-my-record
>>   my-record?
>>   this-record
>>  (first-field my-record-first-field)
>>  (second-field my-record-second-field))
>>
>> And a function that uses such a record, but needs to run on the build
>> side, because it also needs the store path of a package (I can't edit
>> this function):
>>
>> (define (function-of-a-record-and-a-build-time-path rec path)
>>   "Concatenate the path, first, and second field"
>>   (string-append path " " (my-record-first-field rec) " " (car
>>   (my-record-second-field rec)) " " (cdr (my-record-second-field rec))))
>>
>> How can I use this record in the build side. For example, I'm unable to
>> build the following G-exp:
>> (define a-record (my-record
>>               (first-field "first")
>>               (second-field '("second" . "third"))))
>>
>>
>> #~(with-output-to-file (string-append #$output "/file.txt")
>>      (lambda _
>>        (display (function-of-a-record-and-a-build-time-path #$a-record
>>      #$bash)))))
>
> Could you ungexp the record access bits? So something like:
>
>   (string-append #$path " " #$(my-record-first-field rec) " " #$(car
>     (my-record-second-field rec)) " " #$(cdr (my-record-second-field
>     rec)))
>
> Obviously, then the handling of rec would just be on the build side.
>
> I'm not quite sure quite what your code looks like, in the example you
> give, you've got a number of problems.
>
> Ignoring the #$a-record, you'll need to (mkdir #$output) before trying
> to write to a file within that directory.
>
> Secondly, function-of-a-record-and-a-build-time-path isn't defined on
> the build side.
>
> If you want to define function-of-a-record-and-a-build-time-path on the
> host side, then you could have it return a gexp, something like:
>
>   (define (function-of-a-record-and-a-build-time-path rec path)
>     "Concatenate the path, first, and second field"
>     #~(string-append #$path " " #$(my-record-first-field rec) " " #$(car 
> (my-record-second-field rec)) " " #$(cdr (my-record-second-field rec))))
>
>   (build-gexp
>    #~(begin
>        (mkdir #$output)
>        (with-output-to-file (string-append #$output "/file.txt")
>          (lambda _
>            (display #$(function-of-a-record-and-a-build-time-path  a-record 
> bash))))))
>
> With these changes, the example you give works for me.