bug#26023: [PATCH] gexp: Expose functions to allow creating derivation builders.

[Ludovic Courtès]

Roel Janssen <address@hidden> skribis:

> Ludovic Courtès writes:
>
>> Hello!
>>
>> Roel Janssen <address@hidden> skribis:
>>
>>> I would like to add the following two functions to the public interface
>>> of the gexp module.
>>>
>>> This makes writing custom derivation builders easier for external
>>> projects.
>>
>> Out of curiosity, how do you use them?
>>
>> I’m asking because they’re not too bad but not too nice an interface
>> either, especially ‘load-path-expression’, and I’d expect the use cases
>> to be covered by ‘gexp->script’ & ‘gexp->file’.
>>
>> Ludo’.
>
> Here's how I use it:
>
> (define* (process->bash-engine-derivation proc #:key (guile (default-guile)))
>   "Return an executable script that runs the PROCEDURE described in PROC, with
> PROCEDURE's imported modules in its search path."
>   (let ((name (process-full-name proc))
>         (exp (process-procedure proc))
>         (out (process-output-path proc)))
>     (let ((out-str (if out (format #f "(setenv \"out\" ~s)" out) "")))
>       (mlet %store-monad ((set-load-path
>                            (load-path-expression (gexp-modules exp))))
>         (gexp->derivation
>          name
>          (gexp
>           (call-with-output-file (ungexp output)
>             (lambda (port)
>               (use-modules (ice-9 pretty-print))
>               (format port "#!~a/bin/bash~%" (ungexp bash))
>               ;; Now that we've written all of the shell code,
>               ;; We can start writing the Scheme code.
>               ;; We rely on Bash for this to work.
>               (format port "read -d '' CODE <<EOF~%")
>               ;; The destination can be outside of the store.
>               ;; TODO: We have to mount this location when building inside
>               ;; a container.
>               (format port "~a" (ungexp out-str))
>               (format port
>                       "~%;; Code to create a proper Guile environment.~%~a~%"
>                       (with-output-to-string
>                         (lambda _ (pretty-print '(ungexp set-load-path)))))
>               (format port ";; Actual code from the procedure.~%~a~%"
>                       (with-output-to-string
>                         (lambda _ (pretty-print '(ungexp exp)))))
>               (format port "EOF~%")
>               (format port "~a/bin/guile -c \"$CODE\"~%" (ungexp guile))
>               (chmod port #o555)))))))))
>
> If you have a suggestion on how to avoid using them, I'd love to hear it.

In theory you could put the Scheme code in a separate file (created with
‘gexp->script’), or avoid Bash altogether, but perhaps none of these is
an option?

Cheers,
Ludo’.