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

[Roel Janssen]

Ludovic Courtès writes:

> 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?

No, it's meant to build runnable scripts that can be run by a job
scheduling system (Sun Grid Engine).  It's configured to just run a bash
script (yes, really bash, not just a shell script) with some additional
syntax.  (For the sake of the example, I gave a shorter version that
"only" generates the regular bash script.

Perhaps we can come up with a better interface for the gexp module to
handle these cases.  I will give this some thought.

Kind regards,
Roel Janssen