Exposing set-load-path and load-path-expression from Gexp.

[Roel Janssen]

Dear Guix,

To write a derivation builder that can turn a G-expression into a
derivation, it would be handy to expose two functions that are currently
not in the interface of (guix gexp).

Here's an example:

(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)))))))))

Without having set-load-path and load-path-expression, I think it would
be hard (and quite possibly an exact duplicate of the code inside (guix
gexp)) to set up a proper environment for the Guile code to run.

I would like to request exposing these two functions to the public
interface of (guix gexp).

WDYT?

Kind regards,
Roel Janssen