gexp: Add 'with-imported-modules' macro.

* guix/gexp.scm (<gexp>)[modules]: New field.
(gexp-modules): New procedure.
(gexp->derivation): Use it and append the result to %MODULES.
Update docstring to mark #:modules as deprecated.
(current-imported-modules, with-imported-modules): New macros.
(gexp): Pass CURRENT-IMPORTED-MODULES as second argument to 'gexp'.
(gexp->script): Use and honor 'gexp-modules'; define '%modules'.
* tests/gexp.scm ("gexp->derivation & with-imported-modules")
("gexp->derivation & nested with-imported-modules")
("gexp-modules & ungexp", "gexp-modules & ungexp-splicing"):
New tests.
("program-file"): Use 'with-imported-modules'.  Remove #:modules
argument to 'program-file'.
* doc/guix.texi (G-Expressions): Document 'with-imported-modules'.
Mark #:modules of 'gexp->derivation' as deprecated.
* emacs/guix-devel.el: Add syntax for 'with-imported-modules'.
(guix-devel-keywords): Add it.
* .dir-locals.el: Likewise.

1 files changed, 37 insertions, 1 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index c9d9bd8977..b315325034 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -3697,6 +3697,30 @@ In the example above, the native build of @var{coreutils} is used, so
 that @command{ln} can actually run on the host; but then the
 cross-compiled build of @var{emacs} is referenced.
 
+@cindex imported modules, for gexps
+@findex with-imported-modules
+Another gexp feature is @dfn{imported modules}: sometimes you want to be
+able to use certain Guile modules from the ``host environment'' in the
+gexp, so those modules should be imported in the ``build environment''.
+The @code{with-imported-modules} form allows you to express that:
+
+@example
+(let ((build (with-imported-modules '((guix build utils))
+               #~(begin
+                   (use-modules (guix build utils))
+                   (mkdir-p (string-append #$output "/bin"))))))
+  (gexp->derivation "empty-dir"
+                    #~(begin
+                        #$build
+                        (display "success!\n")
+                        #t)))
+@end example
+
+@noindent
+In this example, the @code{(guix build utils)} module is automatically
+pulled into the isolated build environment of our gexp, such that
+@code{(use-modules (guix build utils))} works as expected.
+
 The syntactic form to construct gexps is summarized below.
 
 @deffn {Scheme Syntax} #~@var{exp}
@@ -3756,6 +3780,16 @@ G-expressions created by @code{gexp} or @code{#~} are run-time objects
 of the @code{gexp?} type (see below.)
 @end deffn
 
+@deffn {Scheme Syntax} with-imported-modules @var{modules} @var{body}@dots{}
+Mark the gexps defined in @var{body}@dots{} as requiring @var{modules}
+in their execution environment.  @var{modules} must be a list of Guile
+module names, such as @code{'((guix build utils) (guix build gremlin))}.
+
+This form has @emph{lexical} scope: it has an effect on the gexps
+directly defined in @var{body}@dots{}, but not on those defined, say, in
+procedures called from @var{body}@dots{}.
+@end deffn
+
 @deffn {Scheme Procedure} gexp? @var{obj}
 Return @code{#t} if @var{obj} is a G-expression.
 @end deffn
@@ -3781,7 +3815,9 @@ stored in a file called @var{script-name}.  When @var{target} is true,
 it is used as the cross-compilation target triplet for packages referred
 to by @var{exp}.
 
-Make @var{modules} available in the evaluation context of @var{exp};
+@var{modules} is deprecated in favor of @code{with-imported-modules}.
+Its meaning is to
+make @var{modules} available in the evaluation context of @var{exp};
 @var{modules} is a list of names of Guile modules searched in
 @var{module-path} to be copied in the store, compiled, and made available in
 the load path during the execution of @var{exp}---e.g., @code{((guix