guix build: Allow gexps to be passed to '-e'.

* guix/ui.scm (%guix-user-module): New variable.
  (read/eval): Pass it as the second argument to 'eval'.
* guix/scripts/build.scm (options/resolve-packages): Add case for
  'gexp?'.
* tests/guix-build.sh: Add tests.
* doc/guix.texi (Invoking guix build): Document '-e gexp'.
guxi build: Allow gexps to be passed to '-e'.

* guix/ui.scm (%guix-user-module): New variable.
  (read/eval): Pass it as the second argument to 'eval'.
* guix/scripts/build.scm (options/resolve-packages): Add case for
  'gexp?'.
* tests/guix-build.sh: Add tests.
* doc/guix.texi (Invoking guix build): Document '-e gexp'.

doc/guix.texi

@@ -2244,7 +2244,11 @@ For example, @var{expr} may be @code{(@@ (gnu packages guile)
 guile-1.8)}, which unambiguously designates this specific variant of
 version 1.8 of Guile.
 
-Alternately, @var{expr} may refer to a zero-argument monadic procedure
+Alternately, @var{expr} may be a G-expression, in which case it is used
+as a build program passed to @code{gexp->derivation}
+(@pxref{G-Expressions}).
+
+Lastly, @var{expr} may refer to a zero-argument monadic procedure
 (@pxref{The Store Monad}).  The procedure must return a derivation as a
 monadic value, which is then passed through @code{run-with-store}.
 

guix/scripts/build.scm

@@ -24,6 +24,7 @@ (define-module (guix scripts build)
   #:use-module (guix packages)
   #:use-module (guix utils)
   #:use-module (guix monads)
+  #:use-module (guix gexp)
   #:use-module (ice-9 format)
   #:use-module (ice-9 match)
   #:use-module (ice-9 vlist)
@@ -338,6 +339,11 @@ (define system
             `(argument . ,p))
            ((? procedure? proc)
             (let ((drv (run-with-store store (proc) #:system system)))
+              `(argument . ,drv)))
+           ((? gexp? gexp)
+            (let ((drv (run-with-store store
+                         (gexp->derivation "gexp" gexp
+                                           #:system system))))
               `(argument . ,drv)))))
         (opt opt))
        opts))

guix/ui.scm

@@ -238,6 +238,14 @@ (define (call-with-error-handling thunk)
         (leave (_ "~a: ~a~%") proc
                (apply format #f format-string format-args))))))
 
+(define %guix-user-module
+  ;; Module in which user expressions are evaluated.
+  (let ((module (make-module)))
+    (beautify-user-module! module)
+    ;; Use (guix gexp) so that one can use #~ & co.
+    (module-use! module (resolve-interface '(guix gexp)))
+    module))
+
 (define (read/eval str)
   "Read and evaluate STR, raising an error if something goes wrong."
   (let ((exp (catch #t
@@ -248,7 +256,7 @@ (define (read/eval str)
                         str args)))))
     (catch #t
       (lambda ()
-        (eval exp the-root-module))
+        (eval exp %guix-user-module))
       (lambda args
         (leave (_ "failed to evaluate expression `~a': ~s~%")
                exp args)))))

tests/guix-build.sh

@@ -80,3 +80,7 @@ guix build -e "(begin
                    (gexp->derivation \"test\"
                                      (gexp (mkdir (ungexp output))))))" \
    --dry-run
+
+# Running a gexp.
+guix build -e '#~(mkdir #$output)' -d
+guix build -e '#~(mkdir #$output)' -d | grep 'gexp\.drv'