scripts: build: Add --file option.

* guix/scripts/build.scm (show-help): Add help text for --file option.
  (%options): Add --file option.
  (options/resolve-packages): Handle 'file' options.
* tests/guix-build.sh: Add tests.
* doc/guix.texi ("invoking guix build"): Add doc.

doc/guix.texi

@@ -3615,6 +3615,19 @@ The @var{options} may be zero or more of the following:
 
 @table @code
 
+@item --file=@var{file}
+@itemx -f @var{file}
+
+Build the package or derivation that the code within @var{file}
+evaluates to.
+
+As an example, @var{file} might contain a package definition like this
+(@pxref{Defining Packages}):
+
+@example
+@verbatiminclude package-hello.scm
+@end example
+
 @item --expression=@var{expr}
 @itemx -e @var{expr}
 Build the package or derivation @var{expr} evaluates to.

guix/scripts/build.scm

@@ -290,6 +290,9 @@ Build the given PACKAGE-OR-DERIVATION and return their output paths.\n"))
   (display (_ "
   -e, --expression=EXPR  build the package or derivation EXPR evaluates to"))
   (display (_ "
+  -f, --file=FILE        build the package or derivation that the code within
+                         FILE evaluates to"))
+  (display (_ "
   -S, --source           build the packages' source derivations"))
   (display (_ "
       --sources[=TYPE]   build source derivations; TYPE may optionally be one
@@ -359,6 +362,9 @@ must be one of 'package', 'all', or 'transitive'~%")
          (option '(#\e "expression") #t #f
                  (lambda (opt name arg result)
                    (alist-cons 'expression arg result)))
+         (option '(#\f "file") #t #f
+                 (lambda (opt name arg result)
+                   (alist-cons 'file arg result)))
          (option '(#\n "dry-run") #f #f
                  (lambda (opt name arg result)
                    (alist-cons 'dry-run? #t result)))
@@ -422,29 +428,34 @@ packages."
   (define system
     (or (assoc-ref opts 'system) (%current-system)))
 
+  (define (object->argument obj)
+    (match obj
+      ((? package? p)
+       `(argument . ,p))
+      ((? procedure? proc)
+       (let ((drv (run-with-store store
+                    (mbegin %store-monad
+                      (set-guile-for-build (default-guile))
+                      (proc))
+                    #:system system)))
+         `(argument . ,drv)))
+      ((? gexp? gexp)
+       (let ((drv (run-with-store store
+                    (mbegin %store-monad
+                      (set-guile-for-build (default-guile))
+                      (gexp->derivation "gexp" gexp
+                                        #:system system)))))
+         `(argument . ,drv)))))
+
   (map (match-lambda
         (('argument . (? string? spec))
          (if (store-path? spec)
              `(argument . ,spec)
              `(argument . ,(specification->package spec))))
+        (('file . file)
+         (object->argument (load* file (make-user-module '()))))
         (('expression . str)
-         (match (read/eval str)
-           ((? package? p)
-            `(argument . ,p))
-           ((? procedure? proc)
-            (let ((drv (run-with-store store
-                         (mbegin %store-monad
-                           (set-guile-for-build (default-guile))
-                           (proc))
-                         #:system system)))
-              `(argument . ,drv)))
-           ((? gexp? gexp)
-            (let ((drv (run-with-store store
-                         (mbegin %store-monad
-                           (set-guile-for-build (default-guile))
-                           (gexp->derivation "gexp" gexp
-                                             #:system system)))))
-              `(argument . ,drv)))))
+         (object->argument (read/eval str)))
         (opt opt))
        opts))
 

tests/guix-build.sh

@@ -167,6 +167,33 @@ guix build -e "(begin
 guix build -e '#~(mkdir #$output)' -d
 guix build -e '#~(mkdir #$output)' -d | grep 'gexp\.drv'
 
+# Building from a package file.
+cat > "$module_dir/package.scm"<<EOF
+(use-modules (gnu))
+(use-package-modules bootstrap)
+
+%bootstrap-guile
+EOF
+guix build --file="$module_dir/package.scm"
+
+# Building from a monadic procedure file.
+cat > "$module_dir/proc.scm"<<EOF
+(use-modules (guix gexp))
+(lambda ()
+  (gexp->derivation "test"
+                    (gexp (mkdir (ungexp output)))))
+EOF
+guix build --file="$module_dir/proc.scm" --dry-run
+
+# Building from a gexp file.
+cat > "$module_dir/gexp.scm"<<EOF
+(use-modules (guix gexp))
+
+(gexp (mkdir (ungexp output)))
+EOF
+guix build --file="$module_dir/gexp.scm" -d
+guix build --file="$module_dir/gexp.scm" -d | grep 'gexp\.drv'
+
 # Using 'GUIX_BUILD_OPTIONS'.
 GUIX_BUILD_OPTIONS="--dry-run"
 export GUIX_BUILD_OPTIONS