store: Add monadic access to '%current-system'.

* guix/store.scm (current-system, set-current-system): New procedures.
* tests/store.scm ("current-system"): New test.

2 files changed, 25 insertions, 2 deletions

diff --git a/guix/store.scm b/guix/store.scm
index 3c4d1c0058..8123407816 100644
--- a/guix/store.scm
+++ b/guix/store.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2012, 2013, 2014, 2015 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2012, 2013, 2014, 2015, 2016 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -118,6 +118,8 @@
             store-lower
             run-with-store
             %guile-for-build
+            current-system
+            set-current-system
             text-file
             interned-file
 
@@ -1040,6 +1042,18 @@ permission bits are kept."
 (define set-build-options*
   (store-lift set-build-options))
 
+(define-inlinable (current-system)
+  ;; Consult the %CURRENT-SYSTEM fluid at bind time.  This is equivalent to
+  ;; (lift0 %current-system %store-monad), but inlinable, thus avoiding
+  ;; closure allocation in some cases.
+  (lambda (state)
+    (values (%current-system) state)))
+
+(define-inlinable (set-current-system system)
+  ;; Set the %CURRENT-SYSTEM fluid at bind time.
+  (lambda (state)
+    (values (%current-system system) state)))
+
 (define %guile-for-build
   ;; The derivation of the Guile to be used within the build environment,
   ;; when using 'gexp->derivation' and co.
diff --git a/tests/store.scm b/tests/store.scm
index 394c06bc0f..9d651ce5a9 100644
--- a/tests/store.scm
+++ b/tests/store.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2012, 2013, 2014, 2015 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2012, 2013, 2014, 2015, 2016 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -837,6 +837,15 @@
          (file (add %store "foo" "Lowered.")))
     (call-with-input-file file get-string-all)))
 
+(test-equal "current-system"
+  "bar"
+  (parameterize ((%current-system "frob"))
+    (run-with-store %store
+      (mbegin %store-monad
+        (set-current-system "bar")
+        (current-system))
+      #:system "foo")))
+
 (test-assert "query-path-info"
   (let* ((ref (add-text-to-store %store "ref" "foo"))
          (item (add-text-to-store %store "item" "bar" (list ref)))