store: Add 'store-lower'.

* guix/store.scm (store-lower): New procedure.
* tests/store.scm ("store-lower"): New test.

guix/store.scm

@@ -99,6 +99,7 @@ (define-module (guix store)
             store-bind
             store-return
             store-lift
+            store-lower
             run-with-store
             %guile-for-build
             text-file
@@ -881,6 +882,12 @@ (define result
                         (procedure-property proc 'documentation))
   result)
 
+(define (store-lower proc)
+  "Lower PROC, a monadic procedure in %STORE-MONAD, to a \"normal\" procedure
+taking the store as its first argument."
+  (lambda (store . args)
+    (run-with-store store (apply proc args))))
+
 ;;
 ;; Store monad operators.
 ;;

tests/store.scm

 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2012, 2013, 2014 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2012, 2013, 2014, 2015 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -595,6 +595,12 @@ (define (same? x y)
            (null? (valid-derivers %store file))
            (null? (referrers %store file))))))
 
+(test-equal "store-lower"
+  "Lowered."
+  (let* ((add  (store-lower text-file))
+         (file (add %store "foo" "Lowered.")))
+    (call-with-input-file file get-string-all)))
+
 (test-end "store")