Augment build-expression->derivation' with #:modules; add http-fetch'.

* guix/derivations.scm (imported-modules): New procedure.
  (build-expression->derivation): New keyword argument `modules'.
  Use `imported-modules' when MODULES is non-empty, and pass it with
  `-L' to GUILE.

* guix/build/http.scm, guix/http.scm, tests/builders.scm: New files.

tests/builders.scm

@@ -0,0 +1,50 @@
+;;; Guix --- Nix package management from Guile.         -*- coding: utf-8 -*-
+;;; Copyright (C) 2012 Ludovic Courtès <ludo@gnu.org>
+;;;
+;;; This file is part of Guix.
+;;;
+;;; Guix is free software; you can redistribute it and/or modify it
+;;; under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation; either version 3 of the License, or (at
+;;; your option) any later version.
+;;;
+;;; Guix is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with Guix.  If not, see <http://www.gnu.org/licenses/>.
+
+
+(define-module (test-builders)
+  #:use-module (guix http)
+  #:use-module (guix store)
+  #:use-module (guix utils)
+  #:use-module (guix derivations)
+  #:use-module (srfi srfi-1)
+  #:use-module (srfi srfi-64))
+
+;; Test the higher-level builders.
+
+(define %store
+  (false-if-exception (open-connection)))
+
+(test-begin "builders")
+
+(test-assert "http-fetch"
+  (let* ((url      "http://ftp.gnu.org/gnu/hello/hello-2.8.tar.gz")
+         (hash     (nix-base32-string->bytevector
+                    "0wqd8sjmxfskrflaxywc7gqw7sfawrfvdxd9skxawzfgyy0pzdz6"))
+         (drv-path (http-fetch %store url 'sha256 hash)))
+    (and (build-derivations %store (list drv-path))
+         (file-exists? (derivation-path->output-path drv-path)))))
+
+(test-end "builders")
+
+
+(exit (= (test-runner-fail-count (test-runner-current)) 0))
+
+;;; Local Variables:
+;;; eval: (put 'test-assert 'scheme-indent-function 1)
+;;; End: