* Move `modifyDerivation' from build-support/vm to lib and rename it

to `overrideDerivation'.

svn path=/nixpkgs/trunk/; revision=18466

pkgs/build-support/vm/default.nix

@@ -252,15 +252,6 @@ rec {
   '';
 
 
-  modifyDerivation = f: attrs:
-    let attrsCleaned = removeAttrs attrs ["meta" "passthru" "outPath" "drvPath"];
-        newDrv = derivation (attrsCleaned // (f attrs));
-    in newDrv //
-      { meta = if attrs ? meta then attrs.meta else {};
-        passthru = if attrs ? passthru then attrs.passthru else {};
-      };
-
-
   /* Run a derivation in a Linux virtual machine (using Qemu/KVM).  By
      default, there is no disk image; the root filesystem is a tmpfs,
      and /nix/store is shared with the host (via the CIFS protocol to
@@ -282,7 +273,7 @@ rec {
      `run-vm' will be left behind in the temporary build directory
      that allows you to boot into the VM and debug it interactively. */
      
-  runInLinuxVM = modifyDerivation (attrs: {
+  runInLinuxVM = drv: lib.overrideDerivation drv (attrs: {
     builder = "${bash}/bin/sh";
     args = ["-e" (vmRunCommand qemuCommandLinux)];
     origArgs = attrs.args;
@@ -317,7 +308,7 @@ rec {
      - Reboot to shutdown the machine (because Qemu doesn't seem
        capable of a APM/ACPI VM shutdown).
   */
-  runInGenericVM = modifyDerivation (attrs: {
+  runInGenericVM = drv: lib.overrideDerivation drv (attrs: {
     system = "i686-linux";
     builder = "${bash}/bin/sh";
     args = ["-e" (vmRunCommand qemuCommandGeneric)];

pkgs/lib/attrsets.nix

@@ -13,7 +13,7 @@ rec {
 
   /* Return an attribute from nested attribute sets.  For instance
      ["x" "y"] applied to some set e returns e.x.y, if it exists.  The
-     default value is returned otherwise.  */
+     default value is returned otherwise. */
   attrByPath = attrPath: default: e:
     let attr = head attrPath;
     in

pkgs/lib/customisation.nix

@@ -0,0 +1,40 @@
+{
+
+
+  /* `overrideDerivation drv f' takes a derivation (i.e., the result
+     of a call to the builtin function `derivation') and returns a new
+     derivation in which the attributes of the original are overriden
+     according to the function `f'.  This function is called with the
+     original derivation attributes.
+
+     `overrideDerivation' allows certain "ad-hoc" customisation
+     scenarios (e.g. in ~/.nixpkgs/config.nix).  For instance, if you
+     want to "patch" the derivation returned by a package function in
+     Nixpkgs to build another version than what the function itself
+     provides, you can do something like this:
+
+       mySed = overrideDerivation pkgs.gnused (oldAttrs: {
+         name = "sed-4.2.2-pre";
+         src = fetchurl {
+           url = ftp://alpha.gnu.org/gnu/sed/sed-4.2.2-pre.tar.bz2;
+           sha256 = "11nq06d131y4wmf3drm0yk502d2xc6n5qy82cg88rb9nqd2lj41k";
+         };
+         patches = [];
+       });
+
+     For another application, see build-support/vm, where this
+     function is used to build arbitrary derivations inside a QEMU
+     virtual machine. */
+     
+  overrideDerivation = drv: f:
+    let
+      # Filter out special attributes.
+      attrs = removeAttrs drv ["meta" "passthru" "outPath" "drvPath"];
+      newDrv = derivation (attrs // (f drv));
+    in newDrv //
+      { meta = if drv ? meta then drv.meta else {};
+        passthru = if drv ? passthru then drv.passthru else {};
+      };
+
+
+}

pkgs/lib/default.nix

@@ -15,6 +15,7 @@ let
   misc = import ./misc.nix;
   maintainers = import ./maintainers.nix;
   platforms = import ./platforms.nix;
+  customisation = import ./customisation.nix;
 
 in
   { inherit trivial lists strings stringsWithDeps attrsets sources options
@@ -24,3 +25,4 @@ in
   # commonly used functions.
   // trivial // lists // strings // stringsWithDeps // attrsets // sources
   // properties // options // types // meta // debug // misc // modules
+  // customisation