adding lib function composableDerivation

providing easy setups of flags and configurations for derivations.
configuration is passed via passthru automatically,
additional names can be merged with final attrs in an automatic sensible way
even supporting fix.
fun and funMerge idea by Michael Raskin

svn path=/nixpkgs/trunk/; revision=13546

1 files changed, 19 insertions, 2 deletions

diff --git a/pkgs/lib/default.nix b/pkgs/lib/default.nix
index 01aecfeae8cd..445dcd8ffd0a 100644
--- a/pkgs/lib/default.nix
+++ b/pkgs/lib/default.nix
@@ -43,7 +43,24 @@ rec {
     let arg=(merger init (defaultMergeArg init x)); in  
       # now add the function with composed args already applied to the final attrs
     setAttrMerge "passthru" {} (f arg) ( x : x // { function = foldArgs merger f arg; } );
- 
+
+  # returns f x // { passthru.fun = y : f (merge x y); } while preserving other passthru names.
+  # example: let ex = applyAndFun (x : removeAttrs x ["fixed"])  (mergeOrApply mergeAttr) {name = 6;};
+  #              usage1 = ex.passthru.fun { name = 7; };    # result: { name = 7;}
+  #              usage2 = ex.passthru.fun (a: a // {name = __add a.name 1; }); # result: { a = 7; }
+  # fix usage:
+  #              usage3a = ex.passthru.fun (a: a // {name2 = a.fixed.toBePassed; }); # usage3a will fail because toBePassed is not yet given
+  #              usage3b usage3a.passthru.fun { toBePassed = "foo";}; # result { name = 7; name2 = "foo"; toBePassed = "foo"; fixed = <this attrs>; }
+  applyAndFun = f : merge : x : assert (__isAttrs x || __isFunction x);
+    let takeFix = if (__isFunction x) then x else (attr: merge attr x); in
+    setAttrMerge "passthru" {} (fix (fixed : f (takeFix {inherit fixed;})))
+      ( y : y //
+        {
+          fun = z : applyAndFun f merge (fixed: merge (takeFix fixed) z);
+          funMerge = z : applyAndFun f merge (fixed: let e = takeFix fixed; in e // merge e z);
+        } );
+  mergeOrApply = merge : x : y : if (__isFunction y) then  y x else merge x y;
+
   # rec { # an example of how composedArgsAndFun can be used
   #  a  = composedArgsAndFun (x : x) { a = ["2"]; meta = { d = "bar";}; };
   #  # meta.d will be lost ! It's your task to preserve it (eg using a merge function)
@@ -770,7 +787,7 @@ rec {
                   else throw "assertion of flag ${a} of derivation ${args.name} failed"
                ) args2.flags );
     in removeAttrs
-      (fold (mergeOrApply mergeAttrByFunc) {} ([args] ++ opts))
+      (mergeAttrsByFunc ([args] ++ opts))
       ["flags" "cfg" "mergeAttrBy" "fixed" ]; # fixed may be passed as fix argument or such
   # supportFlag functions for convinience
   sFlagEnable = { name, buildInputs ? [], propagatedBuildInputs ? [] } : {