* Dirty interim hack to make header file dependency determination

reliable (if somewhat inefficient): make the current time an
  attribute of the derivation.  Thus, every call to `nix-build' will
  cause the find-includes derivation to be re-done (but not the actual
  compilations if that's not necessary!).  I added a `curTime' primop
  to do this.

lib/default.nix

@@ -5,7 +5,12 @@ rec {
   compileC = {main, localIncludes ? []}: stdenv.mkDerivation {
     name = "compile-c";
     builder = ./compile-c.sh;
-    inherit main localIncludes;
+    localIncludes =
+      if localIncludes == "auto" then
+        import (findIncludes {main = toString main; hack = curTime;})
+      else
+        localIncludes;
+    inherit main;
   };
 
   /*
@@ -16,10 +21,10 @@ rec {
   };
   */
 
-  findIncludes = {main}: stdenv.mkDerivation {
+  findIncludes = {main, hack}: stdenv.mkDerivation {
     name = "find-includes";
     builder = ./find-includes.sh;
-    inherit main;
+    inherit main hack;
   };
   
   link = {objects, programName ? "program"}: stdenv.mkDerivation {

src/libexpr/eval.cc

@@ -28,6 +28,7 @@ EvalState::EvalState()
     addPrimOp0("true", primTrue);
     addPrimOp0("false", primFalse);
     addPrimOp0("null", primNull);
+    addPrimOp0("curTime", primCurTime);
 
     addPrimOp1("import", primImport);
     addPrimOp1("derivation", primDerivation);

src/libexpr/primops.cc

@@ -1,3 +1,5 @@
+#include <time.h>
+
 #include "primops.hh"
 #include "normalise.hh"
 #include "globals.hh"
@@ -321,3 +323,9 @@ Expr primIsNull(EvalState & state, Expr arg)
     ATMatcher m;
     return makeBool(atMatch(m, arg) >> "Null");
 }
+
+
+Expr primCurTime(EvalState & state)
+{
+    return ATmake("Int(<int>)", time(0));
+}

src/libexpr/primops.hh

@@ -34,5 +34,10 @@ Expr primNull(EvalState & state);
 /* Determine whether the argument is the null value. */
 Expr primIsNull(EvalState & state, Expr arg);
 
+/* Return the current time.  !!! hack, impure - although due to
+   memoization of evaluation results this should always yield the same
+   value for a particular run of the program. */
+Expr primCurTime(EvalState & state);
+
 
 #endif /* !__PRIMOPS_H */