* A new primop `builtins', which returns an attribute set containing

all the primops.  This allows Nix expressions to test for new
  primops and take appropriate action if they're not available.  For
  instance, rather than calling a primop `foo' directly, they could
  say `if builtins ? foo then builtins.foo ... else ...'.

src/libexpr/primops.cc

@@ -7,6 +7,30 @@
 #include "nixexpr-ast.hh"
 
 
+static Expr primBuiltins(EvalState & state, const ATermVector & args)
+{
+    /* Return an attribute set containing all primops.  This allows
+       Nix expressions to test for new primops and take appropriate
+       action if they're not available.  For instance, rather than
+       calling a primop `foo' directly, they could say `if builtins ?
+       foo then builtins.foo ... else ...'. */
+
+    ATermMap builtins(128);
+
+    for (ATermMap::const_iterator i = state.primOps.begin();
+         i != state.primOps.end(); ++i)
+    {
+        string name = aterm2String(i->key);
+        if (string(name, 0, 2) == "__")
+            name = string(name, 2);
+        /* !!! should use makePrimOp here, I guess. */
+        builtins.set(toATerm(name), makeAttrRHS(makeVar(i->key), makeNoPos()));
+    }
+
+    return makeAttrs(builtins);
+}
+
+
 /* Load and evaluate an expression from path specified by the
    argument. */ 
 static Expr primImport(EvalState & state, const ATermVector & args)
@@ -660,6 +684,8 @@ static Expr primRelativise(EvalState & state, const ATermVector & args)
 
 void EvalState::addPrimOps()
 {
+    addPrimOp("builtins", 0, primBuiltins);
+        
     addPrimOp("true", 0, primTrue);
     addPrimOp("false", 0, primFalse);
     addPrimOp("null", 0, primNull);

tests/lang/eval-okay-builtins.exp

@@ -0,0 +1 @@
+Path("/foo")

tests/lang/eval-okay-builtins.nix

@@ -0,0 +1,12 @@
+assert builtins ? currentSystem;
+assert !builtins ? __currentSystem;
+
+let {
+
+  x = if builtins ? dirOf then builtins.dirOf /foo/bar else "";
+
+  y = if builtins ? fnord then builtins.fnord "foo" else "";
+
+  body = x + y;
+  
+}