* 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 ...'.

3 files changed, 39 insertions, 0 deletions

diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index dfc565b4317e..bc4db2d81306 100644
--- a/src/libexpr/primops.cc
+++ b/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);
diff --git a/tests/lang/eval-okay-builtins.exp b/tests/lang/eval-okay-builtins.exp
new file mode 100644
index 000000000000..f4f3ba81a101
--- /dev/null
+++ b/tests/lang/eval-okay-builtins.exp
@@ -0,0 +1 @@
+Path("/foo")
diff --git a/tests/lang/eval-okay-builtins.nix b/tests/lang/eval-okay-builtins.nix
new file mode 100644
index 000000000000..e9d65e88a817
--- /dev/null
+++ b/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;
+  
+}