* Added a builtin function `isList' to test whether a value is a list.

  With this primitive, a list-flattening function can be implemented
  (NIX-55, example is in tests/lang/eval-okay-flatten.nix).

1 files changed, 9 insertions, 0 deletions

diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index b05082293fb7..190d587330d1 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -521,6 +521,14 @@ static Expr primIsNull(EvalState & state, const ATermVector & args)
 }
 
 
+/* Determine whether the argument is a list. */
+static Expr primIsList(EvalState & state, const ATermVector & args)
+{
+    ATermList list;
+    return makeBool(matchList(evalExpr(state, args[0]), list));
+}
+
+
 static Path findDependency(Path dir, string dep)
 {
     if (dep[0] == '/') throw EvalError(
@@ -782,6 +790,7 @@ void EvalState::addPrimOps()
     addPrimOp("__toXML", 1, primToXML);
     addPrimOp("__toFile", 1, primToFile);
     addPrimOp("isNull", 1, primIsNull);
+    addPrimOp("__isList", 1, primIsList);
     addPrimOp("dependencyClosure", 1, primDependencyClosure);
     addPrimOp("abort", 1, primAbort);
     addPrimOp("__head", 1, primHead);