* 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).

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);

tests/lang/eval-okay-flatten.exp

@@ -0,0 +1 @@
+Str("1234567")

tests/lang/eval-okay-flatten.nix

@@ -0,0 +1,19 @@
+let {
+
+  fold = op: nul: list:
+    if list == []
+    then nul
+    else op (builtins.head list) (fold op nul (builtins.tail list));
+
+  concat =
+    fold (x: y: x + y) "";
+    
+  flatten = x:
+    if builtins.isList x
+    then fold (x: y: (flatten x) ++ y) [] x
+    else [x];
+
+  l = ["1" "2" ["3" ["4"] ["5" "6"]] "7"];
+
+  body = concat (flatten l);
+}