expressions: add "length" function

Lists are somtimes used in configurations as replacement for tuples.
Providing length gives an easy way to detect usage errors.

1 files changed, 13 insertions, 0 deletions

diff --git a/test/buildtool/build_engine/expression/expression.test.cpp b/test/buildtool/build_engine/expression/expression.test.cpp
index 18189baf..83550ebd 100644
--- a/test/buildtool/build_engine/expression/expression.test.cpp
+++ b/test/buildtool/build_engine/expression/expression.test.cpp
@@ -1010,6 +1010,19 @@ TEST_CASE("Expression Evaluation", "[expression]") {  // NOLINT
         )"_json));
     }
 
+    SECTION("length expression") {
+        auto expr = Expression::FromJson(R"(
+          { "type": "length"
+          , "$1": ["foo", "bar", "baz"]
+          }
+        )"_json);
+        REQUIRE(expr);
+
+        auto result = expr.Evaluate(env, fcts);
+        REQUIRE(result);
+        CHECK(result == Expression::FromJson("3"_json));
+    }
+
     SECTION("keys expression") {
         auto expr = Expression::FromJson(R"(
             { "type": "keys"