expressions: add "length" function

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

3 files changed, 26 insertions, 0 deletions

diff --git a/doc/concepts/expressions.md b/doc/concepts/expressions.md
index 80378191..a14cd60f 100644
--- a/doc/concepts/expressions.md
+++ b/doc/concepts/expressions.md
@@ -256,6 +256,9 @@ those) argument(s) to obtain the final result.
 - `"reverse"` The argument has to be a list. The result is a new list
   with the entries in reverse order.
 
+- `"length"` The argument has to be a list. The result is the length
+  of the list.
+
  - `"++"` The argument has to be a list of lists. The result is
    the concatenation of those lists.
 
diff --git a/src/buildtool/build_engine/expression/evaluator.cpp b/src/buildtool/build_engine/expression/evaluator.cpp
index f91e5b13..11299bb5 100644
--- a/src/buildtool/build_engine/expression/evaluator.cpp
+++ b/src/buildtool/build_engine/expression/evaluator.cpp
@@ -232,6 +232,15 @@ auto Reverse(ExpressionPtr const& expr) -> ExpressionPtr {
     return ExpressionPtr{reverse_result};
 }
 
+auto Length(ExpressionPtr const& expr) -> ExpressionPtr {
+    if (not expr->IsList()) {
+        throw Evaluator::EvaluationError{fmt::format(
+            "length expects list but instead got: {}.", expr->ToString())};
+    }
+    return ExpressionPtr{
+        static_cast<Expression::number_t>(expr->List().size())};
+}
+
 auto NubRight(ExpressionPtr const& expr) -> ExpressionPtr {
     if (not expr->IsList()) {
         throw Evaluator::EvaluationError{fmt::format(
@@ -1098,6 +1107,7 @@ auto const kBuiltInFunctions =
                           {"enumerate", UnaryExpr(Enumerate)},
                           {"set", UnaryExpr(Set)},
                           {"reverse", UnaryExpr(Reverse)},
+                          {"length", UnaryExpr(Length)},
                           {"values", UnaryExpr(Values)},
                           {"lookup", LookupExpr},
                           {"[]", ArrayAccessExpr},
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"