3 files changed, 68 insertions, 7 deletions
diff --git a/src/mbgl/style/expression/compound_expression.cpp b/src/mbgl/style/expression/compound_expression.cpp
index 86d968c521..b1c2d74a9f 100644
--- a/src/mbgl/style/expression/compound_expression.cpp
+++ b/src/mbgl/style/expression/compound_expression.cpp
@@ -271,13 +271,6 @@ std::unordered_map<std::string, CompoundExpressionRegistry::Definition> initiali
         return object.at(key);
     });
     
-    define("length", [](const std::vector<Value>& arr) -> Result<double> {
-        return arr.size();
-    });
-    define("length", [] (const std::string s) -> Result<double> {
-        return s.size();
-    });
-    
     define("properties", [](const EvaluationContext& params) -> Result<std::unordered_map<std::string, Value>> {
         if (!params.feature) {
             return EvaluationError {
diff --git a/src/mbgl/style/expression/length.cpp b/src/mbgl/style/expression/length.cpp
new file mode 100644
index 0000000000..258353ae4e
--- /dev/null
+++ b/src/mbgl/style/expression/length.cpp
@@ -0,0 +1,66 @@
+#include <mbgl/style/expression/length.hpp>
+#include <mbgl/util/string.hpp>
+
+namespace mbgl {
+namespace style {
+namespace expression {
+
+Length::Length(std::unique_ptr<Expression> input_)
+    : Expression(type::Number),
+      input(std::move(input_)) {
+}
+
+EvaluationResult Length::evaluate(const EvaluationContext& params) const {
+    const EvaluationResult value = input->evaluate(params);
+    if (!value) return value;
+    return value->match(
+        [] (const std::string& s) {
+            return EvaluationResult { double(s.size()) };
+        },
+        [] (const std::vector<Value>& v) {
+            return EvaluationResult { double(v.size()) };
+        },
+        [&] (const auto&) -> EvaluationResult {
+            return EvaluationError { "Expected value to be of type string or array, but found " + toString(typeOf(*value)) + " instead." };
+        });
+}
+
+void Length::eachChild(const std::function<void(const Expression&)>& visit) const {
+    visit(*input);
+}
+
+bool Length::operator==(const Expression& e) const {
+    if (auto eq = dynamic_cast<const Length*>(&e)) {
+        return *eq->input == *input;
+    }
+    return false;
+}
+
+std::vector<optional<Value>> Length::possibleOutputs() const {
+    return { nullopt };
+}
+
+using namespace mbgl::style::conversion;
+ParseResult Length::parse(const Convertible& value, ParsingContext& ctx) {
+    std::size_t length = arrayLength(value);
+
+    if (length != 2) {
+        ctx.error("Expected one argument, but found " + util::toString(length) + " instead.");
+        return ParseResult();
+    }
+
+    ParseResult input = ctx.parse(arrayMember(value, 1), 1);
+    if (!input) return ParseResult();
+
+    type::Type type = (*input)->getType();
+    if (!type.is<type::Array>() && !type.is<type::StringType>() && !type.is<type::ValueType>()) {
+        ctx.error("Expected argument of type string or array, but found " + toString(type) + " instead.");
+        return ParseResult();
+    }
+
+    return ParseResult(std::make_unique<Length>(std::move(*input)));
+}
+
+} // namespace expression
+} // namespace style
+} // namespace mbgl
diff --git a/src/mbgl/style/expression/parsing_context.cpp b/src/mbgl/style/expression/parsing_context.cpp
index 880772d926..364c3f740a 100644
--- a/src/mbgl/style/expression/parsing_context.cpp
+++ b/src/mbgl/style/expression/parsing_context.cpp
@@ -15,6 +15,7 @@
 #include <mbgl/style/expression/compound_expression.hpp>
 #include <mbgl/style/expression/equals.hpp>
 #include <mbgl/style/expression/interpolate.hpp>
+#include <mbgl/style/expression/length.hpp>
 #include <mbgl/style/expression/let.hpp>
 #include <mbgl/style/expression/literal.hpp>
 #include <mbgl/style/expression/match.hpp>
@@ -89,6 +90,7 @@ const ExpressionRegistry& getExpressionRegistry() {
         {"case", Case::parse},
         {"coalesce", Coalesce::parse},
         {"interpolate", parseInterpolate},
+        {"length", Length::parse},
         {"let", Let::parse},
         {"literal", Literal::parse},
         {"match", parseMatch},