Addressed PR feedback

3 files changed, 10 insertions, 6 deletions

diff --git a/include/mbgl/style/filter.hpp b/include/mbgl/style/filter.hpp
index 9347ae9381..00a8163667 100644
--- a/include/mbgl/style/filter.hpp
+++ b/include/mbgl/style/filter.hpp
@@ -236,7 +236,7 @@ public:
     
 class ExpressionFilter {
 public:
-    std::shared_ptr<expression::Expression> expression;
+    std::shared_ptr<const expression::Expression> expression;
     
     friend bool operator==(const ExpressionFilter& lhs, const ExpressionFilter& rhs) {
         return *(lhs.expression) == *(rhs.expression);
@@ -274,7 +274,7 @@ using FilterBase = variant<
 class Filter : public FilterBase {
 public:
     using FilterBase::FilterBase;
-    bool operator()(expression::EvaluationContext context) const;
+    bool operator()(const expression::EvaluationContext &context) const;
 };
 
 } // namespace style
diff --git a/src/mbgl/style/conversion/filter.cpp b/src/mbgl/style/conversion/filter.cpp
index 75f6fffbae..fba149da12 100644
--- a/src/mbgl/style/conversion/filter.cpp
+++ b/src/mbgl/style/conversion/filter.cpp
@@ -18,9 +18,13 @@ static bool isExpressionFilter(const Convertible& filter) {
     
     optional<std::string> op = toString(arrayMember(filter, 0));
     
-    if (*op == "has") {
-        auto operand = toString(arrayMember(filter, 1));
-        return arrayLength(filter) >= 2 && operand && *operand != "$id" && *operand != "$type";
+    if (!op) {
+        return false;
+
+    } else if (*op == "has") {
+        if (arrayLength(filter) < 2) return false;
+        optional<std::string> operand = toString(arrayMember(filter, 1));
+        return operand && *operand != "$id" && *operand != "$type";
         
     } else if (*op == "in" || *op == "!in" || *op == "!has" || *op == "none") {
         return false;
diff --git a/src/mbgl/style/filter.cpp b/src/mbgl/style/filter.cpp
index 99d91cbcb4..51aa6bcf82 100644
--- a/src/mbgl/style/filter.cpp
+++ b/src/mbgl/style/filter.cpp
@@ -5,7 +5,7 @@
 namespace mbgl {
 namespace style {
 
-bool Filter::operator()(expression::EvaluationContext context) const {
+bool Filter::operator()(const expression::EvaluationContext &context) const {
     return FilterBase::visit(*this, FilterEvaluator { context });
 }