1 files changed, 60 insertions, 0 deletions
diff --git a/src/mbgl/style/expression/check_subtype.cpp b/src/mbgl/style/expression/check_subtype.cpp
new file mode 100644
index 0000000000..04a1643f0c
--- /dev/null
+++ b/src/mbgl/style/expression/check_subtype.cpp
@@ -0,0 +1,60 @@
+#include <string>
+#include <mbgl/style/expression/check_subtype.hpp>
+
+namespace mbgl {
+namespace style {
+namespace expression {
+namespace type {
+
+std::string errorMessage(const Type& expected, const Type& t) {
+    return {"Expected " + toString(expected) + " but found " + toString(t) + " instead."};
+}
+
+optional<std::string> checkSubtype(const Type& expected, const Type& t) {
+    if (t.is<ErrorType>()) return {};
+    
+    optional<std::string> result = expected.match(
+        [&] (const Array& expectedArray) -> optional<std::string> {
+            if (!t.is<Array>()) { return {errorMessage(expected, t)}; }
+            const auto& actualArray = t.get<Array>();
+            const auto err = checkSubtype(expectedArray.itemType, actualArray.itemType);
+            if (err) return { errorMessage(expected, t) };
+            if (expectedArray.N && expectedArray.N != actualArray.N) return { errorMessage(expected, t) };
+            return {};
+        },
+        [&] (const ValueType&) -> optional<std::string> {
+            if (t.is<ValueType>()) return {};
+            
+            const Type members[] = {
+                Null,
+                Boolean,
+                Number,
+                String,
+                Object,
+                Color,
+                Array(Value)
+            };
+            
+            for (const auto& member : members) {
+                const auto err = checkSubtype(member, t);
+                if (!err) {
+                    return {};
+                }
+            }
+            return { errorMessage(expected, t) };
+        },
+        [&] (const auto&) -> optional<std::string> {
+            if (expected != t) {
+                return { errorMessage(expected, t) };
+            }
+            return {};
+        }
+    );
+    
+    return result;
+}
+
+} // namespace type
+} // namespace expression
+} // namespace style
+} // namespace mbgl