[core] Implement Expression::serialize()

Issue #10714
- Each expression stores its operator as a string, and default serialization is [operator, serialize(child1), ...]
- Custom implementations of `serialize` for Expression types that don't follow the pattern
- expression::Value -> mbgl::Value converter
- node_expression bindings to expose `serialize`

1 files changed, 11 insertions, 0 deletions

diff --git a/include/mbgl/style/expression/expression.hpp b/include/mbgl/style/expression/expression.hpp
index cf9fa0cb21..c41ac0b5f1 100644
--- a/include/mbgl/style/expression/expression.hpp
+++ b/include/mbgl/style/expression/expression.hpp
@@ -135,6 +135,17 @@ public:
      * complete set of outputs is statically undecidable.
      */
     virtual std::vector<optional<Value>> possibleOutputs() const = 0;
+    
+    virtual mbgl::Value serialize() const {
+        std::vector<mbgl::Value> serialized;
+        serialized.emplace_back(getOperator());
+        eachChild([&](const Expression &child) {
+            serialized.emplace_back(child.serialize());
+        });
+        return serialized;
+    };
+    
+    virtual std::string getOperator() const = 0;
 
 protected:
     template <typename T>