Drop wrapper `{expression:}` object.

4 files changed, 70 insertions, 8 deletions

diff --git a/cmake/core-files.cmake b/cmake/core-files.cmake
index 206a1c8d25..cd66fccef3 100644
--- a/cmake/core-files.cmake
+++ b/cmake/core-files.cmake
@@ -406,6 +406,7 @@ set(MBGL_CORE_FILES
     include/mbgl/style/expression/curve.hpp
     include/mbgl/style/expression/expression.hpp
     include/mbgl/style/expression/is_constant.hpp
+    include/mbgl/style/expression/is_expression.hpp
     include/mbgl/style/expression/let.hpp
     include/mbgl/style/expression/literal.hpp
     include/mbgl/style/expression/match.hpp
@@ -423,6 +424,7 @@ set(MBGL_CORE_FILES
     src/mbgl/style/expression/compound_expression.cpp
     src/mbgl/style/expression/curve.cpp
     src/mbgl/style/expression/is_constant.cpp
+    src/mbgl/style/expression/is_expression.cpp
     src/mbgl/style/expression/let.cpp
     src/mbgl/style/expression/literal.cpp
     src/mbgl/style/expression/match.cpp
diff --git a/include/mbgl/style/conversion/data_driven_property_value.hpp b/include/mbgl/style/conversion/data_driven_property_value.hpp
index aac09aa343..79ea34979e 100644
--- a/include/mbgl/style/conversion/data_driven_property_value.hpp
+++ b/include/mbgl/style/conversion/data_driven_property_value.hpp
@@ -1,10 +1,12 @@
 #pragma once
 
+#include <unordered_set>
 #include <mbgl/style/data_driven_property_value.hpp>
 #include <mbgl/style/conversion.hpp>
 #include <mbgl/style/conversion/constant.hpp>
 #include <mbgl/style/conversion/function.hpp>
 #include <mbgl/style/conversion/expression.hpp>
+#include <mbgl/style/expression/is_expression.hpp>
 #include <mbgl/style/expression/curve.hpp>
 #include <mbgl/style/expression/is_constant.hpp>
 
@@ -14,18 +16,13 @@ namespace conversion {
 
 template <class T>
 struct Converter<DataDrivenPropertyValue<T>> {
+
     optional<DataDrivenPropertyValue<T>> operator()(const Convertible& value, Error& error) const {
         if (isUndefined(value)) {
             return DataDrivenPropertyValue<T>();
-        } else if (!isObject(value)) {
-            optional<T> constant = convert<T>(value, error);
-            if (!constant) {
-                return {};
-            }
-            return DataDrivenPropertyValue<T>(*constant);
-        } else if (objectMember(value, "expression")) {
+        } else if (expression::isExpression(value)) {
             optional<std::unique_ptr<Expression>> expression = convert<std::unique_ptr<Expression>>(
-                *objectMember(value, "expression"),
+                value,
                 error,
                 valueTypeToExpressionType<T>());
             
@@ -44,6 +41,12 @@ struct Converter<DataDrivenPropertyValue<T>> {
             
                 return DataDrivenPropertyValue<T>(CompositeFunction<T>(std::move(*expression)));
             }
+        } else if (!isObject(value)) {
+            optional<T> constant = convert<T>(value, error);
+            if (!constant) {
+                return {};
+            }
+            return DataDrivenPropertyValue<T>(*constant);
         } else if (!objectMember(value, "property")) {
             optional<CameraFunction<T>> function = convert<CameraFunction<T>>(value, error);
             if (!function) {
diff --git a/include/mbgl/style/expression/is_expression.hpp b/include/mbgl/style/expression/is_expression.hpp
new file mode 100644
index 0000000000..7cb2b03863
--- /dev/null
+++ b/include/mbgl/style/expression/is_expression.hpp
@@ -0,0 +1,14 @@
+#pragma once
+#include <unordered_set>
+#include <mbgl/style/expression/expression.hpp>
+#include <mbgl/style/expression/compound_expression.hpp>
+
+namespace mbgl {
+namespace style {
+namespace expression {
+
+bool isExpression(const conversion::Convertible& value);
+
+} // namespace expression
+} // namespace style
+} // namespace mbgl
diff --git a/src/mbgl/style/expression/is_expression.cpp b/src/mbgl/style/expression/is_expression.cpp
new file mode 100644
index 0000000000..1b70c7a096
--- /dev/null
+++ b/src/mbgl/style/expression/is_expression.cpp
@@ -0,0 +1,43 @@
+#include <mbgl/style/expression/is_expression.hpp>
+#include <mbgl/style/conversion.hpp>
+
+namespace mbgl {
+namespace style {
+namespace expression {
+
+bool isExpression(const conversion::Convertible& value) {
+    using namespace mbgl::style::conversion;
+    
+    static std::unordered_set<std::string> specialForms = {
+        "literal",
+        "match",
+        "curve",
+        "coalesce",
+        "array",
+        "let",
+        "var",
+        "at",
+        "string",
+        "number",
+        "color",
+        "boolean",
+        "to-string",
+        "to-number",
+        "to-color",
+        "to-boolean",
+        "any",
+        "all"
+    };
+
+    if (!isArray(value) || arrayLength(value) == 0) return false;
+    optional<std::string> name = toString(arrayMember(value, 0));
+    if (!name) return false;
+    
+    return (specialForms.find(*name) != specialForms.end()) ||
+        (CompoundExpressionRegistry::definitions.find(*name) != CompoundExpressionRegistry::definitions.end());
+}
+
+
+} // namespace expression
+} // namespace style
+} // namespace mbgl