[core] Simplify Value conversions

Use a single template function for each of {to,from}ExpressionValue, always dispatching to ValueConverter.

Remove unused ValueConverter::ExpressionType typedefs.

1 files changed, 23 insertions, 41 deletions

diff --git a/include/mbgl/style/expression/value.hpp b/include/mbgl/style/expression/value.hpp
index fc38c36ff0..b1126118a9 100644
--- a/include/mbgl/style/expression/value.hpp
+++ b/include/mbgl/style/expression/value.hpp
@@ -60,64 +60,39 @@ type::Type valueTypeToExpressionType();
   Conversions between style value types and expression::Value
 */
 
-// no-op overloads
-Value toExpressionValue(const Value&);
-
-// T = Value (just wrap in optional)
-template <typename T>
-std::enable_if_t<std::is_same<T, Value>::value,
-optional<T>> fromExpressionValue(const Value& v)
-{
-    return optional<T>(v);
-}
-
-// T = member type of Value
-template <typename T>
-std::enable_if_t< std::is_convertible<T, Value>::value && !std::is_same<T, Value>::value,
-optional<T>> fromExpressionValue(const Value& v)
-{
-    return v.template is<T>() ? v.template get<T>() : optional<T>();
-}
-
-// real conversions
-template <typename T, typename Enable = std::enable_if_t< !std::is_convertible<T, Value>::value >>
-Value toExpressionValue(const T& value);
-
-template <typename T>
-std::enable_if_t< !std::is_convertible<T, Value>::value,
-optional<T>> fromExpressionValue(const Value& v);
-
-
-
 template <class T, class Enable = void>
 struct ValueConverter {
-    using ExpressionType = T;
-    
     static Value toExpressionValue(const T& value) {
         return Value(value);
     }
+
     static optional<T> fromExpressionValue(const Value& value) {
         return value.template is<T>() ? value.template get<T>() : optional<T>();
     }
 };
 
 template <>
-struct ValueConverter<float> {
-    using ExpressionType = double;
-    static type::Type expressionType() { return type::Number; }
-    static Value toExpressionValue(const float value);
-    static optional<float> fromExpressionValue(const Value& value);
+struct ValueConverter<Value> {
+    static type::Type expressionType() { return type::Value; }
+    static Value toExpressionValue(const Value& value) { return value; }
+    static optional<Value> fromExpressionValue(const Value& value) { return value; }
 };
 
-template<>
+template <>
 struct ValueConverter<mbgl::Value> {
     static Value toExpressionValue(const mbgl::Value& value);
     static mbgl::Value fromExpressionValue(const Value& value);
 };
 
+template <>
+struct ValueConverter<float> {
+    static type::Type expressionType() { return type::Number; }
+    static Value toExpressionValue(const float value);
+    static optional<float> fromExpressionValue(const Value& value);
+};
+
 template <typename T, std::size_t N>
 struct ValueConverter<std::array<T, N>> {
-    using ExpressionType = std::vector<Value>;
     static type::Type expressionType() {
         return type::Array(valueTypeToExpressionType<T>(), N);
     }
@@ -127,7 +102,6 @@ struct ValueConverter<std::array<T, N>> {
 
 template <typename T>
 struct ValueConverter<std::vector<T>> {
-    using ExpressionType = std::vector<Value>;
     static type::Type expressionType() {
         return type::Array(valueTypeToExpressionType<T>());
     }
@@ -137,7 +111,6 @@ struct ValueConverter<std::vector<T>> {
 
 template <>
 struct ValueConverter<Position> {
-    using ExpressionType = std::vector<Value>;
     static type::Type expressionType() { return type::Array(type::Number, 3); }
     static Value toExpressionValue(const mbgl::style::Position& value);
     static optional<Position> fromExpressionValue(const Value& v);
@@ -145,13 +118,22 @@ struct ValueConverter<Position> {
 
 template <typename T>
 struct ValueConverter<T, std::enable_if_t< std::is_enum<T>::value >> {
-    using ExpressionType = std::string;
     static type::Type expressionType() { return type::String; }
     static Value toExpressionValue(const T& value);
     static optional<T> fromExpressionValue(const Value& value);
 };
 
 template <typename T>
+Value toExpressionValue(const T& value) {
+    return ValueConverter<T>::toExpressionValue(value);
+}
+
+template <typename T>
+optional<T> fromExpressionValue(const Value& value) {
+    return ValueConverter<T>::fromExpressionValue(value);
+}
+
+template <typename T>
 std::vector<optional<T>> fromExpressionValues(const std::vector<optional<Value>>& values) {
     std::vector<optional<T>> result;
     for (const auto& value : values) {