1 files changed, 92 insertions, 0 deletions
diff --git a/src/mbgl/style/expression/let.cpp b/src/mbgl/style/expression/let.cpp
new file mode 100644
index 0000000000..561e8515d4
--- /dev/null
+++ b/src/mbgl/style/expression/let.cpp
@@ -0,0 +1,92 @@
+#include <mbgl/style/expression/let.hpp>
+#include <mbgl/style/conversion/get_json_type.hpp>
+#include <mbgl/util/string.hpp>
+
+namespace mbgl {
+namespace style {
+namespace expression {
+
+EvaluationResult Let::evaluate(const EvaluationContext& params) const {
+    return result->evaluate(params);
+}
+
+void Let::eachChild(const std::function<void(const Expression&)>& visit) const {
+    for (auto it = bindings.begin(); it != bindings.end(); it++) {
+        visit(*it->second);
+    }
+    visit(*result);
+}
+
+using namespace mbgl::style::conversion;
+
+ParseResult Let::parse(const Convertible& value, ParsingContext& ctx) {
+    assert(isArray(value));
+
+    std::size_t length = arrayLength(value);
+
+    if (length < 4) {
+        ctx.error("Expected at least 3 arguments, but found " + util::toString(length - 1) + " instead.");
+        return ParseResult();
+    }
+
+    std::map<std::string, std::shared_ptr<Expression>> bindings_;
+    for(std::size_t i = 1; i < length - 1; i += 2) {
+        optional<std::string> name = toString(arrayMember(value, i));
+        if (!name) {
+            ctx.error("Expected string, but found " + getJSONType(arrayMember(value, i)) + " instead.", i);
+            return ParseResult();
+        }
+        
+        bool isValidName = std::all_of(name->begin(), name->end(), [](unsigned char c) {
+            return std::isalnum(c) || c == '_';
+        });
+        if (!isValidName) {
+            ctx.error("Variable names must contain only alphanumeric characters or '_'.", 1);
+            return ParseResult();
+        }
+        
+        ParseResult bindingValue = ctx.parse(arrayMember(value, i + 1), i + 1);
+        if (!bindingValue) {
+            return ParseResult();
+        }
+        
+        bindings_.emplace(*name, std::move(*bindingValue));
+    }
+
+    ParseResult result_ = ctx.parse(arrayMember(value, length - 1), length - 1, ctx.getExpected(), bindings_);
+    if (!result_) {
+        return ParseResult();
+    }
+
+    return ParseResult(std::make_unique<Let>(std::move(bindings_), std::move(*result_)));
+}
+
+EvaluationResult Var::evaluate(const EvaluationContext& params) const {
+    return value->evaluate(params);
+}
+
+void Var::eachChild(const std::function<void(const Expression&)>&) const {}
+
+ParseResult Var::parse(const Convertible& value_, ParsingContext& ctx) {
+    assert(isArray(value_));
+
+    if (arrayLength(value_) != 2 || !toString(arrayMember(value_, 1))) {
+        ctx.error("'var' expression requires exactly one string literal argument.");
+        return ParseResult();
+    }
+
+    std::string name_ = *toString(arrayMember(value_, 1));
+
+    optional<std::shared_ptr<Expression>> bindingValue = ctx.getBinding(name_);
+    if (!bindingValue) {
+        ctx.error(R"(Unknown variable ")" + name_ +  R"(". Make sure ")" +
+            name_ + R"(" has been bound in an enclosing "let" expression before using it.)", 1);
+        return ParseResult();
+    }
+
+    return ParseResult(std::make_unique<Var>(name_, std::move(*bindingValue)));
+}
+
+} // namespace expression
+} // namespace style
+} // namespace mbgl