4 files changed, 64 insertions, 9 deletions
diff --git a/include/mbgl/style/expression/length.hpp b/include/mbgl/style/expression/length.hpp
new file mode 100644
index 0000000000..1d754f1932
--- /dev/null
+++ b/include/mbgl/style/expression/length.hpp
@@ -0,0 +1,32 @@
+#pragma once
+
+#include <mbgl/style/expression/expression.hpp>
+#include <mbgl/style/conversion.hpp>
+#include <mbgl/style/expression/parsing_context.hpp>
+
+#include <memory>
+#include <vector>
+
+namespace mbgl {
+namespace style {
+namespace expression {
+
+class Length : public Expression {
+public:
+    Length(std::unique_ptr<Expression> input);
+
+    static ParseResult parse(const mbgl::style::conversion::Convertible& value, ParsingContext& ctx);
+
+    EvaluationResult evaluate(const EvaluationContext& params) const override;
+    void eachChild(const std::function<void(const Expression&)>& visit) const override;
+    bool operator==(const Expression& e) const override;
+    std::vector<optional<Value>> possibleOutputs() const override;
+    std::string getOperator() const override { return "length"; }
+
+private:
+    std::unique_ptr<Expression> input;
+};
+
+} // namespace expression
+} // namespace style
+} // namespace mbgl
diff --git a/include/mbgl/style/expression/let.hpp b/include/mbgl/style/expression/let.hpp
index 75d2adda62..d0210d8bba 100644
--- a/include/mbgl/style/expression/let.hpp
+++ b/include/mbgl/style/expression/let.hpp
@@ -70,6 +70,9 @@ public:
 
     mbgl::Value serialize() const override;
     std::string getOperator() const override { return "var"; }
+    
+    const std::shared_ptr<Expression>& getBoundExpression() const { return value; }
+    
 private:
     std::string name;
     std::shared_ptr<Expression> value;
diff --git a/include/mbgl/style/expression/literal.hpp b/include/mbgl/style/expression/literal.hpp
index d854b419f4..a00c468efc 100644
--- a/include/mbgl/style/expression/literal.hpp
+++ b/include/mbgl/style/expression/literal.hpp
@@ -40,6 +40,10 @@ public:
     std::vector<optional<Value>> possibleOutputs() const override {
         return {{ value }};
     }
+    
+    Value getValue() const {
+        return value;
+    }
 
     mbgl::Value serialize() const override;
     std::string getOperator() const override { return "literal"; }
diff --git a/include/mbgl/style/expression/parsing_context.hpp b/include/mbgl/style/expression/parsing_context.hpp
index f92a4c95ea..c19974a4f7 100644
--- a/include/mbgl/style/expression/parsing_context.hpp
+++ b/include/mbgl/style/expression/parsing_context.hpp
@@ -55,7 +55,7 @@ class ParsingContext {
 public:
     ParsingContext() : errors(std::make_shared<std::vector<ParsingError>>()) {}
     ParsingContext(std::string key_) : key(std::move(key_)), errors(std::make_shared<std::vector<ParsingError>>()) {}
-    explicit ParsingContext(optional<type::Type> expected_)
+    explicit ParsingContext(type::Type expected_)
         : expected(std::move(expected_)),
           errors(std::make_shared<std::vector<ParsingError>>())
     {}
@@ -67,6 +67,7 @@ public:
     std::string getKey() const { return key; }
     optional<type::Type> getExpected() const { return expected; }
     const std::vector<ParsingError>& getErrors() const { return *errors; }
+    const std::string getCombinedErrors() const;
 
     enum TypeAnnotationOption {
         includeTypeAnnotations,
@@ -74,16 +75,21 @@ public:
     };
 
     /*
-        Parse the given style-spec JSON value into an Expression object.
-        Specifically, this function is responsible for determining the expression
-        type (either Literal, or the one named in value[0]) and dispatching to the
-        appropriate ParseXxxx::parse(const V&, ParsingContext) method.
+        Parse the given style-spec JSON value as an expression.
     */
-    ParseResult parse(const mbgl::style::conversion::Convertible& value,
-                      TypeAnnotationOption typeAnnotationOption = includeTypeAnnotations);
+    ParseResult parseExpression(const mbgl::style::conversion::Convertible& value,
+                                TypeAnnotationOption typeAnnotationOption = includeTypeAnnotations);
 
     /*
-        Parse a child expression.
+        Parse the given style-spec JSON value as an expression intended to be used
+        in a layout or paint property.  This entails checking additional constraints
+        that exist in that context but not, e.g., for filters.
+    */
+    ParseResult parseLayerPropertyExpression(const mbgl::style::conversion::Convertible& value,
+                                TypeAnnotationOption typeAnnotationOption = includeTypeAnnotations);
+
+    /*
+        Parse a child expression. For use by individual Expression::parse() methods.
     */
     ParseResult parse(const mbgl::style::conversion::Convertible&,
                       std::size_t,
@@ -91,7 +97,7 @@ public:
                       TypeAnnotationOption typeAnnotationOption = includeTypeAnnotations);
     
     /*
-        Parse a child expression.
+        Parse a child expression.  For use by individual Expression::parse() methods.
     */
     ParseResult parse(const mbgl::style::conversion::Convertible&,
                       std::size_t index,
@@ -141,6 +147,16 @@ private:
         errors(std::move(errors_))
     {}
     
+    
+    /*
+        Parse the given style-spec JSON value into an Expression object.
+        Specifically, this function is responsible for determining the expression
+        type (either Literal, or the one named in value[0]) and dispatching to the
+        appropriate ParseXxxx::parse(const V&, ParsingContext) method.
+    */
+    ParseResult parse(const mbgl::style::conversion::Convertible& value,
+                      TypeAnnotationOption typeAnnotationOption = includeTypeAnnotations);
+    
     std::string key;
     optional<type::Type> expected;
     std::shared_ptr<detail::Scope> scope;