Reimplement style values atop NSExpression (#10726)

* [ios, macos] Import headers, not implementation files

* [core] Added accessors for various expression parameters

Added missing parameter accessors to various expression operator classes, as well as a method on InterpolatorBase and Step that enumerates the stops and their values.

* [ios, macos] Silenced warning in test of error condition

* [ios, macos] Made MGLSphericalPosition boxable

* [ios, macos] Implemented array enumeration during conversion

* [ios, macos] Temporarily ignore heatmap layer type

* [ios, macos] Migrated MGLSymbolStyleLayer.text to NSExpression

MGLSymbolStyleLayer.text is now of type NSExpression instead of MGLStyleValue, as a first step toward migrating the entire layer API from style values to expressions.

Implemented conversions from NSExpression to JSON arrays and vice versa. The most common NSExpression functions are now converted into style expressions, but not all of the most common style expression operators are supported yet.

* [ios, macos] Implemented string coercion

* [ios, macos] Color literals

* [ios, macos] Null constant expressions

* [ios, macos] Convert dictionary literals

* [ios, macos] Interpolation expressions

* [ios, macos] to-boolean, to-number, get from object

* [ios, macos] Variable expressions

Implemented custom expression functions for assigning and referring to variables within the context of an expression. Variables are assigned via a “context dictionary” and applied to an subexpression that is given as another argument to the same expression. Also implemented built-in variable expressions for zoom level and heatmap density.

* [ios, macos] Convert colors, offsets, padding in expressions to JSON objects

* [ios, macos] Expression-based style property getters

Implemented a conversion from mbgl::style::PropertyValues to Objective-C JSON objects, which are then converted to NSExpressions.

* [ios, macos] Consolidated property value–expression conversion in MGLStyleValueTransformer

* [ios, macos] Predicate and expression guide

Extracted documentation about predicates from a documentation comment in MGLVectorStyleLayer.h to a new jazzy guide. Added details about NSExpression support as well. Began updating the “For Style Authors” guide to reflect the transition from style values to expressions.

* [ios, macos] Updated style authoring guide

Updated the Information for Style Authors guide to discuss expressions instead of style functions. Included a table mapping style specification expression operators to NSExpression syntaxes.

* [ios, macos] Migrated codegen templates to expressions

* [ios, macos] Applied expression changes via codegen

Ran make darwin-style-code.

* [macos] Migrated macosapp to expressions

* [ios, macos] Updated style function guide

This guide needs to be thoroughly rewritten, but for now the example code has been migrated to expressions.

* [ios, macos] Eviscerated style function tests

* [ios, macos] Updated changelogs

* [ios] Migrated iosapp to expressions

* [ios, macos] Exposed JSON conversion methods publicly

* [ios, macos] Removed MGLStyleValue, MGLStyleFunction

1 files changed, 299 insertions, 251 deletions

diff --git a/platform/darwin/test/MGLCircleStyleLayerTests.mm b/platform/darwin/test/MGLCircleStyleLayerTests.mm
index c0c503153a..7677344580 100644
--- a/platform/darwin/test/MGLCircleStyleLayerTests.mm
+++ b/platform/darwin/test/MGLCircleStyleLayerTests.mm
@@ -52,40 +52,44 @@
     {
         XCTAssertTrue(rawLayer->getCircleBlur().isUndefined(),
                       @"circle-blur should be unset initially.");
-        MGLStyleValue<NSNumber *> *defaultStyleValue = layer.circleBlur;
+        NSExpression *defaultExpression = layer.circleBlur;
 
-        MGLStyleValue<NSNumber *> *constantStyleValue = [MGLStyleValue<NSNumber *> valueWithRawValue:@0xff];
-        layer.circleBlur = constantStyleValue;
+        NSExpression *constantExpression = [NSExpression expressionWithFormat:@"0xff"];
+        layer.circleBlur = constantExpression;
         mbgl::style::DataDrivenPropertyValue<float> propertyValue = { 0xff };
         XCTAssertEqual(rawLayer->getCircleBlur(), propertyValue,
-                       @"Setting circleBlur to a constant value should update circle-blur.");
-        XCTAssertEqualObjects(layer.circleBlur, constantStyleValue,
-                              @"circleBlur should round-trip constant values.");
-
-        MGLStyleValue<NSNumber *> * functionStyleValue = [MGLStyleValue<NSNumber *> valueWithInterpolationMode:MGLInterpolationModeInterval cameraStops:@{@18: constantStyleValue} options:nil];
-        layer.circleBlur = functionStyleValue;
-
-        mbgl::style::IntervalStops<float> intervalStops = { {{18, 0xff}} };
+                       @"Setting circleBlur to a constant value expression should update circle-blur.");
+        XCTAssertEqualObjects(layer.circleBlur, constantExpression,
+                              @"circleBlur should round-trip constant value expressions.");
+
+        constantExpression = [NSExpression expressionWithFormat:@"0xff"];
+        NSExpression *functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_stepWithMinimum:stops:', %@, %@)", constantExpression, @{@18: constantExpression}];
+        layer.circleBlur = functionExpression;
+
+        mbgl::style::IntervalStops<float> intervalStops = {{
+            { -INFINITY, 0xff },
+            { 18, 0xff },
+        }};
         propertyValue = mbgl::style::CameraFunction<float> { intervalStops };
         
         XCTAssertEqual(rawLayer->getCircleBlur(), propertyValue,
-                       @"Setting circleBlur to a camera function should update circle-blur.");
-        XCTAssertEqualObjects(layer.circleBlur, functionStyleValue,
-                              @"circleBlur should round-trip camera functions.");
+                       @"Setting circleBlur to a camera expression should update circle-blur.");
+        XCTAssertEqualObjects(layer.circleBlur, functionExpression,
+                              @"circleBlur should round-trip camera expressions.");
 
-        functionStyleValue = [MGLStyleValue<NSNumber *> valueWithInterpolationMode:MGLInterpolationModeExponential sourceStops:@{@18: constantStyleValue} attributeName:@"keyName" options:nil];
-        layer.circleBlur = functionStyleValue;
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION(keyName, 'mgl_interpolateWithCurveType:parameters:stops:', 'linear', nil, %@)", @{@18: constantExpression}];
+        layer.circleBlur = functionExpression;
 
         mbgl::style::ExponentialStops<float> exponentialStops = { {{18, 0xff}}, 1.0 };
         propertyValue = mbgl::style::SourceFunction<float> { "keyName", exponentialStops };
 
         XCTAssertEqual(rawLayer->getCircleBlur(), propertyValue,
-                       @"Setting circleBlur to a source function should update circle-blur.");
-        XCTAssertEqualObjects(layer.circleBlur, functionStyleValue,
-                              @"circleBlur should round-trip source functions.");
+                       @"Setting circleBlur to a data expression should update circle-blur.");
+        XCTAssertEqualObjects(layer.circleBlur, functionExpression,
+                              @"circleBlur should round-trip data expressions.");
 
-        functionStyleValue = [MGLStyleValue<NSNumber *> valueWithInterpolationMode:MGLInterpolationModeExponential compositeStops:@{@10: @{@18: constantStyleValue}} attributeName:@"keyName" options:nil];
-        layer.circleBlur = functionStyleValue;
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_interpolateWithCurveType:parameters:stops:', 'linear', nil, %@)", @{@10: functionExpression}];
+        layer.circleBlur = functionExpression;
 
         std::map<float, float> innerStops { {18, 0xff} };
         mbgl::style::CompositeExponentialStops<float> compositeStops { { {10.0, innerStops} }, 1.0 };
@@ -93,15 +97,15 @@
         propertyValue = mbgl::style::CompositeFunction<float> { "keyName", compositeStops };
 
         XCTAssertEqual(rawLayer->getCircleBlur(), propertyValue,
-                       @"Setting circleBlur to a composite function should update circle-blur.");
-        XCTAssertEqualObjects(layer.circleBlur, functionStyleValue,
-                              @"circleBlur should round-trip composite functions.");                                                                                                          
+                       @"Setting circleBlur to a camera-data expression should update circle-blur.");
+        XCTAssertEqualObjects(layer.circleBlur, functionExpression,
+                              @"circleBlur should round-trip camera-data expressions.");
                               
 
         layer.circleBlur = nil;
         XCTAssertTrue(rawLayer->getCircleBlur().isUndefined(),
                       @"Unsetting circleBlur should return circle-blur to the default value.");
-        XCTAssertEqualObjects(layer.circleBlur, defaultStyleValue,
+        XCTAssertEqualObjects(layer.circleBlur, defaultExpression,
                               @"circleBlur should return the default value after being unset.");
         // Transition property test
         layer.circleBlurTransition = transitionTest;
@@ -118,40 +122,44 @@
     {
         XCTAssertTrue(rawLayer->getCircleColor().isUndefined(),
                       @"circle-color should be unset initially.");
-        MGLStyleValue<MGLColor *> *defaultStyleValue = layer.circleColor;
+        NSExpression *defaultExpression = layer.circleColor;
 
-        MGLStyleValue<MGLColor *> *constantStyleValue = [MGLStyleValue<MGLColor *> valueWithRawValue:[MGLColor redColor]];
-        layer.circleColor = constantStyleValue;
+        NSExpression *constantExpression = [NSExpression expressionWithFormat:@"%@", [MGLColor redColor]];
+        layer.circleColor = constantExpression;
         mbgl::style::DataDrivenPropertyValue<mbgl::Color> propertyValue = { { 1, 0, 0, 1 } };
         XCTAssertEqual(rawLayer->getCircleColor(), propertyValue,
-                       @"Setting circleColor to a constant value should update circle-color.");
-        XCTAssertEqualObjects(layer.circleColor, constantStyleValue,
-                              @"circleColor should round-trip constant values.");
-
-        MGLStyleValue<MGLColor *> * functionStyleValue = [MGLStyleValue<MGLColor *> valueWithInterpolationMode:MGLInterpolationModeInterval cameraStops:@{@18: constantStyleValue} options:nil];
-        layer.circleColor = functionStyleValue;
-
-        mbgl::style::IntervalStops<mbgl::Color> intervalStops = { {{18, { 1, 0, 0, 1 }}} };
+                       @"Setting circleColor to a constant value expression should update circle-color.");
+        XCTAssertEqualObjects(layer.circleColor, constantExpression,
+                              @"circleColor should round-trip constant value expressions.");
+
+        constantExpression = [NSExpression expressionWithFormat:@"%@", [MGLColor redColor]];
+        NSExpression *functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_stepWithMinimum:stops:', %@, %@)", constantExpression, @{@18: constantExpression}];
+        layer.circleColor = functionExpression;
+
+        mbgl::style::IntervalStops<mbgl::Color> intervalStops = {{
+            { -INFINITY, { 1, 0, 0, 1 } },
+            { 18, { 1, 0, 0, 1 } },
+        }};
         propertyValue = mbgl::style::CameraFunction<mbgl::Color> { intervalStops };
         
         XCTAssertEqual(rawLayer->getCircleColor(), propertyValue,
-                       @"Setting circleColor to a camera function should update circle-color.");
-        XCTAssertEqualObjects(layer.circleColor, functionStyleValue,
-                              @"circleColor should round-trip camera functions.");
+                       @"Setting circleColor to a camera expression should update circle-color.");
+        XCTAssertEqualObjects(layer.circleColor, functionExpression,
+                              @"circleColor should round-trip camera expressions.");
 
-        functionStyleValue = [MGLStyleValue<MGLColor *> valueWithInterpolationMode:MGLInterpolationModeExponential sourceStops:@{@18: constantStyleValue} attributeName:@"keyName" options:nil];
-        layer.circleColor = functionStyleValue;
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION(keyName, 'mgl_interpolateWithCurveType:parameters:stops:', 'linear', nil, %@)", @{@18: constantExpression}];
+        layer.circleColor = functionExpression;
 
         mbgl::style::ExponentialStops<mbgl::Color> exponentialStops = { {{18, { 1, 0, 0, 1 }}}, 1.0 };
         propertyValue = mbgl::style::SourceFunction<mbgl::Color> { "keyName", exponentialStops };
 
         XCTAssertEqual(rawLayer->getCircleColor(), propertyValue,
-                       @"Setting circleColor to a source function should update circle-color.");
-        XCTAssertEqualObjects(layer.circleColor, functionStyleValue,
-                              @"circleColor should round-trip source functions.");
+                       @"Setting circleColor to a data expression should update circle-color.");
+        XCTAssertEqualObjects(layer.circleColor, functionExpression,
+                              @"circleColor should round-trip data expressions.");
 
-        functionStyleValue = [MGLStyleValue<MGLColor *> valueWithInterpolationMode:MGLInterpolationModeExponential compositeStops:@{@10: @{@18: constantStyleValue}} attributeName:@"keyName" options:nil];
-        layer.circleColor = functionStyleValue;
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_interpolateWithCurveType:parameters:stops:', 'linear', nil, %@)", @{@10: functionExpression}];
+        layer.circleColor = functionExpression;
 
         std::map<float, mbgl::Color> innerStops { {18, { 1, 0, 0, 1 }} };
         mbgl::style::CompositeExponentialStops<mbgl::Color> compositeStops { { {10.0, innerStops} }, 1.0 };
@@ -159,15 +167,15 @@
         propertyValue = mbgl::style::CompositeFunction<mbgl::Color> { "keyName", compositeStops };
 
         XCTAssertEqual(rawLayer->getCircleColor(), propertyValue,
-                       @"Setting circleColor to a composite function should update circle-color.");
-        XCTAssertEqualObjects(layer.circleColor, functionStyleValue,
-                              @"circleColor should round-trip composite functions.");                                                                                                          
+                       @"Setting circleColor to a camera-data expression should update circle-color.");
+        XCTAssertEqualObjects(layer.circleColor, functionExpression,
+                              @"circleColor should round-trip camera-data expressions.");
                               
 
         layer.circleColor = nil;
         XCTAssertTrue(rawLayer->getCircleColor().isUndefined(),
                       @"Unsetting circleColor should return circle-color to the default value.");
-        XCTAssertEqualObjects(layer.circleColor, defaultStyleValue,
+        XCTAssertEqualObjects(layer.circleColor, defaultExpression,
                               @"circleColor should return the default value after being unset.");
         // Transition property test
         layer.circleColorTransition = transitionTest;
@@ -184,40 +192,44 @@
     {
         XCTAssertTrue(rawLayer->getCircleOpacity().isUndefined(),
                       @"circle-opacity should be unset initially.");
-        MGLStyleValue<NSNumber *> *defaultStyleValue = layer.circleOpacity;
+        NSExpression *defaultExpression = layer.circleOpacity;
 
-        MGLStyleValue<NSNumber *> *constantStyleValue = [MGLStyleValue<NSNumber *> valueWithRawValue:@0xff];
-        layer.circleOpacity = constantStyleValue;
+        NSExpression *constantExpression = [NSExpression expressionWithFormat:@"0xff"];
+        layer.circleOpacity = constantExpression;
         mbgl::style::DataDrivenPropertyValue<float> propertyValue = { 0xff };
         XCTAssertEqual(rawLayer->getCircleOpacity(), propertyValue,
-                       @"Setting circleOpacity to a constant value should update circle-opacity.");
-        XCTAssertEqualObjects(layer.circleOpacity, constantStyleValue,
-                              @"circleOpacity should round-trip constant values.");
-
-        MGLStyleValue<NSNumber *> * functionStyleValue = [MGLStyleValue<NSNumber *> valueWithInterpolationMode:MGLInterpolationModeInterval cameraStops:@{@18: constantStyleValue} options:nil];
-        layer.circleOpacity = functionStyleValue;
-
-        mbgl::style::IntervalStops<float> intervalStops = { {{18, 0xff}} };
+                       @"Setting circleOpacity to a constant value expression should update circle-opacity.");
+        XCTAssertEqualObjects(layer.circleOpacity, constantExpression,
+                              @"circleOpacity should round-trip constant value expressions.");
+
+        constantExpression = [NSExpression expressionWithFormat:@"0xff"];
+        NSExpression *functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_stepWithMinimum:stops:', %@, %@)", constantExpression, @{@18: constantExpression}];
+        layer.circleOpacity = functionExpression;
+
+        mbgl::style::IntervalStops<float> intervalStops = {{
+            { -INFINITY, 0xff },
+            { 18, 0xff },
+        }};
         propertyValue = mbgl::style::CameraFunction<float> { intervalStops };
         
         XCTAssertEqual(rawLayer->getCircleOpacity(), propertyValue,
-                       @"Setting circleOpacity to a camera function should update circle-opacity.");
-        XCTAssertEqualObjects(layer.circleOpacity, functionStyleValue,
-                              @"circleOpacity should round-trip camera functions.");
+                       @"Setting circleOpacity to a camera expression should update circle-opacity.");
+        XCTAssertEqualObjects(layer.circleOpacity, functionExpression,
+                              @"circleOpacity should round-trip camera expressions.");
 
-        functionStyleValue = [MGLStyleValue<NSNumber *> valueWithInterpolationMode:MGLInterpolationModeExponential sourceStops:@{@18: constantStyleValue} attributeName:@"keyName" options:nil];
-        layer.circleOpacity = functionStyleValue;
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION(keyName, 'mgl_interpolateWithCurveType:parameters:stops:', 'linear', nil, %@)", @{@18: constantExpression}];
+        layer.circleOpacity = functionExpression;
 
         mbgl::style::ExponentialStops<float> exponentialStops = { {{18, 0xff}}, 1.0 };
         propertyValue = mbgl::style::SourceFunction<float> { "keyName", exponentialStops };
 
         XCTAssertEqual(rawLayer->getCircleOpacity(), propertyValue,
-                       @"Setting circleOpacity to a source function should update circle-opacity.");
-        XCTAssertEqualObjects(layer.circleOpacity, functionStyleValue,
-                              @"circleOpacity should round-trip source functions.");
+                       @"Setting circleOpacity to a data expression should update circle-opacity.");
+        XCTAssertEqualObjects(layer.circleOpacity, functionExpression,
+                              @"circleOpacity should round-trip data expressions.");
 
-        functionStyleValue = [MGLStyleValue<NSNumber *> valueWithInterpolationMode:MGLInterpolationModeExponential compositeStops:@{@10: @{@18: constantStyleValue}} attributeName:@"keyName" options:nil];
-        layer.circleOpacity = functionStyleValue;
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_interpolateWithCurveType:parameters:stops:', 'linear', nil, %@)", @{@10: functionExpression}];
+        layer.circleOpacity = functionExpression;
 
         std::map<float, float> innerStops { {18, 0xff} };
         mbgl::style::CompositeExponentialStops<float> compositeStops { { {10.0, innerStops} }, 1.0 };
@@ -225,15 +237,15 @@
         propertyValue = mbgl::style::CompositeFunction<float> { "keyName", compositeStops };
 
         XCTAssertEqual(rawLayer->getCircleOpacity(), propertyValue,
-                       @"Setting circleOpacity to a composite function should update circle-opacity.");
-        XCTAssertEqualObjects(layer.circleOpacity, functionStyleValue,
-                              @"circleOpacity should round-trip composite functions.");                                                                                                          
+                       @"Setting circleOpacity to a camera-data expression should update circle-opacity.");
+        XCTAssertEqualObjects(layer.circleOpacity, functionExpression,
+                              @"circleOpacity should round-trip camera-data expressions.");
                               
 
         layer.circleOpacity = nil;
         XCTAssertTrue(rawLayer->getCircleOpacity().isUndefined(),
                       @"Unsetting circleOpacity should return circle-opacity to the default value.");
-        XCTAssertEqualObjects(layer.circleOpacity, defaultStyleValue,
+        XCTAssertEqualObjects(layer.circleOpacity, defaultExpression,
                               @"circleOpacity should return the default value after being unset.");
         // Transition property test
         layer.circleOpacityTransition = transitionTest;
@@ -250,79 +262,88 @@
     {
         XCTAssertTrue(rawLayer->getCirclePitchAlignment().isUndefined(),
                       @"circle-pitch-alignment should be unset initially.");
-        MGLStyleValue<NSValue *> *defaultStyleValue = layer.circlePitchAlignment;
+        NSExpression *defaultExpression = layer.circlePitchAlignment;
 
-        MGLStyleValue<NSValue *> *constantStyleValue = [MGLStyleValue<NSValue *> valueWithRawValue:[NSValue valueWithMGLCirclePitchAlignment:MGLCirclePitchAlignmentViewport]];
-        layer.circlePitchAlignment = constantStyleValue;
+        NSExpression *constantExpression = [NSExpression expressionWithFormat:@"'viewport'"];
+        layer.circlePitchAlignment = constantExpression;
         mbgl::style::PropertyValue<mbgl::style::AlignmentType> propertyValue = { mbgl::style::AlignmentType::Viewport };
         XCTAssertEqual(rawLayer->getCirclePitchAlignment(), propertyValue,
-                       @"Setting circlePitchAlignment to a constant value should update circle-pitch-alignment.");
-        XCTAssertEqualObjects(layer.circlePitchAlignment, constantStyleValue,
-                              @"circlePitchAlignment should round-trip constant values.");
-
-        MGLStyleValue<NSValue *> * functionStyleValue = [MGLStyleValue<NSValue *> valueWithInterpolationMode:MGLInterpolationModeInterval cameraStops:@{@18: constantStyleValue} options:nil];
-        layer.circlePitchAlignment = functionStyleValue;
-
-        mbgl::style::IntervalStops<mbgl::style::AlignmentType> intervalStops = { {{18, mbgl::style::AlignmentType::Viewport}} };
+                       @"Setting circlePitchAlignment to a constant value expression should update circle-pitch-alignment.");
+        XCTAssertEqualObjects(layer.circlePitchAlignment, constantExpression,
+                              @"circlePitchAlignment should round-trip constant value expressions.");
+
+        constantExpression = [NSExpression expressionWithFormat:@"'viewport'"];
+        NSExpression *functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_stepWithMinimum:stops:', %@, %@)", constantExpression, @{@18: constantExpression}];
+        layer.circlePitchAlignment = functionExpression;
+
+        mbgl::style::IntervalStops<mbgl::style::AlignmentType> intervalStops = {{
+            { -INFINITY, mbgl::style::AlignmentType::Viewport },
+            { 18, mbgl::style::AlignmentType::Viewport },
+        }};
         propertyValue = mbgl::style::CameraFunction<mbgl::style::AlignmentType> { intervalStops };
         
         XCTAssertEqual(rawLayer->getCirclePitchAlignment(), propertyValue,
-                       @"Setting circlePitchAlignment to a camera function should update circle-pitch-alignment.");
-        XCTAssertEqualObjects(layer.circlePitchAlignment, functionStyleValue,
-                              @"circlePitchAlignment should round-trip camera functions.");
+                       @"Setting circlePitchAlignment to a camera expression should update circle-pitch-alignment.");
+        XCTAssertEqualObjects(layer.circlePitchAlignment, functionExpression,
+                              @"circlePitchAlignment should round-trip camera expressions.");
 
                               
 
         layer.circlePitchAlignment = nil;
         XCTAssertTrue(rawLayer->getCirclePitchAlignment().isUndefined(),
                       @"Unsetting circlePitchAlignment should return circle-pitch-alignment to the default value.");
-        XCTAssertEqualObjects(layer.circlePitchAlignment, defaultStyleValue,
+        XCTAssertEqualObjects(layer.circlePitchAlignment, defaultExpression,
                               @"circlePitchAlignment should return the default value after being unset.");
 
-        functionStyleValue = [MGLStyleValue<NSValue *> valueWithInterpolationMode:MGLInterpolationModeIdentity sourceStops:nil attributeName:@"" options:nil];
-        XCTAssertThrowsSpecificNamed(layer.circlePitchAlignment = functionStyleValue, NSException, NSInvalidArgumentException, @"MGLStyleValue should raise an exception if it is applied to a property that cannot support it");
-        functionStyleValue = [MGLStyleValue<NSValue *> valueWithInterpolationMode:MGLInterpolationModeInterval compositeStops:@{@18: constantStyleValue} attributeName:@"" options:nil];
-        XCTAssertThrowsSpecificNamed(layer.circlePitchAlignment = functionStyleValue, NSException, NSInvalidArgumentException, @"MGLStyleValue should raise an exception if it is applied to a property that cannot support it");
+        functionExpression = [NSExpression expressionForKeyPath:@"bogus"];
+        XCTAssertThrowsSpecificNamed(layer.circlePitchAlignment = functionExpression, NSException, NSInvalidArgumentException, @"MGLCircleLayer should raise an exception if a camera-data expression is applied to a property that does not support key paths to feature attributes.");
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION(bogus, 'mgl_stepWithMinimum:stops:', %@, %@)", constantExpression, @{@18: constantExpression}];
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_interpolateWithCurveType:parameters:stops:', 'linear', nil, %@)", @{@10: functionExpression}];
+        XCTAssertThrowsSpecificNamed(layer.circlePitchAlignment = functionExpression, NSException, NSInvalidArgumentException, @"MGLCircleLayer should raise an exception if a camera-data expression is applied to a property that does not support key paths to feature attributes.");
     }
 
     // circle-radius
     {
         XCTAssertTrue(rawLayer->getCircleRadius().isUndefined(),
                       @"circle-radius should be unset initially.");
-        MGLStyleValue<NSNumber *> *defaultStyleValue = layer.circleRadius;
+        NSExpression *defaultExpression = layer.circleRadius;
 
-        MGLStyleValue<NSNumber *> *constantStyleValue = [MGLStyleValue<NSNumber *> valueWithRawValue:@0xff];
-        layer.circleRadius = constantStyleValue;
+        NSExpression *constantExpression = [NSExpression expressionWithFormat:@"0xff"];
+        layer.circleRadius = constantExpression;
         mbgl::style::DataDrivenPropertyValue<float> propertyValue = { 0xff };
         XCTAssertEqual(rawLayer->getCircleRadius(), propertyValue,
-                       @"Setting circleRadius to a constant value should update circle-radius.");
-        XCTAssertEqualObjects(layer.circleRadius, constantStyleValue,
-                              @"circleRadius should round-trip constant values.");
-
-        MGLStyleValue<NSNumber *> * functionStyleValue = [MGLStyleValue<NSNumber *> valueWithInterpolationMode:MGLInterpolationModeInterval cameraStops:@{@18: constantStyleValue} options:nil];
-        layer.circleRadius = functionStyleValue;
-
-        mbgl::style::IntervalStops<float> intervalStops = { {{18, 0xff}} };
+                       @"Setting circleRadius to a constant value expression should update circle-radius.");
+        XCTAssertEqualObjects(layer.circleRadius, constantExpression,
+                              @"circleRadius should round-trip constant value expressions.");
+
+        constantExpression = [NSExpression expressionWithFormat:@"0xff"];
+        NSExpression *functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_stepWithMinimum:stops:', %@, %@)", constantExpression, @{@18: constantExpression}];
+        layer.circleRadius = functionExpression;
+
+        mbgl::style::IntervalStops<float> intervalStops = {{
+            { -INFINITY, 0xff },
+            { 18, 0xff },
+        }};
         propertyValue = mbgl::style::CameraFunction<float> { intervalStops };
         
         XCTAssertEqual(rawLayer->getCircleRadius(), propertyValue,
-                       @"Setting circleRadius to a camera function should update circle-radius.");
-        XCTAssertEqualObjects(layer.circleRadius, functionStyleValue,
-                              @"circleRadius should round-trip camera functions.");
+                       @"Setting circleRadius to a camera expression should update circle-radius.");
+        XCTAssertEqualObjects(layer.circleRadius, functionExpression,
+                              @"circleRadius should round-trip camera expressions.");
 
-        functionStyleValue = [MGLStyleValue<NSNumber *> valueWithInterpolationMode:MGLInterpolationModeExponential sourceStops:@{@18: constantStyleValue} attributeName:@"keyName" options:nil];
-        layer.circleRadius = functionStyleValue;
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION(keyName, 'mgl_interpolateWithCurveType:parameters:stops:', 'linear', nil, %@)", @{@18: constantExpression}];
+        layer.circleRadius = functionExpression;
 
         mbgl::style::ExponentialStops<float> exponentialStops = { {{18, 0xff}}, 1.0 };
         propertyValue = mbgl::style::SourceFunction<float> { "keyName", exponentialStops };
 
         XCTAssertEqual(rawLayer->getCircleRadius(), propertyValue,
-                       @"Setting circleRadius to a source function should update circle-radius.");
-        XCTAssertEqualObjects(layer.circleRadius, functionStyleValue,
-                              @"circleRadius should round-trip source functions.");
+                       @"Setting circleRadius to a data expression should update circle-radius.");
+        XCTAssertEqualObjects(layer.circleRadius, functionExpression,
+                              @"circleRadius should round-trip data expressions.");
 
-        functionStyleValue = [MGLStyleValue<NSNumber *> valueWithInterpolationMode:MGLInterpolationModeExponential compositeStops:@{@10: @{@18: constantStyleValue}} attributeName:@"keyName" options:nil];
-        layer.circleRadius = functionStyleValue;
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_interpolateWithCurveType:parameters:stops:', 'linear', nil, %@)", @{@10: functionExpression}];
+        layer.circleRadius = functionExpression;
 
         std::map<float, float> innerStops { {18, 0xff} };
         mbgl::style::CompositeExponentialStops<float> compositeStops { { {10.0, innerStops} }, 1.0 };
@@ -330,15 +351,15 @@
         propertyValue = mbgl::style::CompositeFunction<float> { "keyName", compositeStops };
 
         XCTAssertEqual(rawLayer->getCircleRadius(), propertyValue,
-                       @"Setting circleRadius to a composite function should update circle-radius.");
-        XCTAssertEqualObjects(layer.circleRadius, functionStyleValue,
-                              @"circleRadius should round-trip composite functions.");                                                                                                          
+                       @"Setting circleRadius to a camera-data expression should update circle-radius.");
+        XCTAssertEqualObjects(layer.circleRadius, functionExpression,
+                              @"circleRadius should round-trip camera-data expressions.");
                               
 
         layer.circleRadius = nil;
         XCTAssertTrue(rawLayer->getCircleRadius().isUndefined(),
                       @"Unsetting circleRadius should return circle-radius to the default value.");
-        XCTAssertEqualObjects(layer.circleRadius, defaultStyleValue,
+        XCTAssertEqualObjects(layer.circleRadius, defaultExpression,
                               @"circleRadius should return the default value after being unset.");
         // Transition property test
         layer.circleRadiusTransition = transitionTest;
@@ -355,79 +376,88 @@
     {
         XCTAssertTrue(rawLayer->getCirclePitchScale().isUndefined(),
                       @"circle-pitch-scale should be unset initially.");
-        MGLStyleValue<NSValue *> *defaultStyleValue = layer.circleScaleAlignment;
+        NSExpression *defaultExpression = layer.circleScaleAlignment;
 
-        MGLStyleValue<NSValue *> *constantStyleValue = [MGLStyleValue<NSValue *> valueWithRawValue:[NSValue valueWithMGLCircleScaleAlignment:MGLCircleScaleAlignmentViewport]];
-        layer.circleScaleAlignment = constantStyleValue;
+        NSExpression *constantExpression = [NSExpression expressionWithFormat:@"'viewport'"];
+        layer.circleScaleAlignment = constantExpression;
         mbgl::style::PropertyValue<mbgl::style::CirclePitchScaleType> propertyValue = { mbgl::style::CirclePitchScaleType::Viewport };
         XCTAssertEqual(rawLayer->getCirclePitchScale(), propertyValue,
-                       @"Setting circleScaleAlignment to a constant value should update circle-pitch-scale.");
-        XCTAssertEqualObjects(layer.circleScaleAlignment, constantStyleValue,
-                              @"circleScaleAlignment should round-trip constant values.");
-
-        MGLStyleValue<NSValue *> * functionStyleValue = [MGLStyleValue<NSValue *> valueWithInterpolationMode:MGLInterpolationModeInterval cameraStops:@{@18: constantStyleValue} options:nil];
-        layer.circleScaleAlignment = functionStyleValue;
-
-        mbgl::style::IntervalStops<mbgl::style::CirclePitchScaleType> intervalStops = { {{18, mbgl::style::CirclePitchScaleType::Viewport}} };
+                       @"Setting circleScaleAlignment to a constant value expression should update circle-pitch-scale.");
+        XCTAssertEqualObjects(layer.circleScaleAlignment, constantExpression,
+                              @"circleScaleAlignment should round-trip constant value expressions.");
+
+        constantExpression = [NSExpression expressionWithFormat:@"'viewport'"];
+        NSExpression *functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_stepWithMinimum:stops:', %@, %@)", constantExpression, @{@18: constantExpression}];
+        layer.circleScaleAlignment = functionExpression;
+
+        mbgl::style::IntervalStops<mbgl::style::CirclePitchScaleType> intervalStops = {{
+            { -INFINITY, mbgl::style::CirclePitchScaleType::Viewport },
+            { 18, mbgl::style::CirclePitchScaleType::Viewport },
+        }};
         propertyValue = mbgl::style::CameraFunction<mbgl::style::CirclePitchScaleType> { intervalStops };
         
         XCTAssertEqual(rawLayer->getCirclePitchScale(), propertyValue,
-                       @"Setting circleScaleAlignment to a camera function should update circle-pitch-scale.");
-        XCTAssertEqualObjects(layer.circleScaleAlignment, functionStyleValue,
-                              @"circleScaleAlignment should round-trip camera functions.");
+                       @"Setting circleScaleAlignment to a camera expression should update circle-pitch-scale.");
+        XCTAssertEqualObjects(layer.circleScaleAlignment, functionExpression,
+                              @"circleScaleAlignment should round-trip camera expressions.");
 
                               
 
         layer.circleScaleAlignment = nil;
         XCTAssertTrue(rawLayer->getCirclePitchScale().isUndefined(),
                       @"Unsetting circleScaleAlignment should return circle-pitch-scale to the default value.");
-        XCTAssertEqualObjects(layer.circleScaleAlignment, defaultStyleValue,
+        XCTAssertEqualObjects(layer.circleScaleAlignment, defaultExpression,
                               @"circleScaleAlignment should return the default value after being unset.");
 
-        functionStyleValue = [MGLStyleValue<NSValue *> valueWithInterpolationMode:MGLInterpolationModeIdentity sourceStops:nil attributeName:@"" options:nil];
-        XCTAssertThrowsSpecificNamed(layer.circleScaleAlignment = functionStyleValue, NSException, NSInvalidArgumentException, @"MGLStyleValue should raise an exception if it is applied to a property that cannot support it");
-        functionStyleValue = [MGLStyleValue<NSValue *> valueWithInterpolationMode:MGLInterpolationModeInterval compositeStops:@{@18: constantStyleValue} attributeName:@"" options:nil];
-        XCTAssertThrowsSpecificNamed(layer.circleScaleAlignment = functionStyleValue, NSException, NSInvalidArgumentException, @"MGLStyleValue should raise an exception if it is applied to a property that cannot support it");
+        functionExpression = [NSExpression expressionForKeyPath:@"bogus"];
+        XCTAssertThrowsSpecificNamed(layer.circleScaleAlignment = functionExpression, NSException, NSInvalidArgumentException, @"MGLCircleLayer should raise an exception if a camera-data expression is applied to a property that does not support key paths to feature attributes.");
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION(bogus, 'mgl_stepWithMinimum:stops:', %@, %@)", constantExpression, @{@18: constantExpression}];
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_interpolateWithCurveType:parameters:stops:', 'linear', nil, %@)", @{@10: functionExpression}];
+        XCTAssertThrowsSpecificNamed(layer.circleScaleAlignment = functionExpression, NSException, NSInvalidArgumentException, @"MGLCircleLayer should raise an exception if a camera-data expression is applied to a property that does not support key paths to feature attributes.");
     }
 
     // circle-stroke-color
     {
         XCTAssertTrue(rawLayer->getCircleStrokeColor().isUndefined(),
                       @"circle-stroke-color should be unset initially.");
-        MGLStyleValue<MGLColor *> *defaultStyleValue = layer.circleStrokeColor;
+        NSExpression *defaultExpression = layer.circleStrokeColor;
 
-        MGLStyleValue<MGLColor *> *constantStyleValue = [MGLStyleValue<MGLColor *> valueWithRawValue:[MGLColor redColor]];
-        layer.circleStrokeColor = constantStyleValue;
+        NSExpression *constantExpression = [NSExpression expressionWithFormat:@"%@", [MGLColor redColor]];
+        layer.circleStrokeColor = constantExpression;
         mbgl::style::DataDrivenPropertyValue<mbgl::Color> propertyValue = { { 1, 0, 0, 1 } };
         XCTAssertEqual(rawLayer->getCircleStrokeColor(), propertyValue,
-                       @"Setting circleStrokeColor to a constant value should update circle-stroke-color.");
-        XCTAssertEqualObjects(layer.circleStrokeColor, constantStyleValue,
-                              @"circleStrokeColor should round-trip constant values.");
-
-        MGLStyleValue<MGLColor *> * functionStyleValue = [MGLStyleValue<MGLColor *> valueWithInterpolationMode:MGLInterpolationModeInterval cameraStops:@{@18: constantStyleValue} options:nil];
-        layer.circleStrokeColor = functionStyleValue;
-
-        mbgl::style::IntervalStops<mbgl::Color> intervalStops = { {{18, { 1, 0, 0, 1 }}} };
+                       @"Setting circleStrokeColor to a constant value expression should update circle-stroke-color.");
+        XCTAssertEqualObjects(layer.circleStrokeColor, constantExpression,
+                              @"circleStrokeColor should round-trip constant value expressions.");
+
+        constantExpression = [NSExpression expressionWithFormat:@"%@", [MGLColor redColor]];
+        NSExpression *functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_stepWithMinimum:stops:', %@, %@)", constantExpression, @{@18: constantExpression}];
+        layer.circleStrokeColor = functionExpression;
+
+        mbgl::style::IntervalStops<mbgl::Color> intervalStops = {{
+            { -INFINITY, { 1, 0, 0, 1 } },
+            { 18, { 1, 0, 0, 1 } },
+        }};
         propertyValue = mbgl::style::CameraFunction<mbgl::Color> { intervalStops };
         
         XCTAssertEqual(rawLayer->getCircleStrokeColor(), propertyValue,
-                       @"Setting circleStrokeColor to a camera function should update circle-stroke-color.");
-        XCTAssertEqualObjects(layer.circleStrokeColor, functionStyleValue,
-                              @"circleStrokeColor should round-trip camera functions.");
+                       @"Setting circleStrokeColor to a camera expression should update circle-stroke-color.");
+        XCTAssertEqualObjects(layer.circleStrokeColor, functionExpression,
+                              @"circleStrokeColor should round-trip camera expressions.");
 
-        functionStyleValue = [MGLStyleValue<MGLColor *> valueWithInterpolationMode:MGLInterpolationModeExponential sourceStops:@{@18: constantStyleValue} attributeName:@"keyName" options:nil];
-        layer.circleStrokeColor = functionStyleValue;
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION(keyName, 'mgl_interpolateWithCurveType:parameters:stops:', 'linear', nil, %@)", @{@18: constantExpression}];
+        layer.circleStrokeColor = functionExpression;
 
         mbgl::style::ExponentialStops<mbgl::Color> exponentialStops = { {{18, { 1, 0, 0, 1 }}}, 1.0 };
         propertyValue = mbgl::style::SourceFunction<mbgl::Color> { "keyName", exponentialStops };
 
         XCTAssertEqual(rawLayer->getCircleStrokeColor(), propertyValue,
-                       @"Setting circleStrokeColor to a source function should update circle-stroke-color.");
-        XCTAssertEqualObjects(layer.circleStrokeColor, functionStyleValue,
-                              @"circleStrokeColor should round-trip source functions.");
+                       @"Setting circleStrokeColor to a data expression should update circle-stroke-color.");
+        XCTAssertEqualObjects(layer.circleStrokeColor, functionExpression,
+                              @"circleStrokeColor should round-trip data expressions.");
 
-        functionStyleValue = [MGLStyleValue<MGLColor *> valueWithInterpolationMode:MGLInterpolationModeExponential compositeStops:@{@10: @{@18: constantStyleValue}} attributeName:@"keyName" options:nil];
-        layer.circleStrokeColor = functionStyleValue;
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_interpolateWithCurveType:parameters:stops:', 'linear', nil, %@)", @{@10: functionExpression}];
+        layer.circleStrokeColor = functionExpression;
 
         std::map<float, mbgl::Color> innerStops { {18, { 1, 0, 0, 1 }} };
         mbgl::style::CompositeExponentialStops<mbgl::Color> compositeStops { { {10.0, innerStops} }, 1.0 };
@@ -435,15 +465,15 @@
         propertyValue = mbgl::style::CompositeFunction<mbgl::Color> { "keyName", compositeStops };
 
         XCTAssertEqual(rawLayer->getCircleStrokeColor(), propertyValue,
-                       @"Setting circleStrokeColor to a composite function should update circle-stroke-color.");
-        XCTAssertEqualObjects(layer.circleStrokeColor, functionStyleValue,
-                              @"circleStrokeColor should round-trip composite functions.");                                                                                                          
+                       @"Setting circleStrokeColor to a camera-data expression should update circle-stroke-color.");
+        XCTAssertEqualObjects(layer.circleStrokeColor, functionExpression,
+                              @"circleStrokeColor should round-trip camera-data expressions.");
                               
 
         layer.circleStrokeColor = nil;
         XCTAssertTrue(rawLayer->getCircleStrokeColor().isUndefined(),
                       @"Unsetting circleStrokeColor should return circle-stroke-color to the default value.");
-        XCTAssertEqualObjects(layer.circleStrokeColor, defaultStyleValue,
+        XCTAssertEqualObjects(layer.circleStrokeColor, defaultExpression,
                               @"circleStrokeColor should return the default value after being unset.");
         // Transition property test
         layer.circleStrokeColorTransition = transitionTest;
@@ -460,40 +490,44 @@
     {
         XCTAssertTrue(rawLayer->getCircleStrokeOpacity().isUndefined(),
                       @"circle-stroke-opacity should be unset initially.");
-        MGLStyleValue<NSNumber *> *defaultStyleValue = layer.circleStrokeOpacity;
+        NSExpression *defaultExpression = layer.circleStrokeOpacity;
 
-        MGLStyleValue<NSNumber *> *constantStyleValue = [MGLStyleValue<NSNumber *> valueWithRawValue:@0xff];
-        layer.circleStrokeOpacity = constantStyleValue;
+        NSExpression *constantExpression = [NSExpression expressionWithFormat:@"0xff"];
+        layer.circleStrokeOpacity = constantExpression;
         mbgl::style::DataDrivenPropertyValue<float> propertyValue = { 0xff };
         XCTAssertEqual(rawLayer->getCircleStrokeOpacity(), propertyValue,
-                       @"Setting circleStrokeOpacity to a constant value should update circle-stroke-opacity.");
-        XCTAssertEqualObjects(layer.circleStrokeOpacity, constantStyleValue,
-                              @"circleStrokeOpacity should round-trip constant values.");
-
-        MGLStyleValue<NSNumber *> * functionStyleValue = [MGLStyleValue<NSNumber *> valueWithInterpolationMode:MGLInterpolationModeInterval cameraStops:@{@18: constantStyleValue} options:nil];
-        layer.circleStrokeOpacity = functionStyleValue;
-
-        mbgl::style::IntervalStops<float> intervalStops = { {{18, 0xff}} };
+                       @"Setting circleStrokeOpacity to a constant value expression should update circle-stroke-opacity.");
+        XCTAssertEqualObjects(layer.circleStrokeOpacity, constantExpression,
+                              @"circleStrokeOpacity should round-trip constant value expressions.");
+
+        constantExpression = [NSExpression expressionWithFormat:@"0xff"];
+        NSExpression *functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_stepWithMinimum:stops:', %@, %@)", constantExpression, @{@18: constantExpression}];
+        layer.circleStrokeOpacity = functionExpression;
+
+        mbgl::style::IntervalStops<float> intervalStops = {{
+            { -INFINITY, 0xff },
+            { 18, 0xff },
+        }};
         propertyValue = mbgl::style::CameraFunction<float> { intervalStops };
         
         XCTAssertEqual(rawLayer->getCircleStrokeOpacity(), propertyValue,
-                       @"Setting circleStrokeOpacity to a camera function should update circle-stroke-opacity.");
-        XCTAssertEqualObjects(layer.circleStrokeOpacity, functionStyleValue,
-                              @"circleStrokeOpacity should round-trip camera functions.");
+                       @"Setting circleStrokeOpacity to a camera expression should update circle-stroke-opacity.");
+        XCTAssertEqualObjects(layer.circleStrokeOpacity, functionExpression,
+                              @"circleStrokeOpacity should round-trip camera expressions.");
 
-        functionStyleValue = [MGLStyleValue<NSNumber *> valueWithInterpolationMode:MGLInterpolationModeExponential sourceStops:@{@18: constantStyleValue} attributeName:@"keyName" options:nil];
-        layer.circleStrokeOpacity = functionStyleValue;
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION(keyName, 'mgl_interpolateWithCurveType:parameters:stops:', 'linear', nil, %@)", @{@18: constantExpression}];
+        layer.circleStrokeOpacity = functionExpression;
 
         mbgl::style::ExponentialStops<float> exponentialStops = { {{18, 0xff}}, 1.0 };
         propertyValue = mbgl::style::SourceFunction<float> { "keyName", exponentialStops };
 
         XCTAssertEqual(rawLayer->getCircleStrokeOpacity(), propertyValue,
-                       @"Setting circleStrokeOpacity to a source function should update circle-stroke-opacity.");
-        XCTAssertEqualObjects(layer.circleStrokeOpacity, functionStyleValue,
-                              @"circleStrokeOpacity should round-trip source functions.");
+                       @"Setting circleStrokeOpacity to a data expression should update circle-stroke-opacity.");
+        XCTAssertEqualObjects(layer.circleStrokeOpacity, functionExpression,
+                              @"circleStrokeOpacity should round-trip data expressions.");
 
-        functionStyleValue = [MGLStyleValue<NSNumber *> valueWithInterpolationMode:MGLInterpolationModeExponential compositeStops:@{@10: @{@18: constantStyleValue}} attributeName:@"keyName" options:nil];
-        layer.circleStrokeOpacity = functionStyleValue;
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_interpolateWithCurveType:parameters:stops:', 'linear', nil, %@)", @{@10: functionExpression}];
+        layer.circleStrokeOpacity = functionExpression;
 
         std::map<float, float> innerStops { {18, 0xff} };
         mbgl::style::CompositeExponentialStops<float> compositeStops { { {10.0, innerStops} }, 1.0 };
@@ -501,15 +535,15 @@
         propertyValue = mbgl::style::CompositeFunction<float> { "keyName", compositeStops };
 
         XCTAssertEqual(rawLayer->getCircleStrokeOpacity(), propertyValue,
-                       @"Setting circleStrokeOpacity to a composite function should update circle-stroke-opacity.");
-        XCTAssertEqualObjects(layer.circleStrokeOpacity, functionStyleValue,
-                              @"circleStrokeOpacity should round-trip composite functions.");                                                                                                          
+                       @"Setting circleStrokeOpacity to a camera-data expression should update circle-stroke-opacity.");
+        XCTAssertEqualObjects(layer.circleStrokeOpacity, functionExpression,
+                              @"circleStrokeOpacity should round-trip camera-data expressions.");
                               
 
         layer.circleStrokeOpacity = nil;
         XCTAssertTrue(rawLayer->getCircleStrokeOpacity().isUndefined(),
                       @"Unsetting circleStrokeOpacity should return circle-stroke-opacity to the default value.");
-        XCTAssertEqualObjects(layer.circleStrokeOpacity, defaultStyleValue,
+        XCTAssertEqualObjects(layer.circleStrokeOpacity, defaultExpression,
                               @"circleStrokeOpacity should return the default value after being unset.");
         // Transition property test
         layer.circleStrokeOpacityTransition = transitionTest;
@@ -526,40 +560,44 @@
     {
         XCTAssertTrue(rawLayer->getCircleStrokeWidth().isUndefined(),
                       @"circle-stroke-width should be unset initially.");
-        MGLStyleValue<NSNumber *> *defaultStyleValue = layer.circleStrokeWidth;
+        NSExpression *defaultExpression = layer.circleStrokeWidth;
 
-        MGLStyleValue<NSNumber *> *constantStyleValue = [MGLStyleValue<NSNumber *> valueWithRawValue:@0xff];
-        layer.circleStrokeWidth = constantStyleValue;
+        NSExpression *constantExpression = [NSExpression expressionWithFormat:@"0xff"];
+        layer.circleStrokeWidth = constantExpression;
         mbgl::style::DataDrivenPropertyValue<float> propertyValue = { 0xff };
         XCTAssertEqual(rawLayer->getCircleStrokeWidth(), propertyValue,
-                       @"Setting circleStrokeWidth to a constant value should update circle-stroke-width.");
-        XCTAssertEqualObjects(layer.circleStrokeWidth, constantStyleValue,
-                              @"circleStrokeWidth should round-trip constant values.");
-
-        MGLStyleValue<NSNumber *> * functionStyleValue = [MGLStyleValue<NSNumber *> valueWithInterpolationMode:MGLInterpolationModeInterval cameraStops:@{@18: constantStyleValue} options:nil];
-        layer.circleStrokeWidth = functionStyleValue;
-
-        mbgl::style::IntervalStops<float> intervalStops = { {{18, 0xff}} };
+                       @"Setting circleStrokeWidth to a constant value expression should update circle-stroke-width.");
+        XCTAssertEqualObjects(layer.circleStrokeWidth, constantExpression,
+                              @"circleStrokeWidth should round-trip constant value expressions.");
+
+        constantExpression = [NSExpression expressionWithFormat:@"0xff"];
+        NSExpression *functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_stepWithMinimum:stops:', %@, %@)", constantExpression, @{@18: constantExpression}];
+        layer.circleStrokeWidth = functionExpression;
+
+        mbgl::style::IntervalStops<float> intervalStops = {{
+            { -INFINITY, 0xff },
+            { 18, 0xff },
+        }};
         propertyValue = mbgl::style::CameraFunction<float> { intervalStops };
         
         XCTAssertEqual(rawLayer->getCircleStrokeWidth(), propertyValue,
-                       @"Setting circleStrokeWidth to a camera function should update circle-stroke-width.");
-        XCTAssertEqualObjects(layer.circleStrokeWidth, functionStyleValue,
-                              @"circleStrokeWidth should round-trip camera functions.");
+                       @"Setting circleStrokeWidth to a camera expression should update circle-stroke-width.");
+        XCTAssertEqualObjects(layer.circleStrokeWidth, functionExpression,
+                              @"circleStrokeWidth should round-trip camera expressions.");
 
-        functionStyleValue = [MGLStyleValue<NSNumber *> valueWithInterpolationMode:MGLInterpolationModeExponential sourceStops:@{@18: constantStyleValue} attributeName:@"keyName" options:nil];
-        layer.circleStrokeWidth = functionStyleValue;
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION(keyName, 'mgl_interpolateWithCurveType:parameters:stops:', 'linear', nil, %@)", @{@18: constantExpression}];
+        layer.circleStrokeWidth = functionExpression;
 
         mbgl::style::ExponentialStops<float> exponentialStops = { {{18, 0xff}}, 1.0 };
         propertyValue = mbgl::style::SourceFunction<float> { "keyName", exponentialStops };
 
         XCTAssertEqual(rawLayer->getCircleStrokeWidth(), propertyValue,
-                       @"Setting circleStrokeWidth to a source function should update circle-stroke-width.");
-        XCTAssertEqualObjects(layer.circleStrokeWidth, functionStyleValue,
-                              @"circleStrokeWidth should round-trip source functions.");
+                       @"Setting circleStrokeWidth to a data expression should update circle-stroke-width.");
+        XCTAssertEqualObjects(layer.circleStrokeWidth, functionExpression,
+                              @"circleStrokeWidth should round-trip data expressions.");
 
-        functionStyleValue = [MGLStyleValue<NSNumber *> valueWithInterpolationMode:MGLInterpolationModeExponential compositeStops:@{@10: @{@18: constantStyleValue}} attributeName:@"keyName" options:nil];
-        layer.circleStrokeWidth = functionStyleValue;
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_interpolateWithCurveType:parameters:stops:', 'linear', nil, %@)", @{@10: functionExpression}];
+        layer.circleStrokeWidth = functionExpression;
 
         std::map<float, float> innerStops { {18, 0xff} };
         mbgl::style::CompositeExponentialStops<float> compositeStops { { {10.0, innerStops} }, 1.0 };
@@ -567,15 +605,15 @@
         propertyValue = mbgl::style::CompositeFunction<float> { "keyName", compositeStops };
 
         XCTAssertEqual(rawLayer->getCircleStrokeWidth(), propertyValue,
-                       @"Setting circleStrokeWidth to a composite function should update circle-stroke-width.");
-        XCTAssertEqualObjects(layer.circleStrokeWidth, functionStyleValue,
-                              @"circleStrokeWidth should round-trip composite functions.");                                                                                                          
+                       @"Setting circleStrokeWidth to a camera-data expression should update circle-stroke-width.");
+        XCTAssertEqualObjects(layer.circleStrokeWidth, functionExpression,
+                              @"circleStrokeWidth should round-trip camera-data expressions.");
                               
 
         layer.circleStrokeWidth = nil;
         XCTAssertTrue(rawLayer->getCircleStrokeWidth().isUndefined(),
                       @"Unsetting circleStrokeWidth should return circle-stroke-width to the default value.");
-        XCTAssertEqualObjects(layer.circleStrokeWidth, defaultStyleValue,
+        XCTAssertEqualObjects(layer.circleStrokeWidth, defaultExpression,
                               @"circleStrokeWidth should return the default value after being unset.");
         // Transition property test
         layer.circleStrokeWidthTransition = transitionTest;
@@ -592,84 +630,94 @@
     {
         XCTAssertTrue(rawLayer->getCircleTranslate().isUndefined(),
                       @"circle-translate should be unset initially.");
-        MGLStyleValue<NSValue *> *defaultStyleValue = layer.circleTranslation;
+        NSExpression *defaultExpression = layer.circleTranslation;
 
-        MGLStyleValue<NSValue *> *constantStyleValue = [MGLStyleValue<NSValue *> valueWithRawValue:
+        NSExpression *constantExpression = [NSExpression expressionWithFormat:@"%@",
 #if TARGET_OS_IPHONE
             [NSValue valueWithCGVector:CGVectorMake(1, 1)]
 #else
             [NSValue valueWithMGLVector:CGVectorMake(1, -1)]
 #endif
         ];
-        layer.circleTranslation = constantStyleValue;
+        layer.circleTranslation = constantExpression;
         mbgl::style::PropertyValue<std::array<float, 2>> propertyValue = { { 1, 1 } };
         XCTAssertEqual(rawLayer->getCircleTranslate(), propertyValue,
-                       @"Setting circleTranslation to a constant value should update circle-translate.");
-        XCTAssertEqualObjects(layer.circleTranslation, constantStyleValue,
-                              @"circleTranslation should round-trip constant values.");
-
-        MGLStyleValue<NSValue *> * functionStyleValue = [MGLStyleValue<NSValue *> valueWithInterpolationMode:MGLInterpolationModeInterval cameraStops:@{@18: constantStyleValue} options:nil];
-        layer.circleTranslation = functionStyleValue;
-
-        mbgl::style::IntervalStops<std::array<float, 2>> intervalStops = { {{18, { 1, 1 }}} };
+                       @"Setting circleTranslation to a constant value expression should update circle-translate.");
+        XCTAssertEqualObjects(layer.circleTranslation, constantExpression,
+                              @"circleTranslation should round-trip constant value expressions.");
+
+        constantExpression = [NSExpression expressionWithFormat:@"{1, 1}"];
+        NSExpression *functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_stepWithMinimum:stops:', %@, %@)", constantExpression, @{@18: constantExpression}];
+        layer.circleTranslation = functionExpression;
+
+        mbgl::style::IntervalStops<std::array<float, 2>> intervalStops = {{
+            { -INFINITY, { 1, 1 } },
+            { 18, { 1, 1 } },
+        }};
         propertyValue = mbgl::style::CameraFunction<std::array<float, 2>> { intervalStops };
         
         XCTAssertEqual(rawLayer->getCircleTranslate(), propertyValue,
-                       @"Setting circleTranslation to a camera function should update circle-translate.");
-        XCTAssertEqualObjects(layer.circleTranslation, functionStyleValue,
-                              @"circleTranslation should round-trip camera functions.");
+                       @"Setting circleTranslation to a camera expression should update circle-translate.");
+        XCTAssertEqualObjects(layer.circleTranslation, functionExpression,
+                              @"circleTranslation should round-trip camera expressions.");
 
                               
 
         layer.circleTranslation = nil;
         XCTAssertTrue(rawLayer->getCircleTranslate().isUndefined(),
                       @"Unsetting circleTranslation should return circle-translate to the default value.");
-        XCTAssertEqualObjects(layer.circleTranslation, defaultStyleValue,
+        XCTAssertEqualObjects(layer.circleTranslation, defaultExpression,
                               @"circleTranslation should return the default value after being unset.");
 
-        functionStyleValue = [MGLStyleValue<NSValue *> valueWithInterpolationMode:MGLInterpolationModeIdentity sourceStops:nil attributeName:@"" options:nil];
-        XCTAssertThrowsSpecificNamed(layer.circleTranslation = functionStyleValue, NSException, NSInvalidArgumentException, @"MGLStyleValue should raise an exception if it is applied to a property that cannot support it");
-        functionStyleValue = [MGLStyleValue<NSValue *> valueWithInterpolationMode:MGLInterpolationModeInterval compositeStops:@{@18: constantStyleValue} attributeName:@"" options:nil];
-        XCTAssertThrowsSpecificNamed(layer.circleTranslation = functionStyleValue, NSException, NSInvalidArgumentException, @"MGLStyleValue should raise an exception if it is applied to a property that cannot support it");
+        functionExpression = [NSExpression expressionForKeyPath:@"bogus"];
+        XCTAssertThrowsSpecificNamed(layer.circleTranslation = functionExpression, NSException, NSInvalidArgumentException, @"MGLCircleLayer should raise an exception if a camera-data expression is applied to a property that does not support key paths to feature attributes.");
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION(bogus, 'mgl_stepWithMinimum:stops:', %@, %@)", constantExpression, @{@18: constantExpression}];
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_interpolateWithCurveType:parameters:stops:', 'linear', nil, %@)", @{@10: functionExpression}];
+        XCTAssertThrowsSpecificNamed(layer.circleTranslation = functionExpression, NSException, NSInvalidArgumentException, @"MGLCircleLayer should raise an exception if a camera-data expression is applied to a property that does not support key paths to feature attributes.");
     }
 
     // circle-translate-anchor
     {
         XCTAssertTrue(rawLayer->getCircleTranslateAnchor().isUndefined(),
                       @"circle-translate-anchor should be unset initially.");
-        MGLStyleValue<NSValue *> *defaultStyleValue = layer.circleTranslationAnchor;
+        NSExpression *defaultExpression = layer.circleTranslationAnchor;
 
-        MGLStyleValue<NSValue *> *constantStyleValue = [MGLStyleValue<NSValue *> valueWithRawValue:[NSValue valueWithMGLCircleTranslationAnchor:MGLCircleTranslationAnchorViewport]];
-        layer.circleTranslationAnchor = constantStyleValue;
+        NSExpression *constantExpression = [NSExpression expressionWithFormat:@"'viewport'"];
+        layer.circleTranslationAnchor = constantExpression;
         mbgl::style::PropertyValue<mbgl::style::TranslateAnchorType> propertyValue = { mbgl::style::TranslateAnchorType::Viewport };
         XCTAssertEqual(rawLayer->getCircleTranslateAnchor(), propertyValue,
-                       @"Setting circleTranslationAnchor to a constant value should update circle-translate-anchor.");
-        XCTAssertEqualObjects(layer.circleTranslationAnchor, constantStyleValue,
-                              @"circleTranslationAnchor should round-trip constant values.");
-
-        MGLStyleValue<NSValue *> * functionStyleValue = [MGLStyleValue<NSValue *> valueWithInterpolationMode:MGLInterpolationModeInterval cameraStops:@{@18: constantStyleValue} options:nil];
-        layer.circleTranslationAnchor = functionStyleValue;
-
-        mbgl::style::IntervalStops<mbgl::style::TranslateAnchorType> intervalStops = { {{18, mbgl::style::TranslateAnchorType::Viewport}} };
+                       @"Setting circleTranslationAnchor to a constant value expression should update circle-translate-anchor.");
+        XCTAssertEqualObjects(layer.circleTranslationAnchor, constantExpression,
+                              @"circleTranslationAnchor should round-trip constant value expressions.");
+
+        constantExpression = [NSExpression expressionWithFormat:@"'viewport'"];
+        NSExpression *functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_stepWithMinimum:stops:', %@, %@)", constantExpression, @{@18: constantExpression}];
+        layer.circleTranslationAnchor = functionExpression;
+
+        mbgl::style::IntervalStops<mbgl::style::TranslateAnchorType> intervalStops = {{
+            { -INFINITY, mbgl::style::TranslateAnchorType::Viewport },
+            { 18, mbgl::style::TranslateAnchorType::Viewport },
+        }};
         propertyValue = mbgl::style::CameraFunction<mbgl::style::TranslateAnchorType> { intervalStops };
         
         XCTAssertEqual(rawLayer->getCircleTranslateAnchor(), propertyValue,
-                       @"Setting circleTranslationAnchor to a camera function should update circle-translate-anchor.");
-        XCTAssertEqualObjects(layer.circleTranslationAnchor, functionStyleValue,
-                              @"circleTranslationAnchor should round-trip camera functions.");
+                       @"Setting circleTranslationAnchor to a camera expression should update circle-translate-anchor.");
+        XCTAssertEqualObjects(layer.circleTranslationAnchor, functionExpression,
+                              @"circleTranslationAnchor should round-trip camera expressions.");
 
                               
 
         layer.circleTranslationAnchor = nil;
         XCTAssertTrue(rawLayer->getCircleTranslateAnchor().isUndefined(),
                       @"Unsetting circleTranslationAnchor should return circle-translate-anchor to the default value.");
-        XCTAssertEqualObjects(layer.circleTranslationAnchor, defaultStyleValue,
+        XCTAssertEqualObjects(layer.circleTranslationAnchor, defaultExpression,
                               @"circleTranslationAnchor should return the default value after being unset.");
 
-        functionStyleValue = [MGLStyleValue<NSValue *> valueWithInterpolationMode:MGLInterpolationModeIdentity sourceStops:nil attributeName:@"" options:nil];
-        XCTAssertThrowsSpecificNamed(layer.circleTranslationAnchor = functionStyleValue, NSException, NSInvalidArgumentException, @"MGLStyleValue should raise an exception if it is applied to a property that cannot support it");
-        functionStyleValue = [MGLStyleValue<NSValue *> valueWithInterpolationMode:MGLInterpolationModeInterval compositeStops:@{@18: constantStyleValue} attributeName:@"" options:nil];
-        XCTAssertThrowsSpecificNamed(layer.circleTranslationAnchor = functionStyleValue, NSException, NSInvalidArgumentException, @"MGLStyleValue should raise an exception if it is applied to a property that cannot support it");
+        functionExpression = [NSExpression expressionForKeyPath:@"bogus"];
+        XCTAssertThrowsSpecificNamed(layer.circleTranslationAnchor = functionExpression, NSException, NSInvalidArgumentException, @"MGLCircleLayer should raise an exception if a camera-data expression is applied to a property that does not support key paths to feature attributes.");
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION(bogus, 'mgl_stepWithMinimum:stops:', %@, %@)", constantExpression, @{@18: constantExpression}];
+        functionExpression = [NSExpression expressionWithFormat:@"FUNCTION($zoomLevel, 'mgl_interpolateWithCurveType:parameters:stops:', 'linear', nil, %@)", @{@10: functionExpression}];
+        XCTAssertThrowsSpecificNamed(layer.circleTranslationAnchor = functionExpression, NSException, NSInvalidArgumentException, @"MGLCircleLayer should raise an exception if a camera-data expression is applied to a property that does not support key paths to feature attributes.");
     }
 }