[ios, macos] Addressed Minh's feedback

6 files changed, 487 insertions, 179 deletions

diff --git a/platform/darwin/docs/guides/Migrating to Expressions.md.ejs b/platform/darwin/docs/guides/Migrating to Expressions.md.ejs
new file mode 100644
index 0000000000..b7544c33e1
--- /dev/null
+++ b/platform/darwin/docs/guides/Migrating to Expressions.md.ejs
@@ -0,0 +1,110 @@
+<%
+  const os = locals.os;
+  const iOS = os === 'iOS';
+  const macOS = os === 'macOS';
+  const cocoaPrefix = iOS ? 'UI' : 'NS';
+  const guide = 'MigratingToExpressions';
+-%>
+<!--
+  This file is generated.
+  Edit platform/darwin/scripts/generate-style-code.js, then run `make darwin-style-code`.
+-->
+
+# Migrating from Style Functions to Expressions
+
+[Runtime Styling](runtime-styling.html) enables you to modify every aspect of the map’s appearance dynamically as a user interacts with your application. Users can specify in advance how a layout or paint attribute will vary as the zoom level changes or how the appearance of individual features vary based on metadata provided by a content source. With Maps SDK of iOS v4.0.0, style functions have been replaced with expressions. These provide even more tools for users who want to style their maps dynamically. This guide outlines some tips for migrating from style functions to expressions, and offers an overview of some things that users can do with expressions.
+
+An expression is represented at runtime by the `NSExpression` class. Expressions can be used to style paint and layout properties based on zoom level, data attributes, or a combination of the two.
+
+A constant expression can also be assigned to a style property. For example, the opacity of a fill style layer can be set to a constant value between 0 and 1.
+
+The documentation for each individual style layer property notes which non-constant expressions are enabled for that property.
+
+Style functions supported four interpolation modes: exponential, interval, categorical, and identity. This guide uses earthquake data from the [U.S. Geological Survey](https://earthquake.usgs.gov/earthquakes/feed/v1.0/geojson.php. Under each interpolation mode, the style function implementation will be shown, followed by the current syntax.
+
+For more information about how to work with GeoJSON data in our iOS SDK, please see our [working with GeoJSON data](working-with-geojson-data.html) guide. To learn more about supported expressions, see our [Predicates and Expressions](predicates-and-expressions.html) guide. The Predicates and Expressions guide also outlines Mapbox custom functions that can be used to dynamically style a map.
+
+Mapbox supports some convenience methods that can be used in the place of format string syntax.
+
+## Stops
+Stops are dictionary keys that are associated with layer attribute values. You can use feature attribute values, zoom levels, and heatmap point density as keys, with an constant value or expression for the value. For example, you can use a stop dictionary with the zoom levels 0, 10, and 20 as keys and the colors yellow, orange, and red as the values. Constant values no longer need to be wrapped as style values.
+
+<%- guideExample(guide, 'Stops', os) %>
+
+
+## Interpolation mode
+
+Style functions supported four interpolation modes: exponential/linear, interval, categorical, and identity. For more information about
+
+### Linear
+
+Previously, exponential interpolation mode handled both linear and exponential interpolation. These have been separated in v4.0.0. `mgl_interpolate:withCurveType:parameters:stops:` is a custom function that takes the interpolation type as a parameter. If you previously used the default interpolation base, use curve type `'linear'`.
+The stops dictionary below, for example, shows colors that continuously shift from yellow to orange to red to blue to white based on the attribute value.
+<%- guideExample(guide, 'Linear', os) %>
+
+![exponential mode](img/data-driven-styling/linear.png)
+
+### Exponential
+
+If you previously used an interpolation base greater than `0` (other than `1`), you can use `'exponential'` as the curve type for `mgl_interpolate:withCurveType:parameters:stops:`. The `parameters` argument takes that interpolation base. This interpolates between values exponentially, creating an accelerated ramp effect.
+
+Here’s a visualization from Mapbox Studio (see [Working with Mapbox Studio](working-with-mapbox-studio.html)) comparing interpolation base values of `1.5` and `0.5` based on zoom. In order to convert camera style functions, use `$zoomLevel` or `MGL_FUNCTION('zoomLevel')` as the attribute key.
+
+<img src="img/data-driven-styling/exponential-function.png" height=344/>
+<img src="img/data-driven-styling/exponential-function-1.png" height=344/>
+
+The example below increases a layer’s `circleRadius` exponentially based on a map’s zoom level. The interpolation base is `1.5`.
+<%- guideExample(guide, 'Exponential', os) %>
+
+### Interval
+
+Steps, or intervals, create a range using the keys from the stops dictionary. The range is from the given key to just less than the next key. The attribute values that fall into that range are then styled using the layout or paint value assigned to that key. You can use the format string `'mgl_step:from:stops:'` for cases where you previously used interval interpolation mode. The first parameter takes the feature attribute name and the second parameter (`from:`) optionally takes the default or fallback value for that function. The final parameter takes a stops dictionary as an argument. 
+
+When we use the stops dictionary given above with an `'mgl_step:from:stops:'`, we create ranges where earthquakes with a magnitude of 0 to just less than 2.5 would be yellow, 2.5 to just less than 5 would be orange, and so on.
+<%- guideExample(guide, 'Interval', os) %>
+
+![interval mode](img/data-driven-styling/interval.png)
+
+### Categorical
+
+Categorical interpolation mode took a stops dictionary. If the value for a specified feature attribute name matched one in that stops dictionary, the style value for that attribute value would be used. Categorical style functions can now be replaced with `MGL_MATCH()`.
+
+`MGL_MATCH` takes an initial condition, which in this case is an attribute key. This is follow by possible matches for that key and the value to assign to the layer property if there is a match. The final argument can be a default style value that is to be used if none of the specified values match.
+
+There are three main types of events in the USGS dataset: earthquakes, explosions, and quarry blasts. In this case, the color of the circle layer will be determined by the type of event, with a default value of blue to catch any events that do not fall into any of those categories.
+
+<%- guideExample(guide, 'Categorical', os) %>
+
+![categorical mode](img/data-driven-styling/categorical1.png) ![categorical mode](img/data-driven-styling/categorical2.png)
+
+### Identity
+
+Identity interpolation mode used the attribute’s value as the style layer property value. In this example, you might set the `circleRadius` to the earthquake’s magnitude. In order to use a feature attribute value to style a layer property, set the property value to `[NSExpression expressionForKeyPath:]`, which take the feature attribute name as an argument.
+
+<%- guideExample(guide, 'Identity', os) %>
+
+![identity mode](img/data-driven-styling/identity.png)
+
+Some built-in functions can be applied to attribute values to style layer property values. To set the circle radius to 3 times the earthquake’s magnitude, create a `multiply:by` function that takes the attribute value and the multiplier as arguments.
+
+<%- guideExample(guide, 'Multiply', os) %>
+
+![multiply magnitude](img/data-driven-styling/multiply.png)
+
+You can also cast attribute values in order to use them. One example is to cast an integer as an `NSString` and use it as a text value.
+
+<%- guideExample(guide, 'Cast', os) %>
+
+![cast a value](img/data-driven-styling/cast.png)
+
+## Constant Values
+<% if (iOS) { -%>
+
+For constant values that do not necessarily change based on camera or attribute values, use `[NSExpression expressionForConstantValue:]` (previously `[MGLStyleValue valueWithRawValue:]`).
+
+  <% } -%>
+## Resources
+
+* [USGS](https://earthquake.usgs.gov/earthquakes/feed/v1.0/geojson.php)
+* [For Style Authors](for-style-authors.html)
+* [Predicates and Expressions](predicates-and-expressions.html)
diff --git a/platform/darwin/docs/guides/Style Layers Using Expressions.md.ejs b/platform/darwin/docs/guides/Style Layers Using Expressions.md.ejs
deleted file mode 100644
index f80396bf89..0000000000
--- a/platform/darwin/docs/guides/Style Layers Using Expressions.md.ejs
+++ /dev/null
@@ -1,168 +0,0 @@
-<%
-  const os = locals.os;
-  const iOS = os === 'iOS';
-  const macOS = os === 'macOS';
-  const cocoaPrefix = iOS ? 'UI' : 'NS';
-  const guide = 'StyleLayersUsingExpressions';
--%>
-<!--
-  This file is generated.
-  Edit platform/darwin/scripts/generate-style-code.js, then run `make darwin-style-code`.
--->
-
-# Styling Layers Using Expressions
-
-[Runtime Styling](runtime-styling.html) enables you to modify every aspect of the map’s appearance dynamically as a user interacts with your application. Much of the runtime styling API allows you to specify *non-constant expressions*. A style expression allows users to specify in advance how a layout or paint attribute will vary as the zoom level changes or how the appearance of individual features vary based on metadata provided by a content source.
-
-Runtime styling removes the inconvenience of manually calculating intermediate values between different zoom levels or creating a multitude of style layers to handle homogeneous features in the map content. For example, if your content source indicates the prices of hotels in an area, you can color-code the hotels by price, relying on a data expression to smoothly interpolate among desired colors without having to specify the color for each exact price.
-*Data-driven styling* specifically refers to the use of non-constant expressions to vary the map’s appearance based on data in a content source.
-You can also specify expressions in a style JSON file, to be applied automatically when the map loads. See the [Mapbox Style Specification](https://www.mapbox.com/mapbox-gl-js/style-spec/#types-function) for details.
-
-This guide uses earthquake data from the [U.S. Geological Survey](https://earthquake.usgs.gov/earthquakes/feed/v1.0/geojson.php) and data-driven styling to style a map based on attributes. For more information about how to work with GeoJSON data in our iOS SDK, please see our [working with GeoJSON data](working-with-geojson-data.html) guide. To learn more about supported expressions functions, see our [Predicates and Expressions](predicates-and-expressions.html) guide. The Predicates and Expressions guide also outlines Mapbox custom functions that can be used to dynamically style a map.
-
-An expression is represented at runtime by the `NSExpression` class. There are several types of expressions that can be used to determined paint and layout values.
-
-- A *constant expression* assigns a set value for that style property. For example, the opacity of a fill style layer can be set to a constant value between 0 and 1.
-- A *camera expression* changes a style property with zoom level. For example, you can make the radius of a circle increase according to zoom level. These changes occur either through interpolation or step functions applied to the `$zoom` value.
-- A *data expression* changes a style property with the attributes of a feature. For example, you can adjust the radius of a circle based on the magnitude of an earthquake.
-- A *composite expression* changes a style property with both zoom level and attribute values. For example, you can add a circle layer where each circle has a radius based on both zoom level and the magnitude of an earthquake.
-
-The documentation for each individual style layer property notes which non-constant expressions are enabled for that property.
-
-## Stops
-
-tops are dictionary keys that are associated with layer attribute values. You can use feature attribute values, zoom levels, and heatmap point density as keys, with an constant value or expression for the value. For example, you can use a stop dictionary with the zoom levels 0, 10, and 20 as keys and the colors yellow, orange, and red as the values.
-<%- guideExample(guide, 'Stops', os) %>
-
-Matched values can be used in the place of stops dictionaries with categorical interpolation. The `MGL_MATCH()` custom function creates feature and layer attribute value pairs.
-## Interpolation mode
-
-The effect a key has on the layer style value is determined by the interpolation mode. There are four interpolation modes that can be used with a style expression: exponential/linear, interval, categorical, and identity. You can also use exponential/linear and categorical interpolation modes with a camera expression.
-### Linear
-
-`MGLExpressionInterpolationModeLinear` interpolates linearly between stop values.
-The stops dictionary below, for example, shows colors that continuously shift from yellow to orange to red to blue to white based on the attribute value.
-<%- guideExample(guide, 'Linear', os) %>
-
-![exponential mode](img/data-driven-styling/linear.png)
-
-### Exponential
-
-By combining `MGLExpressionInterpolationModeExponential` with an interpolation base greater than `0` (other than `1`), you can interpolate between values exponentially, create an accelerated ramp effect.
-Here’s a visualization from Mapbox Studio (see [Working with Mapbox Studio](working-with-mapbox-studio.html)) comparing interpolation base values of `1.5` and `0.5` based on zoom.
-
-<img src="img/data-driven-styling/exponential-function.png" height=344/>
-<img src="img/data-driven-styling/exponential-function-1.png" height=344/>
-
-The example below increases a layer’s `circleRadius` exponentially based on a map’s zoom level. The interpolation base is `1.5`.
-<%- guideExample(guide, 'Exponential', os) %>
-
-### Interval
-
-Steps, or intervals, create a range using the keys from the stops dictionary. The range is from the given key to just less than the next key. The attribute values that fall into that range are then styled using the layout or paint value assigned to that key.
-
-When we use the stops dictionary given above with an interval interpolation mode, we create ranges where earthquakes with a magnitude of 0 to just less than 2.5 would be yellow, 2.5 to just less than 5 would be orange, and so on.
-<%- guideExample(guide, 'Interval', os) %>
-
-![interval mode](img/data-driven-styling/interval.png)
-
-### Categorical
-
-At each stop, a categorical, or match expression, produces an output value equal to the function input. We’re going to match different feature attribute and style layer property values than we did for the previous two modes.
-
-There are three main types of events in the USGS dataset: earthquakes, explosions, and quarry blasts. In this case, the color of the circle layer will be determined by the type of event, with a default value of blue to catch any events that do not fall into any of those categories.
-
-<%- guideExample(guide, 'Categorical', os) %>
-
-`MGL_MATCH` takes the attribute key, followed by possible attribute and style value pairs. The final argument is a default style value.
-
-![categorical mode](img/data-driven-styling/categorical1.png) ![categorical mode](img/data-driven-styling/categorical2.png)
-
-### Identity
-
-Identity interpolation mode uses the attribute’s value as the style layer property value. For example, you can set the `circleRadius` to the earthquake’s magnitude. Since the attribute value itself will be used as the style value, `sourceStops` should be set to `nil`.
-
-<%- guideExample(guide, 'Identity', os) %>
-
-![identity mode](img/data-driven-styling/identity.png)
-
-Some built-in expressions functions can be applied to attribute values to style layer property values. To set the circle radius to 3 times the earthquake’s magnitude, create a `multiply:by` function that takes the attribute value and the multiplier as arguments.
-
-<%- guideExample(guide, 'Multiply', os) %>
-
-![multiply magnitude](img/data-driven-styling/multiply.png)
-
-Through expressions, you can also cast attribute values in order to use them. One example is to cast an integer as an `NSString` and use it as a text value.
-
-<%- guideExample(guide, 'Cast', os) %>
-
-![cast a value](img/data-driven-styling/cast.png)
-
-<% if (iOS) { -%>
-
-  ##Migrating from Style Functions to Expressions
-
-  For constant values that do not necessarily change based on camera or attribute values, use `[NSExpression expressionForConstantValue:]` (previously `[MGLStyleValue valueWithRawValue:]`). Expressions support format string syntax, as shown above, as well as convenience methods that include those listed below.
-
-
-<table>
-<thead>
-<tr><th>Interpolation mode</th><th>Syntax prior to v4.0.0</th><th>Current syntax</th></tr>
-</thead>
-<tbody>
-<tr>
-  <td>Linear </td>
-  <td><code>MGLStyleValue(interpolationMode: .exponential,
-                                        sourceStops: stops,
-                                        attributeName: "mag",
-                                        options: nil) </code></td>
-  <td><code>NSExpression(forMGLInterpolating: NSExpression(forKeyPath: "mag"),
-                  curveType: MGLExpressionInterpolationMode.linear,
-                  parameters: nil,
-                  stops: NSExpression(forConstantValue: stops)) </code></td>
-</tr>
-<tr>
-  <td>Exponential</td>
-  <td><code>MGLStyleValue(interpolationMode: .exponential,
-                                        sourceStops: stops,
-                                        attributeName: “mag”,
-                                         options:  [.interpolationBase: 1.5])</code></td>
-  <td><code>layer.circleRadius = NSExpression(format: "FUNCTION($zoomLevel, 'mgl_interpolateWithCurveType:parameters:stops:', 'exponential', 1.5, %@)",
-                                          stops)</code></td>
-</tr>
-<tr>
-  <td>Interval</td>
-  <td><code>MGLStyleValue(interpolationMode: .interval,
-                                        sourceStops: [0: UIColor.yellow, 2.5: UIColor.orange],
-                                        attributeName: "mag",
-                                        options: [.defaultValue: MGLStyleValue<UIColor>(rawValue: .green)])</code></td>
-  <td><code>NSExpression(forMGLStepping: NSExpression(forKeyPath: "mag"),
-                                   from: NSExpression(forConstantValue: UIColor.green),
-                                  stops: [0: UIColor.yellow, 2.5: UIColor.orange])</code></td>
-</tr>
-<tr>
-  <td>Categorical</td>
-  <td><code>MGLStyleValue(interpolationMode: *.*categorical,
-                                    sourceStops: [featureValue : UIColor.green],
-                                    attributeName: "type",
-                                    options: [*.*defaultValue: MGLStyleValue*<*UIColor*>*(rawValue: *.*blue)])</code></td>
-  <td><code>NSExpression(format: “MGL_MATCH(type, ‘earthquake’, %@,  %@)”, UIColor.green, UIColor.blue) </code></td>
-</tr>
-<tr>
-  <td>Identity</td>
-  <td><code>MGLStyleValue(interpolationMode: .identity,
-                                     sourceStops: nil,
-                                     attributeName: "mag",
-                                     options: nil)</code></td>
-  <td><code>NSExpression(forConstantValue: “mag”)</code></td>
-</tbody>
-</table>
-
-  In order to convert camera style functions, use `$zoomLevel` or `MGL_FUNCTION('zoom')` as the attribute key.
-
-  <% } -%>
-##Resources
-
-* [USGS](https://earthquake.usgs.gov/earthquakes/feed/v1.0/geojson.php)
-* [For Style Authors](for-style-authors.html)
-* [Predicates and Expressions](predicates-and-expressions.html)
diff --git a/platform/darwin/scripts/generate-style-code.js b/platform/darwin/scripts/generate-style-code.js
index 384ee0012c..c7b54b326a 100755
--- a/platform/darwin/scripts/generate-style-code.js
+++ b/platform/darwin/scripts/generate-style-code.js
@@ -626,7 +626,7 @@ const layerH = ejs.compile(fs.readFileSync('platform/darwin/src/MGLStyleLayer.h.
 const layerM = ejs.compile(fs.readFileSync('platform/darwin/src/MGLStyleLayer.mm.ejs', 'utf8'), { strict: true});
 const testLayers = ejs.compile(fs.readFileSync('platform/darwin/test/MGLStyleLayerTests.mm.ejs', 'utf8'), { strict: true});
 const forStyleAuthorsMD = ejs.compile(fs.readFileSync('platform/darwin/docs/guides/For Style Authors.md.ejs', 'utf8'), { strict: true });
-const ddsGuideMD = ejs.compile(fs.readFileSync('platform/darwin/docs/guides/Style Layers Using Expressions.md.ejs', 'utf8'), { strict: true });
+const ddsGuideMD = ejs.compile(fs.readFileSync('platform/darwin/docs/guides/Migrating to Expressions.md.ejs', 'utf8'), { strict: true });
 const templatesMD = ejs.compile(fs.readFileSync('platform/darwin/docs/guides/Tile URL Templates.md.ejs', 'utf8'), { strict: true });
 
 const lightH = ejs.compile(fs.readFileSync('platform/darwin/src/MGLLight.h.ejs', 'utf8'), {strict: true});
@@ -757,10 +757,10 @@ writeIfModified(`platform/macos/docs/guides/For Style Authors.md`, forStyleAutho
     renamedProperties: renamedPropertiesByLayerType,
     layers: layers,
 }));
-writeIfModified(`platform/ios/docs/guides/Style Layers Using Expressions.md`, ddsGuideMD({
+writeIfModified(`platform/ios/docs/guides/Migrating to Expressions.md`, ddsGuideMD({
     os: 'iOS',
 }));
-writeIfModified(`platform/macos/docs/guides/Style Layers Using Expressions.md`, ddsGuideMD({
+writeIfModified(`platform/macos/docs/guides/Migrating to Expressions.md`, ddsGuideMD({
     os: 'macOS',
 }));
 writeIfModified(`platform/ios/docs/guides/Tile URL Templates.md`, templatesMD({
diff --git a/platform/darwin/test/MGLDocumentationGuideTests.swift b/platform/darwin/test/MGLDocumentationGuideTests.swift
index df610a8c44..5c0d8164d3 100644
--- a/platform/darwin/test/MGLDocumentationGuideTests.swift
+++ b/platform/darwin/test/MGLDocumentationGuideTests.swift
@@ -50,7 +50,7 @@ class MGLDocumentationGuideTests: XCTestCase, MGLMapViewDelegate {
         styleLoadingExpectation.fulfill()
     }
     
-    func testStyleLayersUsingExpressions$Stops() {
+    func testMigratingToExpressions$Stops() {
         //#-example-code
         #if os(macOS)
             let stops: [NSNumber: NSColor] = [
@@ -75,7 +75,7 @@ class MGLDocumentationGuideTests: XCTestCase, MGLMapViewDelegate {
                              stops[0]!, stops)
     }
     
-    func testStyleLayersUsingExpressions$Linear() {
+    func testMigratingToExpressions$Linear() {
         //#-example-code
         let url = URL(string: "https://earthquake.usgs.gov/earthquakes/feed/v1.0/summary/all_week.geojson")!
         let symbolSource = MGLSource(identifier: "source")
@@ -115,7 +115,37 @@ class MGLDocumentationGuideTests: XCTestCase, MGLMapViewDelegate {
         //#-end-example-code
     }
     
-    func testStyleLayersUsingExpressions$Exponential() {
+    func testMigratingToExpressions$LinearConvenience() {
+        let source = MGLShapeSource(identifier: "circles", shape: nil, options: nil)
+        let layer = MGLCircleStyleLayer(identifier: "circles", source: source)
+        
+        #if os(macOS)
+        let stops: [Float: NSColor] = [
+            0: .yellow,
+            2.5: .orange,
+            5: .red,
+            7.5: .blue,
+            10: .white,
+            ]
+        #else
+        let stops: [Float: UIColor] = [
+            0: .yellow,
+            2.5: .orange,
+            5: .red,
+            7.5: .blue,
+            10: .white,
+            ]
+        #endif
+        
+        //#-example-code
+        layer.circleColor = NSExpression(forMGLInterpolating: NSExpression(forKeyPath: "mag"), curveType: MGLExpressionInterpolationMode.linear, parameters: nil, stops: NSExpression(forConstantValue: stops))
+        //#-end-example-code
+        
+        layer.circleRadius = NSExpression(forConstantValue: 10)
+        mapView.style?.insertLayer(layer, below: symbolLayer)
+        
+    }
+    func testMigratingToExpressions$Exponential() {
         let source = MGLShapeSource(identifier: "circles", shape: nil, options: nil)
         let layer = MGLCircleStyleLayer(identifier: "circles", source: source)
         
@@ -131,7 +161,21 @@ class MGLDocumentationGuideTests: XCTestCase, MGLMapViewDelegate {
         //#-end-example-code
     }
     
-    func testStyleLayersUsingExpressions$Interval() {
+    func testMigratingToExpressions$ExponentialConvenience() {
+        let source = MGLShapeSource(identifier: "circles", shape: nil, options: nil)
+        let layer = MGLCircleStyleLayer(identifier: "circles", source: source)
+        
+        //#-example-code
+        let stops = [
+            12: 0.5,
+            14: 2,
+            18: 18,
+            ]
+        
+        layer.circleRadius =  NSExpression(forMGLInterpolating: NSExpression(forKeyPath: "$zoomLevel"), curveType: MGLExpressionInterpolationMode.exponential, parameters: NSExpression(forConstantValue: 1.5), stops: NSExpression(forConstantValue: stops))
+        //#-end-example-code
+    }
+    func testMigratingToExpressions$Interval() {
         let source = MGLShapeSource(identifier: "circles", shape: nil, options: nil)
         let layer = MGLCircleStyleLayer(identifier: "circles", source: source)
         
@@ -162,7 +206,7 @@ class MGLDocumentationGuideTests: XCTestCase, MGLMapViewDelegate {
         //#-end-example-code
     }
     
-    func testStyleLayersUsingExpressions$Categorical() {
+    func testMigratingToExpressions$Categorical() {
         let source = MGLShapeSource(identifier: "circles", shape: nil, options: nil)
         let layer = MGLCircleStyleLayer(identifier: "circles", source: source)
         
@@ -180,7 +224,7 @@ class MGLDocumentationGuideTests: XCTestCase, MGLMapViewDelegate {
         //#-end-example-code
     }
     
-    func testStyleLayersUsingExpressions$Identity() {
+    func testMigratingToExpressions$Identity() {
         let source = MGLShapeSource(identifier: "circles", shape: nil, options: nil)
         let layer = MGLCircleStyleLayer(identifier: "circles", source: source)
         
@@ -189,7 +233,7 @@ class MGLDocumentationGuideTests: XCTestCase, MGLMapViewDelegate {
         //#-end-example-code
     }
     
-    func testStyleLayersUsingExpressions$Multiply() {
+    func testMigratingToExpressions$Multiply() {
         let source = MGLShapeSource(identifier: "circles", shape: nil, options: nil)
         let layer = MGLCircleStyleLayer(identifier: "circles", source: source)
         
@@ -198,7 +242,7 @@ class MGLDocumentationGuideTests: XCTestCase, MGLMapViewDelegate {
         //#-end-example-code
     }
     
-    func testStyleLayersUsingExpressions$Cast() {
+    func testMigratingToExpressions$Cast() {
         let source = MGLShapeSource(identifier: "circles", shape: nil, options: nil)
         
         //#-example-code
diff --git a/platform/ios/docs/guides/Migrating to Expressions.md b/platform/ios/docs/guides/Migrating to Expressions.md
new file mode 100644
index 0000000000..976225edb9
--- /dev/null
+++ b/platform/ios/docs/guides/Migrating to Expressions.md
@@ -0,0 +1,162 @@
+<!--
+  This file is generated.
+  Edit platform/darwin/scripts/generate-style-code.js, then run `make darwin-style-code`.
+-->
+
+# Migrating from Style Functions to Expressions
+
+[Runtime Styling](runtime-styling.html) enables you to modify every aspect of the map’s appearance dynamically as a user interacts with your application. Users can specify in advance how a layout or paint attribute will vary as the zoom level changes or how the appearance of individual features vary based on metadata provided by a content source. With Maps SDK of iOS v4.0.0, style functions have been replaced with expressions. These provide even more tools for users who want to style their maps dynamically. This guide outlines some tips for migrating from style functions to expressions, and offers an overview of some things that users can do with expressions.
+
+An expression is represented at runtime by the `NSExpression` class. Expressions can be used to style paint and layout properties based on zoom level, data attributes, or a combination of the two.
+
+A constant expression can also be assigned to a style property. For example, the opacity of a fill style layer can be set to a constant value between 0 and 1.
+
+The documentation for each individual style layer property notes which non-constant expressions are enabled for that property.
+
+Style functions supported four interpolation modes: exponential, interval, categorical, and identity. This guide uses earthquake data from the [U.S. Geological Survey](https://earthquake.usgs.gov/earthquakes/feed/v1.0/geojson.php. Under each interpolation mode, the style function implementation will be shown, followed by the current syntax.
+
+For more information about how to work with GeoJSON data in our iOS SDK, please see our [working with GeoJSON data](working-with-geojson-data.html) guide. To learn more about supported expressions, see our [Predicates and Expressions](predicates-and-expressions.html) guide. The Predicates and Expressions guide also outlines Mapbox custom functions that can be used to dynamically style a map.
+
+Mapbox supports some convenience methods that can be used in the place of format string syntax.
+
+## Stops
+Stops are dictionary keys that are associated with layer attribute values. You can use feature attribute values, zoom levels, and heatmap point density as keys, with an constant value or expression for the value. For example, you can use a stop dictionary with the zoom levels 0, 10, and 20 as keys and the colors yellow, orange, and red as the values. Constant values no longer need to be wrapped as style values.
+
+```swift
+let stops: [NSNumber: UIColor] = [
+    0: .yellow,
+    2.5: .orange,
+    5: .red,
+    7.5: .blue,
+    10: .white,
+]
+```
+
+
+## Interpolation mode
+
+Style functions supported four interpolation modes: exponential/linear, interval, categorical, and identity. For more information about
+
+### Linear
+
+Previously, exponential interpolation mode handled both linear and exponential interpolation. These have been separated in v4.0.0. `mgl_interpolate:withCurveType:parameters:stops:` is a custom function that takes the interpolation type as a parameter. If you previously used the default interpolation base, use curve type `'linear'`.
+The stops dictionary below, for example, shows colors that continuously shift from yellow to orange to red to blue to white based on the attribute value.
+```swift
+let url = URL(string: "https://earthquake.usgs.gov/earthquakes/feed/v1.0/summary/all_week.geojson")!
+let symbolSource = MGLSource(identifier: "source")
+let symbolLayer = MGLSymbolStyleLayer(identifier: "place-city-sm", source: symbolSource)
+
+let source = MGLShapeSource(identifier: "earthquakes", url: url, options: nil)
+mapView.style?.addSource(source)
+
+let stops: [Float: UIColor] = [
+    0: .yellow,
+    2.5: .orange,
+    5: .red,
+    7.5: .blue,
+    10: .white,
+]
+
+let layer = MGLCircleStyleLayer(identifier: "circles", source: source)
+layer.circleColor = NSExpression(format: "mgl_interpolate:withCurveType:parameters:stops:(mag, 'linear', nil, %@)",
+                                 stops)
+layer.circleRadius = NSExpression(forConstantValue: 10)
+mapView.style?.insertLayer(layer, below: symbolLayer)
+```
+
+![exponential mode](img/data-driven-styling/linear.png)
+
+### Exponential
+
+If you previously used an interpolation base greater than `0` (other than `1`), you can use `'exponential'` as the curve type for `mgl_interpolate:withCurveType:parameters:stops:`. The `parameters` argument takes that interpolation base. This interpolates between values exponentially, creating an accelerated ramp effect.
+
+Here’s a visualization from Mapbox Studio (see [Working with Mapbox Studio](working-with-mapbox-studio.html)) comparing interpolation base values of `1.5` and `0.5` based on zoom. In order to convert camera style functions, use `$zoomLevel` or `MGL_FUNCTION('zoomLevel')` as the attribute key.
+
+<img src="img/data-driven-styling/exponential-function.png" height=344/>
+<img src="img/data-driven-styling/exponential-function-1.png" height=344/>
+
+The example below increases a layer’s `circleRadius` exponentially based on a map’s zoom level. The interpolation base is `1.5`.
+```swift
+let stops = [
+    12: 0.5,
+    14: 2,
+    18: 18,
+]
+
+layer.circleRadius = NSExpression(format: "mgl_interpolate:withCurveType:parameters:stops:($zoomLevel, 'exponential', 1.5, %@)",
+                                  stops)
+```
+
+### Interval
+
+Steps, or intervals, create a range using the keys from the stops dictionary. The range is from the given key to just less than the next key. The attribute values that fall into that range are then styled using the layout or paint value assigned to that key. You can use the format string `'mgl_step:from:stops:'` for cases where you previously used interval interpolation mode. The first parameter takes the feature attribute name and the second parameter (`from:`) optionally takes the default or fallback value for that function. The final parameter takes a stops dictionary as an argument. 
+
+When we use the stops dictionary given above with an `'mgl_step:from:stops:'`, we create ranges where earthquakes with a magnitude of 0 to just less than 2.5 would be yellow, 2.5 to just less than 5 would be orange, and so on.
+```swift
+let stops: [Float: UIColor] = [
+    0: .yellow,
+    2.5: .orange,
+    5: .red,
+    7.5: .blue,
+    10: .white,
+]
+
+layer.circleColor = NSExpression(format: "mgl_step:from:stops:(mag, %@, %@)",
+                                 UIColor.green, stops)
+```
+
+![interval mode](img/data-driven-styling/interval.png)
+
+### Categorical
+
+Categorical interpolation mode took a stops dictionary. If the value for a specified feature attribute name matched one in that stops dictionary, the style value for that attribute value would be used. Categorical style functions can now be replaced with `MGL_MATCH()`.
+
+`MGL_MATCH` takes an initial condition, which in this case is an attribute key. This is follow by possible matches for that key and the value to assign to the layer property if there is a match. The final argument can be a default style value that is to be used if none of the specified values match.
+
+There are three main types of events in the USGS dataset: earthquakes, explosions, and quarry blasts. In this case, the color of the circle layer will be determined by the type of event, with a default value of blue to catch any events that do not fall into any of those categories.
+
+```swift
+let defaultColor = UIColor.blue
+layer.circleColor = NSExpression(format: "MGL_MATCH(type, 'earthquake', %@, 'explosion', %@, 'quarry blast', %@, %@)",
+    UIColor.orange, UIColor.red, UIColor.yellow, defaultColor)
+```
+
+![categorical mode](img/data-driven-styling/categorical1.png) ![categorical mode](img/data-driven-styling/categorical2.png)
+
+### Identity
+
+Identity interpolation mode used the attribute’s value as the style layer property value. In this example, you might set the `circleRadius` to the earthquake’s magnitude. In order to use a feature attribute value to style a layer property, set the property value to `[NSExpression expressionForKeyPath:]`, which take the feature attribute name as an argument.
+
+```swift
+layer.circleRadius = NSExpression(forKeyPath: "mag")
+```
+
+![identity mode](img/data-driven-styling/identity.png)
+
+Some built-in functions can be applied to attribute values to style layer property values. To set the circle radius to 3 times the earthquake’s magnitude, create a `multiply:by` function that takes the attribute value and the multiplier as arguments.
+
+```swift
+layer.circleRadius = NSExpression(forFunction: "multiply:by:", arguments: [NSExpression(forKeyPath: "mag"), 3])
+```
+
+![multiply magnitude](img/data-driven-styling/multiply.png)
+
+You can also cast attribute values in order to use them. One example is to cast an integer as an `NSString` and use it as a text value.
+
+```swift
+let magnitudeLayer = MGLSymbolStyleLayer(identifier: "mag-layer", source: source)
+magnitudeLayer.text = NSExpression(format: "CAST(mag, 'NSString')")
+mapView.style?.addLayer(magnitudeLayer)
+```
+
+![cast a value](img/data-driven-styling/cast.png)
+
+## Constant Values
+
+For constant values that do not necessarily change based on camera or attribute values, use `[NSExpression expressionForConstantValue:]` (previously `[MGLStyleValue valueWithRawValue:]`).
+
+  ## Resources
+
+* [USGS](https://earthquake.usgs.gov/earthquakes/feed/v1.0/geojson.php)
+* [For Style Authors](for-style-authors.html)
+* [Predicates and Expressions](predicates-and-expressions.html)
diff --git a/platform/macos/docs/guides/Migrating to Expressions.md b/platform/macos/docs/guides/Migrating to Expressions.md
new file mode 100644
index 0000000000..56cc313f93
--- /dev/null
+++ b/platform/macos/docs/guides/Migrating to Expressions.md
@@ -0,0 +1,160 @@
+<!--
+  This file is generated.
+  Edit platform/darwin/scripts/generate-style-code.js, then run `make darwin-style-code`.
+-->
+
+# Migrating from Style Functions to Expressions
+
+[Runtime Styling](runtime-styling.html) enables you to modify every aspect of the map’s appearance dynamically as a user interacts with your application. Users can specify in advance how a layout or paint attribute will vary as the zoom level changes or how the appearance of individual features vary based on metadata provided by a content source. With Maps SDK of iOS v4.0.0, style functions have been replaced with expressions. These provide even more tools for users who want to style their maps dynamically. This guide outlines some tips for migrating from style functions to expressions, and offers an overview of some things that users can do with expressions.
+
+An expression is represented at runtime by the `NSExpression` class. Expressions can be used to style paint and layout properties based on zoom level, data attributes, or a combination of the two.
+
+A constant expression can also be assigned to a style property. For example, the opacity of a fill style layer can be set to a constant value between 0 and 1.
+
+The documentation for each individual style layer property notes which non-constant expressions are enabled for that property.
+
+Style functions supported four interpolation modes: exponential, interval, categorical, and identity. This guide uses earthquake data from the [U.S. Geological Survey](https://earthquake.usgs.gov/earthquakes/feed/v1.0/geojson.php. Under each interpolation mode, the style function implementation will be shown, followed by the current syntax.
+
+For more information about how to work with GeoJSON data in our iOS SDK, please see our [working with GeoJSON data](working-with-geojson-data.html) guide. To learn more about supported expressions, see our [Predicates and Expressions](predicates-and-expressions.html) guide. The Predicates and Expressions guide also outlines Mapbox custom functions that can be used to dynamically style a map.
+
+Mapbox supports some convenience methods that can be used in the place of format string syntax.
+
+## Stops
+Stops are dictionary keys that are associated with layer attribute values. You can use feature attribute values, zoom levels, and heatmap point density as keys, with an constant value or expression for the value. For example, you can use a stop dictionary with the zoom levels 0, 10, and 20 as keys and the colors yellow, orange, and red as the values. Constant values no longer need to be wrapped as style values.
+
+```swift
+let stops: [NSNumber: NSColor] = [
+    0: .yellow,
+    2.5: .orange,
+    5: .red,
+    7.5: .blue,
+    10: .white,
+]
+```
+
+
+## Interpolation mode
+
+Style functions supported four interpolation modes: exponential/linear, interval, categorical, and identity. For more information about
+
+### Linear
+
+Previously, exponential interpolation mode handled both linear and exponential interpolation. These have been separated in v4.0.0. `mgl_interpolate:withCurveType:parameters:stops:` is a custom function that takes the interpolation type as a parameter. If you previously used the default interpolation base, use curve type `'linear'`.
+The stops dictionary below, for example, shows colors that continuously shift from yellow to orange to red to blue to white based on the attribute value.
+```swift
+let url = URL(string: "https://earthquake.usgs.gov/earthquakes/feed/v1.0/summary/all_week.geojson")!
+let symbolSource = MGLSource(identifier: "source")
+let symbolLayer = MGLSymbolStyleLayer(identifier: "place-city-sm", source: symbolSource)
+
+let source = MGLShapeSource(identifier: "earthquakes", url: url, options: nil)
+mapView.style?.addSource(source)
+
+let stops: [Float: NSColor] = [
+    0: .yellow,
+    2.5: .orange,
+    5: .red,
+    7.5: .blue,
+    10: .white,
+]
+
+let layer = MGLCircleStyleLayer(identifier: "circles", source: source)
+layer.circleColor = NSExpression(format: "mgl_interpolate:withCurveType:parameters:stops:(mag, 'linear', nil, %@)",
+                                 stops)
+layer.circleRadius = NSExpression(forConstantValue: 10)
+mapView.style?.insertLayer(layer, below: symbolLayer)
+```
+
+![exponential mode](img/data-driven-styling/linear.png)
+
+### Exponential
+
+If you previously used an interpolation base greater than `0` (other than `1`), you can use `'exponential'` as the curve type for `mgl_interpolate:withCurveType:parameters:stops:`. The `parameters` argument takes that interpolation base. This interpolates between values exponentially, creating an accelerated ramp effect.
+
+Here’s a visualization from Mapbox Studio (see [Working with Mapbox Studio](working-with-mapbox-studio.html)) comparing interpolation base values of `1.5` and `0.5` based on zoom. In order to convert camera style functions, use `$zoomLevel` or `MGL_FUNCTION('zoomLevel')` as the attribute key.
+
+<img src="img/data-driven-styling/exponential-function.png" height=344/>
+<img src="img/data-driven-styling/exponential-function-1.png" height=344/>
+
+The example below increases a layer’s `circleRadius` exponentially based on a map’s zoom level. The interpolation base is `1.5`.
+```swift
+let stops = [
+    12: 0.5,
+    14: 2,
+    18: 18,
+]
+
+layer.circleRadius = NSExpression(format: "mgl_interpolate:withCurveType:parameters:stops:($zoomLevel, 'exponential', 1.5, %@)",
+                                  stops)
+```
+
+### Interval
+
+Steps, or intervals, create a range using the keys from the stops dictionary. The range is from the given key to just less than the next key. The attribute values that fall into that range are then styled using the layout or paint value assigned to that key. You can use the format string `'mgl_step:from:stops:'` for cases where you previously used interval interpolation mode. The first parameter takes the feature attribute name and the second parameter (`from:`) optionally takes the default or fallback value for that function. The final parameter takes a stops dictionary as an argument. 
+
+When we use the stops dictionary given above with an `'mgl_step:from:stops:'`, we create ranges where earthquakes with a magnitude of 0 to just less than 2.5 would be yellow, 2.5 to just less than 5 would be orange, and so on.
+```swift
+let stops: [Float: NSColor] = [
+    0: .yellow,
+    2.5: .orange,
+    5: .red,
+    7.5: .blue,
+    10: .white,
+]
+
+layer.circleColor = NSExpression(format: "mgl_step:from:stops:(mag, %@, %@)",
+                                 NSColor.green, stops)
+```
+
+![interval mode](img/data-driven-styling/interval.png)
+
+### Categorical
+
+Categorical interpolation mode took a stops dictionary. If the value for a specified feature attribute name matched one in that stops dictionary, the style value for that attribute value would be used. Categorical style functions can now be replaced with `MGL_MATCH()`.
+
+`MGL_MATCH` takes an initial condition, which in this case is an attribute key. This is follow by possible matches for that key and the value to assign to the layer property if there is a match. The final argument can be a default style value that is to be used if none of the specified values match.
+
+There are three main types of events in the USGS dataset: earthquakes, explosions, and quarry blasts. In this case, the color of the circle layer will be determined by the type of event, with a default value of blue to catch any events that do not fall into any of those categories.
+
+```swift
+let defaultColor = NSColor.blue
+layer.circleColor = NSExpression(
+format: "MGL_MATCH(type, 'earthquake', %@, 'explosion', %@, 'quarry blast', %@, %@)",
+    NSColor.orange, NSColor.red, NSColor.yellow, defaultColor)
+```
+
+![categorical mode](img/data-driven-styling/categorical1.png) ![categorical mode](img/data-driven-styling/categorical2.png)
+
+### Identity
+
+Identity interpolation mode used the attribute’s value as the style layer property value. In this example, you might set the `circleRadius` to the earthquake’s magnitude. In order to use a feature attribute value to style a layer property, set the property value to `[NSExpression expressionForKeyPath:]`, which take the feature attribute name as an argument.
+
+```swift
+layer.circleRadius = NSExpression(forKeyPath: "mag")
+```
+
+![identity mode](img/data-driven-styling/identity.png)
+
+Some built-in functions can be applied to attribute values to style layer property values. To set the circle radius to 3 times the earthquake’s magnitude, create a `multiply:by` function that takes the attribute value and the multiplier as arguments.
+
+```swift
+layer.circleRadius = NSExpression(forFunction: "multiply:by:", arguments: [NSExpression(forKeyPath: "mag"), 3])
+```
+
+![multiply magnitude](img/data-driven-styling/multiply.png)
+
+You can also cast attribute values in order to use them. One example is to cast an integer as an `NSString` and use it as a text value.
+
+```swift
+let magnitudeLayer = MGLSymbolStyleLayer(identifier: "mag-layer", source: source)
+magnitudeLayer.text = NSExpression(format: "CAST(mag, 'NSString')")
+mapView.style?.addLayer(magnitudeLayer)
+```
+
+![cast a value](img/data-driven-styling/cast.png)
+
+## Constant Values
+## Resources
+
+* [USGS](https://earthquake.usgs.gov/earthquakes/feed/v1.0/geojson.php)
+* [For Style Authors](for-style-authors.html)
+* [Predicates and Expressions](predicates-and-expressions.html)