[ios, macos] Add NSExpression convenience constructors and helper methods. (#11278)

* [ios, macos] Refactor convinience expression initializers.

# Conflicts:
#	platform/darwin/src/NSExpression+MGLAdditions.h
#	platform/darwin/src/NSExpression+MGLAdditions.mm
#	platform/ios/app/MBXViewController.m

* [ios, macos] Add MGL_MATCH convenience method.

# Conflicts:
#	platform/darwin/src/NSExpression+MGLAdditions.h
#	platform/darwin/src/NSExpression+MGLAdditions.mm
#	platform/darwin/test/MGLExpressionTests.mm

* [ios, macos] Add Swift friendly convenience methods name syntax..

# Conflicts:
#	platform/darwin/src/NSExpression+MGLAdditions.h

* [ios, macos] Update runtime styling examples.

# Conflicts:
#	platform/ios/app/MBXViewController.m

* [ios, macos] Add NSExpression variable expressions properties.

* [ios, macos] Clarify convenience initializer names.

* [ios, macos] Update Style Authors guides.

* [ios, macos] Fix cherry-pick merge.

* [ios, macos] Update convenience initializers documentation.

7 files changed, 295 insertions, 43 deletions

diff --git a/platform/darwin/docs/guides/For Style Authors.md.ejs b/platform/darwin/docs/guides/For Style Authors.md.ejs
index 4fa70ff807..a3083571cf 100644
--- a/platform/darwin/docs/guides/For Style Authors.md.ejs
+++ b/platform/darwin/docs/guides/For Style Authors.md.ejs
@@ -339,9 +339,9 @@ In style specification | Method, function, or predicate type | Format string syn
 `to-number`            | `mgl_numberWithFallbackValues:` | `CAST(zipCode, 'NSNumber')`
 `to-string`            | `stringValue` | `CAST(ele, 'NSString')`
 `typeof`               | |
-`geometry-type`        | |`$geometryType`
-`id`                   | |`$featureIdentifier`
-`properties`           | |`$featureProperties`
+`geometry-type`        | `NSExpression.geometryTypeVariableExpression` | `$geometryType`
+`id`                   | `NSExpression.featureIdentifierVariableExpression` | `$featureIdentifier`
+`properties`           | `NSExpression.featurePropertiesVariableExpression` | `$featureProperties`
 `at`                   | `objectFrom:withIndex:` | `array[n]`
 `get`                  | `+[NSExpression expressionForKeyPath:]` | Key path
 `has`                  | `mgl_does:have:` | `mgl_does:have:(self, 'key')`
@@ -355,14 +355,14 @@ In style specification | Method, function, or predicate type | Format string syn
 `>=`                   | `NSGreaterThanOrEqualToPredicateOperatorType` | `key >= value`
 `all`                  | `NSAndPredicateType` | `p0 AND … AND pn`
 `any`                  | `NSOrPredicateType` | `p0 OR … OR pn`
-`case`                 | `+[NSExpression expressionForConditional:trueExpression:falseExpression:]` or `MGL_IF` | `TERNARY(1 = 2, YES, NO)` or `MGL_IF(1 = 2, YES, 2 = 2, YES, NO)`
+`case`                 | `+[NSExpression expressionForConditional:trueExpression:falseExpression:]` or `MGL_IF` or `+[NSExpression mgl_expressionForConditional:trueExpression:falseExpresssion:]` | `TERNARY(1 = 2, YES, NO)` or `MGL_IF(1 = 2, YES, 2 = 2, YES, NO)`
 `coalesce`             | `mgl_coalesce:` | `mgl_coalesce({x, y, z})`
-`match`                | `MGL_MATCH` | `MGL_MATCH(x, 0, 'zero match', 1, 'one match', 'two match', 'default')`
-`interpolate`          | `mgl_interpolate:withCurveType:parameters:stops:` |
-`step`                 | `mgl_step:withMinimum:stops:` |
+`match`                | `MGL_MATCH` or `+[NSExpression mgl_expressionForMatchingExpression:inDictionary:defaultExpression:]` | `MGL_MATCH(x, 0, 'zero match', 1, 'one match', 'two match', 'default')`
+`interpolate`          | `mgl_interpolate:withCurveType:parameters:stops:` or `+[NSExpression mgl_expressionForInterpolatingExpression:withCurveType:parameters:stops:]` |
+`step`                 | `mgl_step:withMinimum:stops:` or `+[NSExpression mgl_expressionForSteppingExpression:fromExpression:stops:]` |
 `let`                  | `mgl_expressionWithContext:` | `MGL_LET('ios', 11, 'macos', 10.13, $ios + $macos)`
 `var`                  | `+[NSExpression expressionForVariable:]` | `$variable`
-`concat`               | `mgl_join:` | `mgl_join({'Old', ' ', 'MacDonald'})`
+`concat`               | `mgl_join:` or `-[NSExpression mgl_expressionByAppendingExpression:]` | `mgl_join({'Old', ' ', 'MacDonald'})`
 `downcase`             | `lowercase:` | `lowercase('DOWNTOWN')`
 `upcase`               | `uppercase:` | `uppercase('Elysian Fields')`
 <% if (macOS) { -%>
@@ -398,8 +398,8 @@ In style specification | Method, function, or predicate type | Format string syn
 `sin`                  | |
 `sqrt`                 | `sqrt:` | `sqrt(2)`
 `tan`                  | |
-`zoom`                 | | `$zoom`
-`heatmap-density`      | | `$heatmapDensity`
+`zoom`                 | `NSExpression.zoomLevelVariableExpression` | `$zoom`
+`heatmap-density`      | `NSExpression.heatmapDensityVariableExpression` | `$heatmapDensity`
 
 For operators that have no corresponding `NSExpression` symbol, use the
 `MGL_FUNCTION()` format string syntax.
diff --git a/platform/darwin/src/NSExpression+MGLAdditions.h b/platform/darwin/src/NSExpression+MGLAdditions.h
index a0081cb428..39a841543d 100644
--- a/platform/darwin/src/NSExpression+MGLAdditions.h
+++ b/platform/darwin/src/NSExpression+MGLAdditions.h
@@ -9,6 +9,130 @@
 
 NS_ASSUME_NONNULL_BEGIN
 
+typedef NSString *MGLExpressionInterpolationMode NS_TYPED_ENUM;
+
+/**
+ An `NSString` identifying the `linear` interpolation type in an `NSExpression`.
+ 
+ This attribute corresponds to the `linear` value in the
+ <a href="https://www.mapbox.com/mapbox-gl-js/style-spec/#expressions-interpolate"><code>interpolate</code></a>
+ expression reference in the Mapbox Style Specification.
+ */
+extern MGL_EXPORT const MGLExpressionInterpolationMode MGLExpressionInterpolationModeLinear;
+
+/**
+ An `NSString` identifying the `expotential` interpolation type in an `NSExpression`.
+ 
+ This attribute corresponds to the `exponential` value in the
+ <a href="https://www.mapbox.com/mapbox-gl-js/style-spec/#expressions-interpolate"><code>interpolate</code></a>
+ expression reference in the Mapbox Style Specification.
+ */
+extern MGL_EXPORT const MGLExpressionInterpolationMode MGLExpressionInterpolationModeExponential;
+
+/**
+ An `NSString` identifying the `cubic-bezier` interpolation type in an `NSExpression`.
+ 
+ This attribute corresponds to the `cubic-bezier` value in the
+ <a href="https://www.mapbox.com/mapbox-gl-js/style-spec/#expressions-interpolate"><code>interpolate</code></a>
+ expression reference in the Mapbox Style Specification.
+ */
+extern MGL_EXPORT const MGLExpressionInterpolationMode MGLExpressionInterpolationModeCubicBezier;
+
+@interface NSExpression (MGLVariableAdditions)
+
+/**
+ `NSExpression` variable that corresponds to the
+ <a href="https://www.mapbox.com/mapbox-gl-js/style-spec/#expressions-zoom"><code>zoom</code></a>
+ expression reference in the Mapbox Style Specification.
+ */
+@property (class, nonatomic, readonly) NSExpression *zoomLevelVariableExpression;
+
+/**
+ `NSExpression` variable that corresponds to the
+ <a href="https://www.mapbox.com/mapbox-gl-js/style-spec/#expressions-heatmap-density"><code>heatmap-density</code></a>
+ expression reference in the Mapbox Style Specification.
+ */
+@property (class, nonatomic, readonly) NSExpression *heatmapDensityVariableExpression;
+
+/**
+ `NSExpression` variable that corresponds to the
+ <a href="https://www.mapbox.com/mapbox-gl-js/style-spec/#eexpressions-geometry-type"><code>geometry-type</code></a>
+ expression reference in the Mapbox Style Specification.
+ */
+@property (class, nonatomic, readonly) NSExpression *geometryTypeVariableExpression;
+
+/**
+ `NSExpression` variable that corresponds to the
+ <a href="https://www.mapbox.com/mapbox-gl-js/style-spec/#expressions-id"><code>id</code></a>
+ expression reference in the Mapbox Style Specification.
+ */
+@property (class, nonatomic, readonly) NSExpression *featureIdentifierVariableExpression;
+
+/**
+ `NSExpression` variable that corresponds to the
+ <a href="https://www.mapbox.com/mapbox-gl-js/style-spec/#expressions-properties"><code>properties</code></a>
+ expression reference in the Mapbox Style Specification.
+ */
+@property (class, nonatomic, readonly) NSExpression *featurePropertiesVariableExpression;
+
+@end
+
+@interface NSExpression (MGLInitializerAdditions)
+
+/**
+ Returns a conditional function expression specifying the string predicate, and
+ expressions for each condition.
+ 
+ @param conditionPredicate The predicate to get evaluated.
+ @param trueExpression The expression for conditions equal to true.
+ @param falseExpression The expression for conditions equal to false.
+ */
++ (instancetype)mgl_expressionForConditional:(nonnull NSPredicate *)conditionPredicate trueExpression:(nonnull NSExpression *)trueExpression falseExpresssion:(nonnull NSExpression *)falseExpression NS_SWIFT_NAME(init(forMGLConditional:trueExpression:falseExpression:));
+
+/**
+ Returns a step function expression specifying the stepping, from expression
+ and stops.
+ 
+ @param steppingExpression The stepping expression.
+ @param minimumExpression The expression which could be a constant or function expression.
+ @param stops The stops must be an `NSDictionary` constant `NSExpression`.
+ */
++ (instancetype)mgl_expressionForSteppingExpression:(nonnull NSExpression*)steppingExpression fromExpression:(nonnull NSExpression *)minimumExpression stops:(nonnull NSExpression*)stops NS_SWIFT_NAME(init(forMGLStepping:from:stops:));
+
+/**
+ Returns an interpolated function expression specifying the function operator, curve type,
+ parameters and steps.
+ 
+ @param inputExpression The interpolating expression input.
+ @param curveType The curve type could be `MGLExpressionInterpolationModeLinear`,
+ `MGLExpressionInterpolationModeExponential` and
+ `MGLExpressionInterpolationModeCubicBezier`.
+ @param parameters The parameters expression.
+ @param stops The stops expression.
+ */
++ (instancetype)mgl_expressionForInterpolatingExpression:(nonnull NSExpression*)inputExpression withCurveType:(nonnull MGLExpressionInterpolationMode)curveType parameters:(nullable NSExpression *)parameters stops:(nonnull NSExpression*)stops NS_SWIFT_NAME(init(forMGLInterpolating:curveType:parameters:stops:));
+
+/**
+ Returns a match function expression specifying the input, matching values,
+ and default value.
+ 
+ @param inputExpression The matching expression.
+ @param matchedExpressions The matched values expression dictionary must be condition : value.
+ @param defaultExpression The defaultValue expression to be used in case there is no match.
+ */
++ (instancetype)mgl_expressionForMatchingExpression:(nonnull NSExpression *)inputExpression inDictionary:(nonnull NSDictionary<NSExpression *, NSExpression *> *)matchedExpressions defaultExpression:(nonnull NSExpression *)defaultExpression NS_SWIFT_NAME(init(forMGLMatchingKey:in:default:));
+/**
+ Returns a constant expression appending the passed expression.
+ 
+ @note Both the receiver and the given expression must be an `NSString` constant
+ expression type; otherwise, an exception is rised.
+ 
+ @param expression The expression to append to the receiver.
+ */
+- (instancetype)mgl_expressionByAppendingExpression:(nonnull NSExpression *)expression NS_SWIFT_NAME(mgl_appending(_:));
+
+@end
+
 @interface NSExpression (MGLAdditions)
 
 /**
diff --git a/platform/darwin/src/NSExpression+MGLAdditions.mm b/platform/darwin/src/NSExpression+MGLAdditions.mm
index be6eef7f47..54f98c6ab9 100644
--- a/platform/darwin/src/NSExpression+MGLAdditions.mm
+++ b/platform/darwin/src/NSExpression+MGLAdditions.mm
@@ -16,6 +16,10 @@
 
 #import <mbgl/style/expression/expression.hpp>
 
+const MGLExpressionInterpolationMode MGLExpressionInterpolationModeLinear = @"linear";
+const MGLExpressionInterpolationMode MGLExpressionInterpolationModeExponential = @"exponential";
+const MGLExpressionInterpolationMode MGLExpressionInterpolationModeCubicBezier = @"cubic-bezier";
+
 @interface MGLAftermarketExpressionInstaller: NSObject
 @end
 
@@ -557,6 +561,72 @@ NS_DICTIONARY_OF(NSNumber *, NSExpression *) *MGLStopDictionaryByReplacingTokens
     [NSException raise:NSInvalidArgumentException
                 format:@"Has expressions lack underlying Objective-C implementations."];
     return nil;
+
+}
+
+@end
+@implementation NSExpression (MGLVariableAdditions)
+
++ (NSExpression *)zoomLevelVariableExpression {
+    return [NSExpression expressionForVariable:@"zoomLevel"];
+}
+
++ (NSExpression *)heatmapDensityVariableExpression {
+    return [NSExpression expressionForVariable:@"heatmapDensity"];
+}
+
++ (NSExpression *)geometryTypeVariableExpression {
+    return [NSExpression expressionForVariable:@"geometryType"];
+}
+
++ (NSExpression *)featureIdentifierVariableExpression {
+    return [NSExpression expressionForVariable:@"featureIdentifier"];
+}
+
++ (NSExpression *)featurePropertiesVariableExpression {
+    return [NSExpression expressionForVariable:@"featureProperties"];
+}
+
+@end
+
+@implementation NSExpression (MGLInitializerAdditions)
+
++ (instancetype)mgl_expressionForConditional:(nonnull NSPredicate *)conditionPredicate trueExpression:(nonnull NSExpression *)trueExpression falseExpresssion:(nonnull NSExpression *)falseExpression {
+    if (@available(iOS 9.0, *)) {
+        return [NSExpression expressionForConditional:conditionPredicate trueExpression:trueExpression falseExpression:falseExpression];
+    } else {
+        return [NSExpression expressionForFunction:@"MGL_IF" arguments:@[[NSExpression expressionWithFormat:@"%@", conditionPredicate], trueExpression, falseExpression]];
+    }
+}
+
++ (instancetype)mgl_expressionForSteppingExpression:(nonnull NSExpression*)steppingExpression fromExpression:(nonnull NSExpression *)minimumExpression stops:(nonnull NSExpression*)stops {
+    return [NSExpression expressionForFunction:@"mgl_step:from:stops:"
+                                     arguments:@[steppingExpression, minimumExpression, stops]];
+}
+
++ (instancetype)mgl_expressionForInterpolatingExpression:(nonnull NSExpression*)inputExpression withCurveType:(nonnull MGLExpressionInterpolationMode)curveType parameters:(nullable NSExpression *)parameters stops:(nonnull NSExpression*)stops {
+    NSExpression *sanitizeParams = parameters ? parameters : [NSExpression expressionForConstantValue:nil];
+    return [NSExpression expressionForFunction:@"mgl_interpolate:withCurveType:parameters:stops:"
+                                     arguments:@[inputExpression, [NSExpression expressionForConstantValue:curveType], sanitizeParams, stops]];
+}
+
++ (instancetype)mgl_expressionForMatchingExpression:(nonnull NSExpression *)inputExpression inDictionary:(nonnull NSDictionary<NSExpression *, NSExpression *> *)matchedExpressions defaultExpression:(nonnull NSExpression *)defaultExpression {
+    NSMutableArray *optionsArray = [NSMutableArray arrayWithObjects:inputExpression, nil];
+    
+    NSEnumerator *matchEnumerator = matchedExpressions.keyEnumerator;
+    while (NSExpression *key = matchEnumerator.nextObject) {
+        [optionsArray addObject:key];
+        [optionsArray addObject:[matchedExpressions objectForKey:key]];
+    }
+    
+    [optionsArray addObject:defaultExpression];
+    return [NSExpression expressionForFunction:@"MGL_MATCH"
+                                     arguments:optionsArray];
+}
+
+- (instancetype)mgl_expressionByAppendingExpression:(nonnull NSExpression *)expression {
+    NSExpression *subexpression = [NSExpression expressionForAggregate:@[self, expression]];
+    return [NSExpression expressionForFunction:@"mgl_join:" arguments:@[subexpression]];
 }
 
 @end
@@ -767,15 +837,15 @@ NSArray *MGLSubexpressionsWithJSONObjects(NSArray *objects) {
             return [NSExpression expressionForFunction:@"mgl_step:from:stops:"
                                              arguments:@[inputExpression, minimum, stopExpression]];
         } else if ([op isEqualToString:@"zoom"]) {
-            return [NSExpression expressionForVariable:@"zoomLevel"];
+            return NSExpression.zoomLevelVariableExpression;
         } else if ([op isEqualToString:@"heatmap-density"]) {
-            return [NSExpression expressionForVariable:@"heatmapDensity"];
+            return NSExpression.heatmapDensityVariableExpression;
         } else if ([op isEqualToString:@"geometry-type"]) {
-            return [NSExpression expressionForVariable:@"geometryType"];
+            return NSExpression.geometryTypeVariableExpression;
         } else if ([op isEqualToString:@"id"]) {
-            return [NSExpression expressionForVariable:@"featureIdentifier"];
+            return NSExpression.featureIdentifierVariableExpression;
         }  else if ([op isEqualToString:@"properties"]) {
-            return [NSExpression expressionForVariable:@"featureProperties"];
+            return NSExpression.featurePropertiesVariableExpression;
         } else if ([op isEqualToString:@"var"]) {
             return [NSExpression expressionForVariable:argumentObjects.firstObject];
         } else if ([op isEqualToString:@"case"]) {
diff --git a/platform/darwin/test/MGLExpressionTests.mm b/platform/darwin/test/MGLExpressionTests.mm
index c91cf36d1a..e078c4f4b6 100644
--- a/platform/darwin/test/MGLExpressionTests.mm
+++ b/platform/darwin/test/MGLExpressionTests.mm
@@ -1006,4 +1006,54 @@ using namespace std::string_literals;
     }
 }
 
+- (void)testConvenienceInitializers {
+    {
+        NSExpression *expression = [NSExpression mgl_expressionForConditional:[NSPredicate predicateWithFormat:@"1 = 2"]
+                                                               trueExpression:MGLConstantExpression(@YES)
+                                                             falseExpresssion:MGLConstantExpression(@NO)];
+        
+        NSArray *jsonExpression = @[@"case", @[@"==", @1, @2], @YES, @NO];
+        XCTAssertEqualObjects(expression.mgl_jsonExpressionObject, jsonExpression);
+        XCTAssertEqualObjects([NSExpression expressionWithMGLJSONObject:jsonExpression], expression);
+        XCTAssertEqualObjects([expression expressionValueWithObject:nil context:nil], @NO);
+    }
+    {
+        NSDictionary *stops = @{@0: MGLConstantExpression(@111), @1: MGLConstantExpression(@1111)};
+        NSExpression *expression = [NSExpression mgl_expressionForSteppingExpression:[NSExpression expressionForKeyPath:@"x"]
+                                                                      fromExpression:[NSExpression expressionForConstantValue:@11]
+                                                                               stops:[NSExpression expressionForConstantValue:stops]];
+        NSArray *jsonExpression = @[@"step", @[@"get", @"x"], @11, @0, @111, @1, @1111];
+        XCTAssertEqualObjects(expression.mgl_jsonExpressionObject, jsonExpression);
+        XCTAssertEqualObjects([NSExpression expressionWithMGLJSONObject:jsonExpression], expression);
+    }
+    {
+        NSDictionary *stops = @{@0: MGLConstantExpression(@100), @10: MGLConstantExpression(@200)};
+        NSExpression *expression = [NSExpression mgl_expressionForInterpolatingExpression:[NSExpression expressionForKeyPath:@"x"]
+                                                                            withCurveType:MGLExpressionInterpolationModeLinear
+                                                                               parameters:nil
+                                                                                    stops:[NSExpression expressionForConstantValue:stops]];
+        NSArray *jsonExpression = @[@"interpolate", @[@"linear"], @[@"get", @"x"], @0, @100, @10, @200];
+        XCTAssertEqualObjects(expression.mgl_jsonExpressionObject, jsonExpression);
+        XCTAssertEqualObjects([NSExpression expressionWithMGLJSONObject:jsonExpression], expression);
+    }
+    {
+        NSExpression *expression = [[NSExpression expressionForConstantValue:@"Old"] mgl_expressionByAppendingExpression:[NSExpression expressionForConstantValue:@"MacDonald"]];
+
+        NSArray *jsonExpression = @[@"concat", @"Old", @"MacDonald"];
+        XCTAssertEqualObjects(expression.mgl_jsonExpressionObject, jsonExpression);
+        XCTAssertEqualObjects([expression expressionValueWithObject:nil context:nil], @"OldMacDonald");
+        XCTAssertEqualObjects([NSExpression expressionWithMGLJSONObject:jsonExpression], expression);
+    }
+    {
+        NSDictionary *values = @{ MGLConstantExpression(@1): MGLConstantExpression(@"one") };
+        NSExpression *expression = [NSExpression mgl_expressionForMatchingExpression:[NSExpression expressionWithFormat:@"2 * 1"]
+                                                                        inDictionary:values
+                                                                   defaultExpression:[NSExpression expressionForConstantValue:@"default"]];
+        NSArray *jsonExpression =  @[@"match", @[@"*", @2, @1], @1, @"one", @"default"];
+        XCTAssertEqualObjects(expression.mgl_jsonExpressionObject, jsonExpression);
+        XCTAssertEqualObjects([NSExpression expressionWithMGLJSONObject:jsonExpression], expression);
+    }
+}
+
+
 @end
diff --git a/platform/ios/app/MBXViewController.m b/platform/ios/app/MBXViewController.m
index 0a76f8c0c5..391af5ea05 100644
--- a/platform/ios/app/MBXViewController.m
+++ b/platform/ios/app/MBXViewController.m
@@ -967,18 +967,20 @@ typedef NS_ENUM(NSInteger, MBXSettingsMiscellaneousRows) {
                                       @10.0f: [UIColor redColor],
                                       @12.0f: [UIColor greenColor],
                                       @14.0f: [UIColor blueColor]};
-    waterLayer.fillColor = [NSExpression expressionWithFormat:
-                            @"mgl_interpolate:withCurveType:parameters:stops:($zoomLevel, 'linear', nil, %@)",
-                            waterColorStops];
+    NSExpression *fillColorExpression = [NSExpression mgl_expressionForInterpolatingExpression:NSExpression.zoomLevelVariableExpression
+                                                                                 withCurveType:MGLExpressionInterpolationModeLinear
+                                                                                    parameters:nil
+                                                                                         stops:[NSExpression expressionForConstantValue:waterColorStops]];
+    waterLayer.fillColor = fillColorExpression;
 
     NSDictionary *fillAntialiasedStops = @{@11: @YES,
                                            @12: @NO,
                                            @13: @YES,
                                            @14: @NO,
                                            @15: @YES};
-    waterLayer.fillAntialiased = [NSExpression expressionWithFormat:
-                                  @"mgl_step:from:stops:($zoomLevel, false, %@)",
-                                  fillAntialiasedStops];
+    waterLayer.fillAntialiased = [NSExpression mgl_expressionForSteppingExpression:NSExpression.zoomLevelVariableExpression
+                                                                    fromExpression:[NSExpression expressionForConstantValue:@NO]
+                                                                             stops:[NSExpression expressionForConstantValue:fillAntialiasedStops]];
 }
 
 - (void)styleRoadLayer
@@ -1467,10 +1469,16 @@ typedef NS_ENUM(NSInteger, MBXSettingsMiscellaneousRows) {
 
     // source, categorical function that sets any feature with a "fill" attribute value of true to red color and anything without to green
     MGLFillStyleLayer *fillStyleLayer = [[MGLFillStyleLayer alloc] initWithIdentifier:@"fill-layer" source:shapeSource];
-    fillStyleLayer.fillColor = [NSExpression expressionWithFormat:@"TERNARY(fill == YES, %@, %@)", [UIColor greenColor], [UIColor redColor]];
+    fillStyleLayer.fillColor = [NSExpression mgl_expressionForConditional:[NSPredicate predicateWithFormat:@"fill == YES"]
+                                                           trueExpression:[NSExpression expressionForConstantValue:[UIColor greenColor]]
+                                                         falseExpresssion:[NSExpression expressionForConstantValue:[UIColor redColor]]];
+                                                               
+    
 
     // source, identity function that sets any feature with an "opacity" attribute to use that value and anything without to 1.0
-    fillStyleLayer.fillOpacity = [NSExpression expressionWithFormat:@"TERNARY(opacity != nil, opacity, 1.0)"];
+    fillStyleLayer.fillOpacity = [NSExpression mgl_expressionForConditional:[NSPredicate predicateWithFormat:@"opacity != nil"]
+                                                             trueExpression:[NSExpression expressionForKeyPath:@"opacity"] 
+                                                           falseExpresssion:[NSExpression expressionForConstantValue:@1.0]];
     [self.mapView.style addLayer:fillStyleLayer];
 }
 
diff --git a/platform/ios/docs/guides/For Style Authors.md b/platform/ios/docs/guides/For Style Authors.md
index 852a9c2ef4..1ce0a4bf4f 100644
--- a/platform/ios/docs/guides/For Style Authors.md
+++ b/platform/ios/docs/guides/For Style Authors.md
@@ -329,9 +329,9 @@ In style specification | Method, function, or predicate type | Format string syn
 `to-number`            | `mgl_numberWithFallbackValues:` | `CAST(zipCode, 'NSNumber')`
 `to-string`            | `stringValue` | `CAST(ele, 'NSString')`
 `typeof`               | |
-`geometry-type`        | |`$geometryType`
-`id`                   | |`$featureIdentifier`
-`properties`           | |`$featureProperties`
+`geometry-type`        | `NSExpression.geometryTypeVariableExpression` | `$geometryType`
+`id`                   | `NSExpression.featureIdentifierVariableExpression` | `$featureIdentifier`
+`properties`           | `NSExpression.featurePropertiesVariableExpression` | `$featureProperties`
 `at`                   | `objectFrom:withIndex:` | `array[n]`
 `get`                  | `+[NSExpression expressionForKeyPath:]` | Key path
 `has`                  | `mgl_does:have:` | `mgl_does:have:(self, 'key')`
@@ -345,14 +345,14 @@ In style specification | Method, function, or predicate type | Format string syn
 `>=`                   | `NSGreaterThanOrEqualToPredicateOperatorType` | `key >= value`
 `all`                  | `NSAndPredicateType` | `p0 AND … AND pn`
 `any`                  | `NSOrPredicateType` | `p0 OR … OR pn`
-`case`                 | `+[NSExpression expressionForConditional:trueExpression:falseExpression:]` or `MGL_IF` | `TERNARY(1 = 2, YES, NO)` or `MGL_IF(1 = 2, YES, 2 = 2, YES, NO)`
+`case`                 | `+[NSExpression expressionForConditional:trueExpression:falseExpression:]` or `MGL_IF` or `+[NSExpression mgl_expressionForConditional:trueExpression:falseExpresssion:]` | `TERNARY(1 = 2, YES, NO)` or `MGL_IF(1 = 2, YES, 2 = 2, YES, NO)`
 `coalesce`             | `mgl_coalesce:` | `mgl_coalesce({x, y, z})`
-`match`                | `MGL_MATCH` | `MGL_MATCH(x, 0, 'zero match', 1, 'one match', 'two match', 'default')`
-`interpolate`          | `mgl_interpolate:withCurveType:parameters:stops:` |
-`step`                 | `mgl_step:withMinimum:stops:` |
+`match`                | `MGL_MATCH` or `+[NSExpression mgl_expressionForMatchingExpression:inDictionary:defaultExpression:]` | `MGL_MATCH(x, 0, 'zero match', 1, 'one match', 'two match', 'default')`
+`interpolate`          | `mgl_interpolate:withCurveType:parameters:stops:` or `+[NSExpression mgl_expressionForInterpolatingExpression:withCurveType:parameters:stops:]` |
+`step`                 | `mgl_step:withMinimum:stops:` or `+[NSExpression mgl_expressionForSteppingExpression:fromExpression:stops:]` |
 `let`                  | `mgl_expressionWithContext:` | `MGL_LET('ios', 11, 'macos', 10.13, $ios + $macos)`
 `var`                  | `+[NSExpression expressionForVariable:]` | `$variable`
-`concat`               | `mgl_join:` | `mgl_join({'Old', ' ', 'MacDonald'})`
+`concat`               | `mgl_join:` or `-[NSExpression mgl_expressionByAppendingExpression:]` | `mgl_join({'Old', ' ', 'MacDonald'})`
 `downcase`             | `lowercase:` | `lowercase('DOWNTOWN')`
 `upcase`               | `uppercase:` | `uppercase('Elysian Fields')`
 `rgb`                  | `+[UIColor colorWithRed:green:blue:alpha:]` |
@@ -383,8 +383,8 @@ In style specification | Method, function, or predicate type | Format string syn
 `sin`                  | |
 `sqrt`                 | `sqrt:` | `sqrt(2)`
 `tan`                  | |
-`zoom`                 | | `$zoom`
-`heatmap-density`      | | `$heatmapDensity`
+`zoom`                 | `NSExpression.zoomLevelVariableExpression` | `$zoom`
+`heatmap-density`      | `NSExpression.heatmapDensityVariableExpression` | `$heatmapDensity`
 
 For operators that have no corresponding `NSExpression` symbol, use the
 `MGL_FUNCTION()` format string syntax.
diff --git a/platform/macos/docs/guides/For Style Authors.md b/platform/macos/docs/guides/For Style Authors.md
index dea4eb845a..c8e517a5bd 100644
--- a/platform/macos/docs/guides/For Style Authors.md
+++ b/platform/macos/docs/guides/For Style Authors.md
@@ -322,9 +322,9 @@ In style specification | Method, function, or predicate type | Format string syn
 `to-number`            | `mgl_numberWithFallbackValues:` | `CAST(zipCode, 'NSNumber')`
 `to-string`            | `stringValue` | `CAST(ele, 'NSString')`
 `typeof`               | |
-`geometry-type`        | |`$geometryType`
-`id`                   | |`$featureIdentifier`
-`properties`           | |`$featureProperties`
+`geometry-type`        | `NSExpression.geometryTypeVariableExpression` | `$geometryType`
+`id`                   | `NSExpression.featureIdentifierVariableExpression` | `$featureIdentifier`
+`properties`           | `NSExpression.featurePropertiesVariableExpression` | `$featureProperties`
 `at`                   | `objectFrom:withIndex:` | `array[n]`
 `get`                  | `+[NSExpression expressionForKeyPath:]` | Key path
 `has`                  | `mgl_does:have:` | `mgl_does:have:(self, 'key')`
@@ -338,14 +338,14 @@ In style specification | Method, function, or predicate type | Format string syn
 `>=`                   | `NSGreaterThanOrEqualToPredicateOperatorType` | `key >= value`
 `all`                  | `NSAndPredicateType` | `p0 AND … AND pn`
 `any`                  | `NSOrPredicateType` | `p0 OR … OR pn`
-`case`                 | `+[NSExpression expressionForConditional:trueExpression:falseExpression:]` or `MGL_IF` | `TERNARY(1 = 2, YES, NO)` or `MGL_IF(1 = 2, YES, 2 = 2, YES, NO)`
+`case`                 | `+[NSExpression expressionForConditional:trueExpression:falseExpression:]` or `MGL_IF` or `+[NSExpression mgl_expressionForConditional:trueExpression:falseExpresssion:]` | `TERNARY(1 = 2, YES, NO)` or `MGL_IF(1 = 2, YES, 2 = 2, YES, NO)`
 `coalesce`             | `mgl_coalesce:` | `mgl_coalesce({x, y, z})`
-`match`                | `MGL_MATCH` | `MGL_MATCH(x, 0, 'zero match', 1, 'one match', 'two match', 'default')`
-`interpolate`          | `mgl_interpolate:withCurveType:parameters:stops:` |
-`step`                 | `mgl_step:withMinimum:stops:` |
+`match`                | `MGL_MATCH` or `+[NSExpression mgl_expressionForMatchingExpression:inDictionary:defaultExpression:]` | `MGL_MATCH(x, 0, 'zero match', 1, 'one match', 'two match', 'default')`
+`interpolate`          | `mgl_interpolate:withCurveType:parameters:stops:` or `+[NSExpression mgl_expressionForInterpolatingExpression:withCurveType:parameters:stops:]` |
+`step`                 | `mgl_step:withMinimum:stops:` or `+[NSExpression mgl_expressionForSteppingExpression:fromExpression:stops:]` |
 `let`                  | `mgl_expressionWithContext:` | `MGL_LET('ios', 11, 'macos', 10.13, $ios + $macos)`
 `var`                  | `+[NSExpression expressionForVariable:]` | `$variable`
-`concat`               | `mgl_join:` | `mgl_join({'Old', ' ', 'MacDonald'})`
+`concat`               | `mgl_join:` or `-[NSExpression mgl_expressionByAppendingExpression:]` | `mgl_join({'Old', ' ', 'MacDonald'})`
 `downcase`             | `lowercase:` | `lowercase('DOWNTOWN')`
 `upcase`               | `uppercase:` | `uppercase('Elysian Fields')`
 `rgb`                  | `+[NSColor colorWithCalibratedRed:green:blue:alpha:]` |
@@ -376,8 +376,8 @@ In style specification | Method, function, or predicate type | Format string syn
 `sin`                  | |
 `sqrt`                 | `sqrt:` | `sqrt(2)`
 `tan`                  | |
-`zoom`                 | | `$zoom`
-`heatmap-density`      | | `$heatmapDensity`
+`zoom`                 | `NSExpression.zoomLevelVariableExpression` | `$zoom`
+`heatmap-density`      | `NSExpression.heatmapDensityVariableExpression` | `$heatmapDensity`
 
 For operators that have no corresponding `NSExpression` symbol, use the
 `MGL_FUNCTION()` format string syntax.