[ios, macos] Aftermarket let expressions

3 files changed, 31 insertions, 28 deletions

diff --git a/platform/darwin/docs/guides/Predicates and Expressions.md b/platform/darwin/docs/guides/Predicates and Expressions.md
index 97834b4f71..5e6af06dc4 100644
--- a/platform/darwin/docs/guides/Predicates and Expressions.md
+++ b/platform/darwin/docs/guides/Predicates and Expressions.md
@@ -193,6 +193,7 @@ string syntax:
 Initializer parameter | Format string syntax | Arguments | Returns
 ----------------------|----------------------|-----------|--------
 `mgl_join:` | `mgl_join({'Old', 'MacDonald'})` | An aggregate expression or `NSArray` constant value expression containing one or more `NSExpression`s, each evaluating to a string. | An `NSString` object (the result of concatenating together all the elements of an array in order).
+`MGL_LET` | `MGL_LET('age', uppercase('old'), 'name', uppercase('MacDonald'), mgl_join({$age, $name}))` | Any number of variable names interspersed with their assigned `NSExpression` values, followed by an `NSExpression` that may contain references to those variables.
 
 The following custom functions are also available with the
 `+[NSExpression expressionForFunction:selectorName:arguments:]` method or the
@@ -215,22 +216,6 @@ The following custom functions are also available with the
    </td>
 </tr>
 <tr>
-   <td><code>mgl_expressionWithContext:</code></td>
-   <td>
-      An `NSExpression` that may contain references to the variables defined in
-      the context dictionary.
-   </td>
-   <td>
-      An `NSDictionary` with `NSString`s as keys and `NSExpression`s as values.
-      Each key is a variable name and each value is the variable’s value within
-      the target expression.
-   </td>
-   <td>
-      The target expression with variable subexpressions replaced with the
-      values defined in the context dictionary.
-   </td>
-</tr>
-<tr>
    <td><code>mgl_interpolateWithCurveType:parameters:stops:</code></td>
    <td>
       An `NSExpression` that evaluates to a number and contains a variable or
diff --git a/platform/darwin/src/NSExpression+MGLAdditions.mm b/platform/darwin/src/NSExpression+MGLAdditions.mm
index 43661a5502..16c1656fb8 100644
--- a/platform/darwin/src/NSExpression+MGLAdditions.mm
+++ b/platform/darwin/src/NSExpression+MGLAdditions.mm
@@ -24,6 +24,16 @@ NSString *MGLJoinComponents(Class self, SEL _cmd, NSArray<NSString *> *component
 }
 
 /**
+ A placeholder for a method that evaluates an expression based on an arbitrary
+ number of variable names and assigned expressions.
+ */
+id MGLEvaluateWithContext(Class self, SEL _cmd, NSString *firstVariableName, ...) {
+    [NSException raise:NSInvalidArgumentException
+                format:@"Assignment expressions lack underlying Objective-C implementations."];
+    return nil;
+};
+
+/**
  Adds to NSExpression’s built-in repertoire of functions.
  */
 void MGLInstallAftermarketExpressionFunctions() {
@@ -38,6 +48,10 @@ void MGLInstallAftermarketExpressionFunctions() {
 #pragma clang push
 #pragma clang diagnostic ignored "-Wundeclared-selector"
     class_addMethod(NSPredicateUtilities, @selector(mgl_join:), (IMP)MGLJoinComponents, "@@:@");
+    
+    // Vararg aftermarket expressions need to be declared with an explicit and implicit first argument.
+    class_addMethod(NSPredicateUtilities, @selector(MGL_LET), (IMP)MGLEvaluateWithContext, "@@:@");
+    class_addMethod(NSPredicateUtilities, @selector(MGL_LET:), (IMP)MGLEvaluateWithContext, "@@:@");
 #pragma clang pop
 }
 
@@ -369,6 +383,7 @@ NSArray *MGLSubexpressionsWithJSONObjects(NSArray *objects) {
             @"^": @"raise:toPower:",
             @"upcase": @"uppercase:",
             @"downcase": @"lowercase:",
+            @"let": @"MGL_LET",
         };
     });
     
@@ -530,17 +545,6 @@ NSArray *MGLSubexpressionsWithJSONObjects(NSArray *objects) {
             return [NSExpression expressionForVariable:@"mgl_featureIdentifier"];
         }  else if ([op isEqualToString:@"properties"]) {
             return [NSExpression expressionForVariable:@"mgl_featureProperties"];
-        } else if ([op isEqualToString:@"let"]) {
-            NSExpression *operand = [NSExpression mgl_expressionWithJSONObject:argumentObjects.lastObject];
-            NSArray *bindingObjects = [argumentObjects subarrayWithRange:NSMakeRange(0, argumentObjects.count - 1)];
-            NSMutableDictionary *context = [NSMutableDictionary dictionaryWithCapacity:bindingObjects.count / 2];
-            NSEnumerator *bindingEnumerator = bindingObjects.objectEnumerator;
-            while (NSString *key = bindingEnumerator.nextObject) {
-                context[key] = [NSExpression mgl_expressionWithJSONObject:bindingEnumerator.nextObject];
-            }
-            return [NSExpression expressionForFunction:operand
-                                          selectorName:@"mgl_expressionWithContext:"
-                                             arguments:@[[NSExpression expressionForConstantValue:context]]];
         } else if ([op isEqualToString:@"var"]) {
             return [NSExpression expressionForVariable:argumentObjects.firstObject];
         } else if ([op isEqualToString:@"case"]) {
@@ -608,6 +612,9 @@ NSArray *MGLSubexpressionsWithJSONObjects(NSArray *objects) {
             @"uppercase:": @"upcase",
             @"lowercase:": @"downcase",
             @"length:": @"length",
+            // Vararg aftermarket expressions need to be declared with an explicit and implicit first argument.
+            @"MGL_LET": @"let",
+            @"MGL_LET:": @"let",
         };
     });
     
diff --git a/platform/darwin/test/MGLExpressionTests.mm b/platform/darwin/test/MGLExpressionTests.mm
index 5451174e24..94c9410892 100644
--- a/platform/darwin/test/MGLExpressionTests.mm
+++ b/platform/darwin/test/MGLExpressionTests.mm
@@ -199,9 +199,11 @@ using namespace std::string_literals;
     }
     {
         NSDictionary *context = @{@"loremIpsum": MGLConstantExpression(@"Lorem ipsum dolor sit amet")};
-        NSExpression *expression = [NSExpression expressionWithFormat:@"FUNCTION(uppercase($loremIpsum), 'mgl_expressionWithContext:', %@)", context];
+        NSExpression *expression = [NSExpression expressionWithFormat:@"MGL_LET('loremIpsum', 'Lorem ipsum dolor sit amet', uppercase($loremIpsum))", context];
+        NSExpression *compatibilityExpression = [NSExpression expressionWithFormat:@"FUNCTION(uppercase($loremIpsum), 'mgl_expressionWithContext:', %@)", context];
         NSArray *jsonExpression = @[@"let", @"loremIpsum", @"Lorem ipsum dolor sit amet", @[@"upcase", @[@"var", @"loremIpsum"]]];
         XCTAssertEqualObjects(expression.mgl_jsonExpressionObject, jsonExpression);
+        XCTAssertEqualObjects(compatibilityExpression.mgl_jsonExpressionObject, jsonExpression);
         XCTAssertEqualObjects([NSExpression mgl_expressionWithJSONObject:jsonExpression], expression);
     }
 }
@@ -353,6 +355,15 @@ using namespace std::string_literals;
         XCTAssertEqualObjects([NSExpression mgl_expressionWithJSONObject:jsonExpression], expression);
     }
     {
+        NSArray *arguments = @[MGLConstantExpression(@1), MGLConstantExpression(@1), MGLConstantExpression(@1)];
+        NSExpression *expression = [NSExpression expressionForFunction:@"add:to:" arguments:arguments];
+        NSArray *jsonExpression = @[@"+", @1, @1, @1];
+        XCTAssertEqualObjects(expression.mgl_jsonExpressionObject, jsonExpression);
+        jsonExpression = @[@"+", @[@"+", @1, @1], @1];
+        XCTAssertEqualObjects([NSExpression expressionWithFormat:@"1 + 1 + 1"].mgl_jsonExpressionObject, jsonExpression);
+        XCTAssertEqualObjects([NSExpression mgl_expressionWithJSONObject:jsonExpression], [NSExpression expressionWithFormat:@"1 + 1 + 1"]);
+    }
+    {
         NSExpression *expression = [NSExpression expressionForFunction:@"from:subtract:" arguments:arguments];
         NSArray *jsonExpression = @[@"-", @1, @1];
         XCTAssertEqualObjects(expression.mgl_jsonExpressionObject, jsonExpression);