Merge pull request realm#2885 from realm/mf-flatmap

Add `flatmap_over_map_reduce` opt-in rule

.swiftlint.yml

@@ -27,6 +27,7 @@ opt_in_rules:
   - file_header
   - file_name
   - first_where
+  - flatmap_over_map_reduce
   - identical_operands
   - joined_default_parameter
   - legacy_random

CHANGELOG.md

@@ -23,6 +23,11 @@
   [nvanfleet](https://github.com/nvanfleet)
   [#2866](https://github.com/realm/SwiftLint/issues/2866)
 
+* Add `flatmap_over_map_reduce` opt-in rule to prefer
+  using `flatMap` over `map { ... }.reduce([], +)`.  
+  [Marcelo Fabri](https://github.com/marcelofabri)
+  [#2883](https://github.com/realm/SwiftLint/issues/2883)
+
 #### Bug Fixes
 
 * None.

Rules.md

@@ -54,6 +54,7 @@
 * [File Name](#file-name)
 * [File Types Order](#file-types-order)
 * [First Where](#first-where)
+* [FlatMap over map and reduce](#flatmap-over-map-and-reduce)
 * [For Where](#for-where)
 * [Force Cast](#force-cast)
 * [Force Try](#force-try)
@@ -8429,6 +8430,39 @@ if let pause = timeTracker.pauses.filter("beginDate < %@", beginDate).first { pr
 
 
 
+## FlatMap over map and reduce
+
+Identifier | Enabled by default | Supports autocorrection | Kind | Analyzer | Minimum Swift Compiler Version
+--- | --- | --- | --- | --- | ---
+`flatmap_over_map_reduce` | Disabled | No | performance | No | 3.0.0 
+
+Prefer `flatMap` over `map` followed by `reduce([], +)`.
+
+### Examples
+
+<details>
+<summary>Non Triggering Examples</summary>
+
+```swift
+let foo = bar.map { $0.count }.reduce(0, +)
+```
+
+```swift
+let foo = bar.flatMap { $0.array }
+```
+
+</details>
+<details>
+<summary>Triggering Examples</summary>
+
+```swift
+let foo = ↓bar.map { $0.array }.reduce([], +)
+```
+
+</details>
+
+
+
 ## For Where
 
 Identifier | Enabled by default | Supports autocorrection | Kind | Analyzer | Minimum Swift Compiler Version

Source/SwiftLintFramework/Models/MasterRuleList.swift

@@ -55,6 +55,7 @@ public let masterRuleList = RuleList(rules: [
     FileNameRule.self,
     FileTypesOrderRule.self,
     FirstWhereRule.self,
+    FlatMapOverMapReduceRule.self,
     ForWhereRule.self,
     ForceCastRule.self,
     ForceTryRule.self,

Source/SwiftLintFramework/Rules/Performance/FlatMapOverMapReduceRule.swift

@@ -0,0 +1,27 @@
+import SourceKittenFramework
+
+public struct FlatMapOverMapReduceRule: CallPairRule, OptInRule, ConfigurationProviderRule, AutomaticTestableRule {
+    public var configuration = SeverityConfiguration(.warning)
+
+    public init() {}
+
+    public static let description = RuleDescription(
+        identifier: "flatmap_over_map_reduce",
+        name: "FlatMap over map and reduce",
+        description: "Prefer `flatMap` over `map` followed by `reduce([], +)`.",
+        kind: .performance,
+        nonTriggeringExamples: [
+            "let foo = bar.map { $0.count }.reduce(0, +)",
+            "let foo = bar.flatMap { $0.array }"
+        ],
+        triggeringExamples: [
+            "let foo = ↓bar.map { $0.array }.reduce([], +)"
+        ]
+    )
+
+    public func validate(file: File) -> [StyleViolation] {
+        let pattern = "[\\}\\)]\\s*\\.reduce\\s*\\(\\[\\s*\\],\\s*\\+\\s*\\)"
+        return validate(file: file, pattern: pattern, patternSyntaxKinds: [.identifier],
+                        callNameSuffix: ".map", severity: configuration.severity)
+    }
+}

SwiftLint.xcodeproj/project.pbxproj

@@ -282,6 +282,7 @@
 		D45255C81F0932F8003C9B56 /* RuleDescription+Examples.swift in Sources */ = {isa = PBXBuildFile; fileRef = D45255C71F0932F8003C9B56 /* RuleDescription+Examples.swift */; };
 		D462021F1E15F52D0027AAD1 /* NumberSeparatorRuleExamples.swift in Sources */ = {isa = PBXBuildFile; fileRef = D462021E1E15F52D0027AAD1 /* NumberSeparatorRuleExamples.swift */; };
 		D46252541DF63FB200BE2CA1 /* NumberSeparatorRule.swift in Sources */ = {isa = PBXBuildFile; fileRef = D46252531DF63FB200BE2CA1 /* NumberSeparatorRule.swift */; };
+		D466B620233D229F0068190B /* FlatMapOverMapReduceRule.swift in Sources */ = {isa = PBXBuildFile; fileRef = D466B61F233D229F0068190B /* FlatMapOverMapReduceRule.swift */; };
 		D46A317F1F1CEDCD00AF914A /* UnneededParenthesesInClosureArgumentRule.swift in Sources */ = {isa = PBXBuildFile; fileRef = D46A317E1F1CEDCD00AF914A /* UnneededParenthesesInClosureArgumentRule.swift */; };
 		D46E041D1DE3712C00728374 /* TrailingCommaRule.swift in Sources */ = {isa = PBXBuildFile; fileRef = D46E041C1DE3712C00728374 /* TrailingCommaRule.swift */; };
 		D47079A71DFCEB2D00027086 /* EmptyParenthesesWithTrailingClosureRule.swift in Sources */ = {isa = PBXBuildFile; fileRef = D47079A61DFCEB2D00027086 /* EmptyParenthesesWithTrailingClosureRule.swift */; };
@@ -779,6 +780,7 @@
 		D45255C71F0932F8003C9B56 /* RuleDescription+Examples.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = "RuleDescription+Examples.swift"; sourceTree = "<group>"; };
 		D462021E1E15F52D0027AAD1 /* NumberSeparatorRuleExamples.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = NumberSeparatorRuleExamples.swift; sourceTree = "<group>"; };
 		D46252531DF63FB200BE2CA1 /* NumberSeparatorRule.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = NumberSeparatorRule.swift; sourceTree = "<group>"; };
+		D466B61F233D229F0068190B /* FlatMapOverMapReduceRule.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = FlatMapOverMapReduceRule.swift; sourceTree = "<group>"; };
 		D46A317E1F1CEDCD00AF914A /* UnneededParenthesesInClosureArgumentRule.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = UnneededParenthesesInClosureArgumentRule.swift; sourceTree = "<group>"; };
 		D46E041C1DE3712C00728374 /* TrailingCommaRule.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = TrailingCommaRule.swift; sourceTree = "<group>"; };
 		D47079A61DFCEB2D00027086 /* EmptyParenthesesWithTrailingClosureRule.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = EmptyParenthesesWithTrailingClosureRule.swift; sourceTree = "<group>"; };
@@ -1088,6 +1090,7 @@
 				E847F0A81BFBBABD00EA9363 /* EmptyCountRule.swift */,
 				740DF1AF203F5AFC0081F694 /* EmptyStringRule.swift */,
 				D42D2B371E09CC0D00CD7A2E /* FirstWhereRule.swift */,
+				D466B61F233D229F0068190B /* FlatMapOverMapReduceRule.swift */,
 				D414D6AD21D22FF500960935 /* LastWhereRule.swift */,
 				756C0777222EA49400A111F4 /* ReduceIntoRule.swift */,
 				429644B41FB0A99E00D75128 /* SortedFirstLastRule.swift */,
@@ -2197,6 +2200,7 @@
 				4A9A3A3A1DC1D75F00DF5183 /* HTMLReporter.swift in Sources */,
 				D40F83881DE9179200524C62 /* TrailingCommaConfiguration.swift in Sources */,
 				827169B31F488181003FB9AF /* ExplicitEnumRawValueRule.swift in Sources */,
+				D466B620233D229F0068190B /* FlatMapOverMapReduceRule.swift in Sources */,
 				D41985E921FAB62F003BE2B7 /* DeploymentTargetRule.swift in Sources */,
 				62FE5D32200CABDD00F68793 /* DiscouragedOptionalCollectionExamples.swift in Sources */,
 				D49896F12026B36C00814A83 /* RedundantSetAccessControlRule.swift in Sources */,

Tests/LinuxMain.swift

@@ -528,6 +528,12 @@ extension FirstWhereRuleTests {
     ]
 }
 
+extension FlatMapOverMapReduceRuleTests {
+    static var allTests: [(String, (FlatMapOverMapReduceRuleTests) -> () throws -> Void)] = [
+        ("testWithDefaultConfiguration", testWithDefaultConfiguration)
+    ]
+}
+
 extension ForWhereRuleTests {
     static var allTests: [(String, (ForWhereRuleTests) -> () throws -> Void)] = [
         ("testWithDefaultConfiguration", testWithDefaultConfiguration)
@@ -1624,6 +1630,7 @@ XCTMain([
     testCase(FileNameRuleTests.allTests),
     testCase(FileTypesOrderRuleTests.allTests),
     testCase(FirstWhereRuleTests.allTests),
+    testCase(FlatMapOverMapReduceRuleTests.allTests),
     testCase(ForWhereRuleTests.allTests),
     testCase(ForceCastRuleTests.allTests),
     testCase(ForceTryRuleTests.allTests),

Tests/SwiftLintFrameworkTests/AutomaticRuleTests.generated.swift

@@ -228,6 +228,12 @@ class FirstWhereRuleTests: XCTestCase {
     }
 }
 
+class FlatMapOverMapReduceRuleTests: XCTestCase {
+    func testWithDefaultConfiguration() {
+        verifyRule(FlatMapOverMapReduceRule.description)
+    }
+}
+
 class ForWhereRuleTests: XCTestCase {
     func testWithDefaultConfiguration() {
         verifyRule(ForWhereRule.description)