Tamar Nachmany - Linting in Swift

Linters define and enforce syntactical conventions across codebases. They enable teams of developers to convey shared values to one another, making new contributions easier and more fun to review, and existing code better to build off of. This talk will explore how Swift linting works, why it’s valuable, and whether linting can go beyond fixing syntax to analyze code for more complex shared patterns across iOS teams.

Recording from the December 2015 Brooklyn Swift Developers Meetup: meetup.com/Brooklyn-Swift-Developers/events/226935450/

Video: https://vimeo.com/152932872

Transcript

1. Linting in Swift Tamar Nachmany
2. Listing is analyzing code and enforcing custom stylistic rules What is linting?
3. let greeting:String
 let greeting: String Rule: Leave space after colons
4. for item in library { if let movie = item as! Movie { print(“5 rotten tomatoes”) } } Rule: Don’t Allow Force Unwrapping
5. Why lint? Patterns & Onboarding
 Better code reviews
 Tooling
6. Hi, I’m Tamar. 1. I’m an engineer on Tumblr’s core iOS team. 2. I’ve directed about seven plays and collaborated in other forms as well.
7. Let’s explore linting by:
 • Investigating the Swift Compiler
 • Seeing how SwiftLint uses it
 • Answering the question, ‘Why Lint?’
8. What do linters get us during the compilation process?
9. What do linters get us during the compilation process? Extra errors.
10. Where do Xcode errors come from?
11. Before building, where do Xcode errors in Swift code come from? SourceKit, using an instance of the Swift Compiler
12. The Swift compiler is now open source
13. Basically
14. Credit: Chris Lattner and Joe Groff Swift Compilation
15. Credit: Chris Lattner and Joe Groff Swift Compilation
16. // Make AST and return any errors.
 CompilerInstance TextCI; if (makeParserAST(IFaceGenCtx->Impl.TextCI, IFaceGenCtx- >Impl.Info.Text)) {
 ErrMsg = "Error during syntactic parsing"; return nullptr; 
 } SwiftEditorInterfaceGen
17. // Perform semantic analysis
 
 CompilerInstance CI; CompilerInvocation Invocation; bool Failed = Lang.getASTManager().initCompilerInvocation( Invocation, Args, CI.getDiags(), InputFile- >getBufferIdentifier(), Error); if (Failed) { return false; } CI.performSema(); SwiftCompletion.cpp
18. Let’s make an error: println(“yo”)
20. What information do we need to display this? Error location Error message Fix-it suggestion Whether it’s an error or warning
21. When you enter new code into your project, Xcode sends messages to SourceKit.
 SourceKit responds with some info.
22. SourceKit Response key.diagnostics: [ { key.line: 5, key.column: 9, key.filepath: "/Users/SourceKit-Logging.playground", key.severity: source.diagnostic.severity.error, key.description: "'println' has been renamed to 'print'", key.diagnostic_stage: source.diagnostic.stage.swift.sema, key.fixits: [ { key.offset: 0, key.length: 7, key.sourcetext: "print" } ]
23. lib/Parse Response filename.swift:5:9: error: 'println' has been renamed to 'print' println("yo") ^~~~~~~ print
25. Let’s look at a linter
26. SwiftLint by
27. How does SwiftLint enforce new rules in SourceKit?
28. How does SwiftLint enforce new rules in SourceKit? Let’s step back for a second.
29. Why is this a hard problem?
30. Why is this a hard problem? • Print stuff in Xcode, and other formats • Tap into compilation process • Establish rules • ‘Parse’ files and assess them 
 based on new rules
31. Three Rule Protocols Rule, ParameterizedRule, & ASTRule
32. Three Rule Protocols Rule, ParameterizedRule, & ASTRule Each includes a set of triggering examples and non-triggering examples, which is very effective documentation.
33. validate(file: File) public func validateFile(file: File) -> [StyleViolation] {...}
 
 func validateFile(file: File, kind: SwiftDeclarationKind, dictionary: XPCDictionary) -> [StyleViolation] {...}
34. validate(file: File) 
 public final class File { public let path: String? public var contents: String public var lines: [Line]
35. public func validateFile(file: File) -> [StyleViolation] { let pattern = "(\\,[^\\s])|(\\s\\,)" let excludingKinds = SyntaxKind.commentAndStringKinds() return file.matchPattern(pattern, excludingSyntaxKinds: excludingKinds).map { StyleViolation(ruleDescription: self.dynamicType.description, location: Location(file: file, offset: $0.location)) } } CommaRule analyzes the file using regular expressions
36. public func validateFile(file: File, kind: SwiftDeclarationKind, dictionary: XPCDictionary) -> [StyleViolation] { return file.validateVariableName(dictionary, kind: kind).map { name, offset in let charCount = name.characters.count for parameter in self.parameters.reverse() where charCount > parameter.value { return [StyleViolation(ruleDescription: self.dynamicType.description, severity: parameter.severity, location: Location(file: file, offset: offset), reason: "Variable name should be \(parameter.value) characters " + "or less: currently \(charCount) characters")] } return [] } ?? [] } VariableNameMaxLengthRule analyzes the file as an XPCDictionary
37. SwiftLint uses SourceKit’s responses to generate an XPCDictionary for each file
38. SwiftLint (Realm) SourceKit (Apple) SwiftKitten (Realm) Swift Compiler Instance (Apple)
39. Works nicely in our app.
40. Linting is about collaboration and effectively, passively sharing code patterns.
41. I heard you like feedback, so I have some feedback on your feedback
42. In theatre directing, the first thing you say matters.
43. In code reviews, the content of your comments matters. Linting can help keep the collaboration between your team members pleasant and focused on important stuff.
44. What do you think would be a great collaboration tool for your team?
 

45. What do you think would be a great collaboration tool for your team?
 
 You should build it!
46. @tamarshmallows Say hi
47. Reading Materials Swift.org & Apple’s Github Repos SwiftLint, SourceKitten, and SwiftXPC
 
 JP Simard on Source Kit: http://www.jpsim.com/uncovering- sourcekit/ Chris Eidhof on the Objective-C Compiler: https://www.objc.io/ issues/6-build-tools/compiler Thinking In Swift Part II: Compiler: https:// www.accelebrate.com/blog/thinking-swift-part-ii/ OS X 10.10 Yosemite: The Ars Technica Review: http:// arstechnica.com/apple/2014/10/os-x-10-10/22/ 
 Hello SIL: http://swiftc.io/post/132002781944/hello-sil