Linting in Swift Tamar Nachmany

Listing is analyzing code and enforcing custom stylistic rules What is linting?

let greeting:String
 let greeting: String Rule: Leave space after colons

for item in library { if let movie = item as! Movie { print(“5 rotten tomatoes”) } } Rule: Don’t Allow Force Unwrapping

Why lint? Patterns & Onboarding
 Better code reviews
 Tooling

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.

Let’s explore linting by:
 • Investigating the Swift Compiler
 • Seeing how SwiftLint uses it
 • Answering the question, ‘Why Lint?’

What do linters get us during the compilation process?

What do linters get us during the compilation process? Extra errors.

Where do Xcode errors come from?

Before building, where do Xcode errors in Swift code come from? SourceKit, using an instance of the Swift Compiler

The Swift compiler is now open source

Basically

Credit: Chris Lattner and Joe Groff Swift Compilation

Credit: Chris Lattner and Joe Groff Swift Compilation

// 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

// 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

Let’s make an error: println(“yo”)

No content

What information do we need to display this? Error location Error message Fix-it suggestion Whether it’s an error or warning

When you enter new code into your project, Xcode sends messages to SourceKit.
 SourceKit responds with some info.

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" } ]

lib/Parse Response filename.swift:5:9: error: 'println' has been renamed to 'print' println("yo") ^~~~~~~ print

No content

Let’s look at a linter

SwiftLint by

How does SwiftLint enforce new rules in SourceKit?

How does SwiftLint enforce new rules in SourceKit? Let’s step back for a second.

Why is this a hard problem?

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

Three Rule Protocols Rule, ParameterizedRule, & ASTRule

Three Rule Protocols Rule, ParameterizedRule, & ASTRule Each includes a set of triggering examples and non-triggering examples, which is very effective documentation.

validate(file: File) public func validateFile(file: File) -> [StyleViolation] {...}
 
 func validateFile(file: File, kind: SwiftDeclarationKind, dictionary: XPCDictionary) -> [StyleViolation] {...}

validate(file: File) 
 public final class File { public let path: String? public var contents: String public var lines: [Line]

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

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

SwiftLint uses SourceKit’s responses to generate an XPCDictionary for each file

SwiftLint (Realm) SourceKit (Apple) SwiftKitten (Realm) Swift Compiler Instance (Apple)

Works nicely in our app.

Linting is about collaboration and effectively, passively sharing code patterns.

I heard you like feedback, so I have some feedback on your feedback

In theatre directing, the first thing you say matters.

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.

What do you think would be a great collaboration tool for your team?
 


What do you think would be a great collaboration tool for your team?
 
 You should build it!

@tamarshmallows Say hi

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