Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

[SR-15588] Generic/overload type-checking slowness #57891

stephencelis opened this issue Dec 13, 2021 · 0 comments

[SR-15588] Generic/overload type-checking slowness #57891

stephencelis opened this issue Dec 13, 2021 · 0 comments


Copy link

@stephencelis stephencelis commented Dec 13, 2021

Previous ID SR-15588
Radar None
Original Reporter @stephencelis
Type Improvement
Additional Detail from JIRA
Votes 0
Component/s Compiler
Labels Improvement
Assignee None
Priority Medium

md5: 55b28a544364316a0bf385d9e2b129c2

Issue Description:

I wanted to contribute the following code that uses generics, overloads, closures, and String initializers to produce a relatively short expression take 10 seconds or so to compile:

protocol Parser {
  associatedtype Input
  associatedtype Output
  func parse(_ input: inout Input) -> Output?

struct Prefix<Input>: Parser
  Input: Collection,
  Input.SubSequence == Input
  let predicate: (Input.Element) -> Bool
  func parse(_ input: inout Input) -> Input? {
    let output = input.prefix(while: predicate)
    return output

extension Prefix where Input == Substring {
  init(predicate: @escaping (Character) -> Bool) {
    self.predicate = predicate

struct Map<Upstream, Output>: Parser
  Upstream: Parser
  let upstream: Upstream
  let transform: (Upstream.Output) -> Output
  func parse(_ input: inout Upstream.Input) -> Output? {

extension Parser {
  func map<NewOutput>(
    _ transform: @escaping (Output) -> NewOutput
  ) -> Map<Self, NewOutput> {
    .init(upstream: self, transform: transform)

struct Zip<A, B>: Parser where A: Parser, B: Parser, A.Input == B.Input {
  let a: A
  let b: B

  init(_ a: A, _ b: B) {
    self.a = a
    self.b = b

  func parse(_ input: inout A.Input) -> (A.Output, B.Output)? {
    let original = input
      let a = self.a.parse(&input),
      let b = self.b.parse(&input)
    else {
      input = original
      return nil
    return (a, b)

func f() {
  // Expression took 9680ms to type-check
  _ = Zip(Prefix { $0 != "-" }, Prefix { $0 != "-" })
    .map { (String($0), String($1)) }

  // Expression took 12457ms to type-check
  _ = Zip(
    Prefix { $0 != "-" }.map(String.init),
    Prefix { $0 != "-" }.map(String.init)

There are tricks one can employ to improve the compilation time by a lot:

  • Explicitly specify a generic (in the Prefix type, for example)

  • Explicitly define the types in the map closure

  • Use string interpolation instead of a string initializer

Hopefully, though, improvements can be made to the type checker to get things building a bit more quickly.

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment
None yet

No branches or pull requests

1 participant