Rust API Design Learnings

Transcript

1. Armin @mitsuhiko Ronacher Rust API Design Learnings Lessons learned from building Rust libraries
2. Who am I • Armin Ronacher • twitter.com/@mitsuhiko | hachyderm.io/@mitsuhiko • Python since time immemorial, Rust since 2012 • Python: Flask, Jinja, Werkzeug, … • Rust: Insta, MiniJinja, Console, Indicatif, Similar
3. unnamed developer of a popular Rust crate “Sorry, I have no interest in making that style of coding easier. I want users to consciously choose what config they're using. I view blindly picking a default as a mistake […]”
4. APIs are Important • A library's author's true success metrics are: • how successful all users are in using the API • the quality of the output that users achieve by using the API • the percentage of users making the correct choices
5. Your User Matters • When you build a library you should treat it like any other thing • De fi ne success metrics • Measure yourself
6. But we are Flying Blind • Library developers typically fl y blind • The only metrics we have is download stats, which mostly correlate with CI setups, and not true utilization • User frustration is often the only other form of feedback we get • We need extrapolation from user surveys and interviews • In the absence of this, personal frustration and issues is a good proxy
7. Values: Metrics without Measuring • If we have trouble measuring, metrics are not useless • Metrics often express what we believe is important • Values can steer us
8. Values and Metrics
9. My Values • Concise: easy to get started • Good Defaults: easy to get started, trivial to stay on the golden path as it changes • Small Surface Area: enable room to breath and innovate, without breaking users • Backwards compatible: avoid unnecessary churn to keep users on the golden path
10. The Golden Path
11. The Golden Path • An opinionated path for how to build • That path might change over time • Change requires adjustment by users • Fast change means users being left behind • Measuring success: users on the golden path (not churning, not staying on old versions, not hating the upgrade experience, not using old patterns)
12. Defaults Matter
13. Use Defaults to Fight Cargo Cult • Defaults are hard and of two types: • Absolute defaults that cannot be changed (i32::default() -> 0) • Defaults that allow a level of fl exibility (Default Hasher: SipHash) • For defaults to allow fl exibility, care has to be taken: • Set rules and expectations about stability • Aim for some level of change
14. Good Defaults • Default Hasher: • Hasher is documented to be non portable • Hasher is documented to change • No expectation around cross-version/process stability • A better hasher can be picked, all code ever written bene fi ts at once
15. Cargo Cult • Imagine mandatory hasher • People would cargo cult some default
 hasher that they see elsewhere or in
 the docs. • New hasher comes around, lots of code
 stuck with the old choice.
16. Defaults and Protocols • What if this hash becomes part of a protocol? • If you have an API that drives a protocol, consider that protocol to consider defaults • This approach can only be guidance, a lot of situations do not allow it.
17. Less is More
18. More API = More Problems • The larger the surface, the more of it ends up used • Less commonly used APIs have the most leaky abstractions • Inhibits future change: "does someone even use this?"
19. Hide API Behind Common Abstractions • Developers are used to these patterns, they are worth exploring: • Into<T> • AsRef<T> • Careful: surface area stays large, but large bound to common and simple patterns
20. Into • Common pairs: • Into<String> • Into<Cow<'_, T>> • Into<YourRuntimeType> • ToString can be sometimes an interesting alternative to Into<String>
21. AsRef<T> • Related in Into, but for borrowing • Abstracts over • &String/&str/&Cow<'_, str> • &PathBuf/&Path • &[u8]/&Vec<u8>/&String/&str
22. Monomorphization & Compile Times • Rust loves to inline • All those di ff erent types create
 duplicated generated code • Example: isolate conversions and
 call into shared functions to
 reduce the total amount of copied
 code.
23. Hide the Onion but create the Onion • Good APIs are Layered Like Onions • Only provide the outermost layer fi rst • Keeps the inner layers fl exibility to change • Over time, consider exposing internal layers under separate stability guarantees
24. Layer 2 and 3 • Example: CompiledTemplate is
 entirely private, so is the
 CodeGenerator or the parser. • It's still layered, and over time
 some functionality could be
 exposed.
25. Crate Structure
26. Explicit Exports • Hide your internal structure, re-export sensibly • Your folder structure does not matter to your users
27. Explicit Fake Modules • Consider creating modules on the spot for utilities • For instance "insta" has utility
 functions and types that are rarely
 useful. The ones I subscribe stability
 to are re-exported under a speci fi c
 module.
28. Public but Hidden • Sometimes stu ff needs to be public,
 but you don't want anyone to use it. • Common example: utility functionality
 for macros. • Here both __context and
 __context_pair! are public but hidden
29. Traits
30. Traits are Tricky • Traits are super useful, but they are tricky • Fall into two categories: • Sealed (user should not implement) • Open (user should implement)
31. Sealed Traits • Not really supported, doc hidden
 and hackery • Example in MiniJinja: want to
 abstract over types, but I don't
 really want to let the user do that.
32. Full Seal • Uses a private zero sized marker type somewhere • User cannot implement or invoke as the type is private
33. Traits are Hard to Discover • I avoid traits unless I know abstraction over implementations is necessary • Did you notice that BTreeMap and HashMap are not expressed via traits? • The usefulness of abstraction even for interchangeable types is sometimes unclear • You can always add traits later
34. Common Traits
35. Debug • Put it on all public types • Consider it on your internal types behind a feature fl ag • Super valuable for dbg!() and co
36. Display • Makes the type have a representation in format!() • It also gives it the `.to_string()` method • Certain types need it in the contract (eg: all errors) • Recommendation: avoid in most cases unless you implement a custom integer, string etc.
37. Copy and Clone • Once granted, impossible to take away • Neither can be universally provided • Clone: really useful, consider adding • If you ever feel you need to take it away, consider Arc<T> internally • Copy: might inhibit future change, but really useful • Some types regrettably do not have Copy (eg: Range) and people hate it
38. Sync and Send • I cannot give recommendations • The only one I have: non Send/Sync types are not that bad • Consider them seriously
39. Lifetimes
40. Lifetimes and Libraries • Try to avoid too clever setups • Consider "Session" abstractions where people only need to temporarily hold on to data.
41. Borrowing to Self • Rust is really bad at this, sometimes you build yourself into a corner • Best tool I found to date for this is the self_cell crate • Bu ff er can be held into itself
42. Erroring
43. Panic vs Error • Try to avoid panics • If you do need to panic, consider #[track_caller]
44. Errors Matter • Spend some time designing your errors • Errors deserve attention just as much as your other types • A talk all by itself, so here the basics: • Implement std::error::Error on your errors • Implement source() if you think someone might want to peak into
45. Questions!