%title: impl 3rdTrait for 3rdType? %author: Ivan Tham %date: 2021-05-12 %usage: mdp slide.md -> Implementing 3rd party trait for 3rd party type <- ===================================================== Remember our last talk (at the end)? We talked about inline_python and how to pass data between rust and python. --- ``` impl ToPyObject for StockRecord<'_> { fn to_object(&self, py: Python<'_>) -> PyObject { [ self.day.to_string().to_object(py), // the hack self.rule_id.to_object(py), self.stock_id.to_object(py), self.price.to_object(py), self.volume.to_object(py), self.open.to_object(py), self.close.to_object(py), self.value.to_object(py), self.smi.to_object(py), self.power.to_object(py), self.power6m.to_object(py), ] .to_object(py) } } ``` --- The hack T_T ------------ - `chrono::NaiveDate` cannot be converted to `pyo3::PyObject` - Why? `chrono::NaiveDate` does not implement `ToPyObject` (demo) - How? --- Let's try implementing that ourselves. ``` impl ToPyObject for chrono::NaiveDate { fn to_object(&self, py: Python<'_>) -> PyObject { self.to_string().to_object(py) } } ``` --- > **お前はもう死んでいる。** > (You are already dead.) --- ``` ``:::::'' ``` --- ``` ``::::||||||||:'' ``:::::'' ``` --- ``` :::::::::::::||||::' ``::::||||||||:'' ``:::::'' ``` --- ``` ::... ..::||||:' :::::::::::::||||::' ``::::||||||||:'' ``:::::'' ``` --- ``` .. .::|||:' ::... ..::||||:' :::::::::::::||||::' ``::::||||||||:'' ``:::::'' ``` --- ``` ~~~ ::|||: .. .::|||:' ::... ..::||||:' :::::::::::::||||::' ``::::||||||||:'' ``:::::'' ``` --- ``` . `. .' ,::||: ~~~ ::|||: .. .::|||:' ::... ..::||||:' :::::::::::::||||::' ``::::||||||||:'' ``:::::'' ``` --- ``` | | ::|:: . `. .' ,::||: ~~~ ::|||: .. .::|||:' ::... ..::||||:' :::::::::::::||||::' ``::::||||||||:'' ``:::::'' ``` --- ``` . | | :::: | | ::|:: . `. .' ,::||: ~~~ ::|||: .. .::|||:' ::... ..::||||:' :::::::::::::||||::' ``::::||||||||:'' ``:::::'' ``` --- ``` | | `::: . | | :::: | | ::|:: . `. .' ,::||: ~~~ ::|||: .. .::|||:' ::... ..::||||:' :::::::::::::||||::' ``::::||||||||:'' ``:::::'' ``` --- ``` . | | `:::. | | `::: . | | :::: | | ::|:: . `. .' ,::||: ~~~ ::|||: .. .::|||:' ::... ..::||||:' :::::::::::::||||::' ``::::||||||||:'' ``:::::'' ``` --- ``` . `| |':.. . | | `:::. | | `::: . | | :::: | | ::|:: . `. .' ,::||: ~~~ ::|||: .. .::|||:' ::... ..::||||:' :::::::::::::||||::' ``::::||||||||:'' ``:::::'' ``` --- ``` _ ~~~ _ . `| |':.. . | | `:::. | | `::: . | | :::: | | ::|:: . `. .' ,::||: ~~~ ::|||: .. .::|||:' ::... ..::||||:' :::::::::::::||||::' ``::::||||||||:'' ``:::::'' ``` --- ``` `...::' _ ~~~ _ . `| |':.. . | | `:::. | | `::: . | | :::: | | ::|:: . `. .' ,::||: ~~~ ::|||: .. .::|||:' ::... ..::||||:' :::::::::::::||||::' ``::::||||||||:'' ``:::::'' ``` --- ``` | .:| `...::' _ ~~~ _ . `| |':.. . | | `:::. | | `::: . | | :::: | | ::|:: . `. .' ,::||: ~~~ ::|||: .. .::|||:' ::... ..::||||:' :::::::::::::||||::' ``::::||||||||:'' ``:::::'' ``` --- ``` | :| | .:| `...::' _ ~~~ _ . `| |':.. . | | `:::. | | `::: . | | :::: | | ::|:: . `. .' ,::||: ~~~ ::|||: .. .::|||:' ::... ..::||||:' :::::::::::::||||::' ``::::||||||||:'' ``:::::'' ``` --- ``` | :| | :| | .:| `...::' _ ~~~ _ . `| |':.. . | | `:::. | | `::: . | | :::: | | ::|:: . `. .' ,::||: ~~~ ::|||: .. .::|||:' ::... ..::||||:' :::::::::::::||||::' ``::::||||||||:'' ``:::::'' ``` --- ``` | :| | :| | :| | .:| `...::' _ ~~~ _ . `| |':.. . | | `:::. | | `::: . | | :::: | | ::|:: . `. .' ,::||: ~~~ ::|||: .. .::|||:' ::... ..::||||:' :::::::::::::||||::' ``::::||||||||:'' ``:::::'' ``` --- ``` | :| | :| | :| | :| | .:| `...::' _ ~~~ _ . `| |':.. . | | `:::. | | `::: . | | :::: | | ::|:: . `. .' ,::||: ~~~ ::|||: .. .::|||:' ::... ..::||||:' :::::::::::::||||::' ``::::||||||||:'' ``:::::'' ``` --- ``` _____ | :| | :| | :| | :| | .:| `...::' _ ~~~ _ . `| |':.. . | | `:::. | | `::: . | | :::: | | ::|:: . `. .' ,::||: ~~~ ::|||: .. .::|||:' ::... ..::||||:' :::::::::::::||||::' ``::::||||||||:'' ``` --- ``` ____ __,-~~/~ `---. _/_,---( , ) __ / < / ) \___ - ------===;;;'====------------------===;;;===----- - - \/ ~"~"~"~"~"~\~"~)~"/ (_ ( \ ( > \) \_( _ < >_>' ~ `-i' ::>|--" I;|.|.| <|i::|i|`. (` ^'"`-' ") ``` --- > **何？** > (What?) --- **error[E0117]**: only traits defined in the current crate can be implemented for arbitrary types --> src/main.rs:247:1 | 247 | impl ToPyObject for chrono::NaiveDate { | ^^^^^^^^^^^^^^^^^^^^----------------- | | | | | _*\`NaiveDate\` is not defined in the current crate*_ | **impl doesn't use only types from inside the current crate** | = note: define and implement a trait or new type instead **error**: aborting due to previous error For more information about this error, try \`rustc --explain E0117\`. **error**: could not compile \`hello\` To learn more, run the command again with --verbose. (colors replicated to some extent but next time) --- -> Orphan Rule <- ================= ^ Caused by ``` ____ ____ ____ __ _____ _ __ ____ __ ____ ________ / __ \/ __ \/ __ \/ / / / | / | / / / __ \/ / / / / / ____/ / / / / / /_/ / /_/ / /_/ / /| | / |/ / / /_/ / / / / / / __/ / / / /_/ / _, _/ ____/ __ / ___ |/ /| / / _, _/ /_/ / /___/ /___/_/ \____/_/ |_/_/ /_/ /_/_/ |_/_/ |_/ /_/ |_|\____/_____/_____(_) ``` I repeat. ^ Before going into that ... > https://github.com/Ixrec/rust-orphan-rules --- -> Coherence <- =============== Trait coherence (or coherence): > **At most one implementation** of a trait for any given type. Any programming language that has a feature like traits or interfaces must either: - Enforce coherence by simply refusing to compile programs that contain conflicting implementations - Embrace incoherence and give programmers a way to manually choose an implementation when there's a conflict Rust chooses to enforce coherence. --- -> Orphan Rule <- ================= Two rules - "overlap rules" - no two `impl` blocks that "overlap" - `impl Trait for T` - `impl Trait for T` - current workaround is through "specialization" (unstable) - "orphan rules" - either the type or trait must be from the same crate - Why? - prevent "dependency hell", what if both crates have the same impl? - allow `impl` to be added without being a breaking change - In our case, `pyo3` and `chrono` both comes from different crates --- -> So how? <- ============= Note that I only show some solutions that was done here. - newtype (chrono-pyo3 crate) - https://github.com/kangalioo/pyo3-chrono - easy solution, just `struct NaiveDate(NaiveDate)` - then just delegate everything but add an additional `impl` - may not be good for popular/core crates like pyo3 - integrate it directly with the library (we will discuss this) - WIP https://github.com/chronotope/chrono/pull/542 - can utilize underlying data (implementation details) directly - in this case, troublesome since MSRV for chrono is low --- -> Change chrono upstream <- ============================ Why not change pyo3? I started out with chrono and think it is a good idea. But basically could be added to either side. --- -> Cargo features <- ==================== Conditional compilation for pyo3 integration, using crates features flag. ``` [dependencies] libc = { version = "0.2.69", optional = true } time = { version = "0.1.43", optional = true } num-integer = { version = "0.1.36", default-features = false } num-traits = { version = "0.2", default-features = false } rustc-serialize = { version = "0.3.20", optional = true } serde = { version = "1.0.99", default-features = false, optional = true } pure-rust-locales = { version = "0.5.2", optional = true } pyo3 = { version = "0.13.2", optional = true } ``` \^ implicit feature infered from dependency I guess -> Usage <- ----------- ``` [dependencies] chrono = { version = "0.4", features = ["pyo3"] } ``` --- -> Code <- ========== > Talk is cheap. Show me the code. -- Linus Torvalds ``` #[cfg(feature = "pyo3")] mod pyo3 { use ...; impl ToPyObject for NaiveDate { fn to_object(&self, py: pyo3::Python) -> pyo3::PyObject { let mdf = self.mdf(); let date = PyDate::new(py, self.year(), mdf.month() as u8, mdf.day() as u8) .expect("Failed to construct date"); date.into() } } impl IntoPy for NaiveDate { ... } impl FromPyObject<'_> for NaiveDate { ... } } ``` --- -> Progress <- ============== Pull request still not done yet due to pyo3 upstream missing python datetime FFI components for timezone. https://github.com/PyO3/pyo3/pull/1588 That is way harder to talk about and probably not for meetup. T_T --- -> Q&A <- =========