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178 lines
10 KiB
Plaintext
---
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slug: 2018/06/dateutil-parser-to-rust
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title: "What I learned porting dateutil to Rust"
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date: 2018-06-25 12:00:00
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authors: [bspeice]
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tags: []
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---
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I've mostly been a lurker in Rust for a while, making a couple small contributions here and there.
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So launching [dtparse](https://github.com/bspeice/dtparse) feels like nice step towards becoming a
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functioning member of society. But not too much, because then you know people start asking you to
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pay bills, and ain't nobody got time for that.
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<!-- truncate -->
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But I built dtparse, and you can read about my thoughts on the process. Or don't. I won't tell you
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what to do with your life (but you should totally keep reading).
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## Slow down, what?
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OK, fine, I guess I should start with _why_ someone would do this.
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[Dateutil](https://github.com/dateutil/dateutil) is a Python library for handling dates. The
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standard library support for time in Python is kinda dope, but there are a lot of extras that go
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into making it useful beyond just the [datetime](https://docs.python.org/3.6/library/datetime.html)
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module. `dateutil.parser` specifically is code to take all the super-weird time formats people come
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up with and turn them into something actually useful.
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Date/time parsing, it turns out, is just like everything else involving
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[computers](https://infiniteundo.com/post/25326999628/falsehoods-programmers-believe-about-time) and
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[time](https://infiniteundo.com/post/25509354022/more-falsehoods-programmers-believe-about-time): it
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feels like it shouldn't be that difficult to do, until you try to do it, and you realize that people
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suck and this is why
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[we can't we have nice things](https://zachholman.com/talk/utc-is-enough-for-everyone-right). But
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alas, we'll try and make contemporary art out of the rubble and give it a pretentious name like
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_Time_.
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> [Time](https://www.goodfreephotos.com/united-states/montana/elkhorn/remains-of-the-mining-operation-elkhorn.jpg.php)
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What makes `dateutil.parser` great is that there's single function with a single argument that
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drives what programmers interact with:
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[`parse(timestr)`](https://github.com/dateutil/dateutil/blob/6dde5d6298cfb81a4c594a38439462799ed2aef2/dateutil/parser/_parser.py#L1258).
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It takes in the time as a string, and gives you back a reasonable "look, this is the best anyone can
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possibly do to make sense of your input" value. It doesn't expect much of you.
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[And now it's in Rust.](https://github.com/bspeice/dtparse/blob/7d565d3a78876dbebd9711c9720364fe9eba7915/src/lib.rs#L1332)
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## Lost in Translation
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Having worked at a bulge-bracket bank watching Java programmers try to be Python programmers, I'm
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admittedly hesitant to publish Python code that's trying to be Rust. Interestingly, Rust code can
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actually do a great job of mimicking Python. It's certainly not idiomatic Rust, but I've had better
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experiences than
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[this guy](https://webcache.googleusercontent.com/search?q=cache:wkYMpktJtnUJ:https://jackstouffer.com/blog/porting_dateutil.html+&cd=3&hl=en&ct=clnk&gl=us)
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who attempted the same thing for D. These are the actual take-aways:
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When transcribing code, **stay as close to the original library as possible**. I'm talking about
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using the same variable names, same access patterns, the whole shebang. It's way too easy to make a
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couple of typos, and all of a sudden your code blows up in new and exciting ways. Having a reference
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manual for verbatim what your code should be means that you don't spend that long debugging
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complicated logic, you're more looking for typos.
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Also, **don't use nice Rust things like enums**. While
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[one time it worked out OK for me](https://github.com/bspeice/dtparse/blob/7d565d3a78876dbebd9711c9720364fe9eba7915/src/lib.rs#L88-L94),
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I also managed to shoot myself in the foot a couple times because `dateutil` stores AM/PM as a
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boolean and I mixed up which was true, and which was false (side note: AM is false, PM is true). In
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general, writing nice code _should not be a first-pass priority_ when you're just trying to recreate
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the same functionality.
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**Exceptions are a pain.** Make peace with it. Python code is just allowed to skip stack frames. So
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when a co-worker told me "Rust is getting try-catch syntax" I properly freaked out. Turns out
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[he's not quite right](https://github.com/rust-lang/rfcs/pull/243), and I'm OK with that. And while
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`dateutil` is pretty well-behaved about not skipping multiple stack frames,
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[130-line try-catch blocks](https://github.com/dateutil/dateutil/blob/16561fc99361979e88cccbd135393b06b1af7e90/dateutil/parser/_parser.py#L730-L865)
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take a while to verify.
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As another Python quirk, **be very careful about
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[long nested if-elif-else blocks](https://github.com/dateutil/dateutil/blob/16561fc99361979e88cccbd135393b06b1af7e90/dateutil/parser/_parser.py#L494-L568)**.
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I used to think that Python's whitespace was just there to get you to format your code correctly. I
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think that no longer. It's way too easy to close a block too early and have incredibly weird issues
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in the logic. Make sure you use an editor that displays indentation levels so you can keep things
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straight.
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**Rust macros are not free.** I originally had the
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[main test body](https://github.com/bspeice/dtparse/blob/b0e737f088eca8e83ab4244c6621a2797d247697/tests/compat.rs#L63-L217)
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wrapped up in a macro using [pyo3](https://github.com/PyO3/PyO3). It took two minutes to compile.
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After
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[moving things to a function](https://github.com/bspeice/dtparse/blob/e017018295c670e4b6c6ee1cfff00dbb233db47d/tests/compat.rs#L76-L205)
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compile times dropped down to ~5 seconds. Turns out 150 lines \* 100 tests = a lot of redundant code
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to be compiled. My new rule of thumb is that any macros longer than 10-15 lines are actually
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functions that need to be liberated, man.
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Finally, **I really miss list comprehensions and dictionary comprehensions.** As a quick comparison,
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see
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[this dateutil code](https://github.com/dateutil/dateutil/blob/16561fc99361979e88cccbd135393b06b1af7e90/dateutil/parser/_parser.py#L476)
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and
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[the implementation in Rust](https://github.com/bspeice/dtparse/blob/7d565d3a78876dbebd9711c9720364fe9eba7915/src/lib.rs#L619-L629).
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I probably wrote it wrong, and I'm sorry. Ultimately though, I hope that these comprehensions can be
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added through macros or syntax extensions. Either way, they're expressive, save typing, and are
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super-readable. Let's get more of that.
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## Using a young language
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Now, Rust is exciting and new, which means that there's opportunity to make a substantive impact. On
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more than one occasion though, I've had issues navigating the Rust ecosystem.
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What I'll call the "canonical library" is still being built. In Python, if you need datetime
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parsing, you use `dateutil`. If you want `decimal` types, it's already in the
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[standard library](https://docs.python.org/3.6/library/decimal.html). While I might've gotten away
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with `f64`, `dateutil` uses decimals, and I wanted to follow the principle of **staying as close to
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the original library as possible**. Thus began my quest to find a decimal library in Rust. What I
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quickly found was summarized in a comment:
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> Writing a BigDecimal is easy. Writing a _good_ BigDecimal is hard.
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>
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> [-cmr](https://github.com/rust-lang/rust/issues/8937#issuecomment-34582794)
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In practice, this means that there are at least [4](https://crates.io/crates/bigdecimal)
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[different](https://crates.io/crates/rust_decimal)
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[implementations](https://crates.io/crates/decimal) [available](https://crates.io/crates/decimate).
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And that's a lot of decisions to worry about when all I'm thinking is "why can't
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[calendar reform](https://en.wikipedia.org/wiki/Calendar_reform) be a thing" and I'm forced to dig
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through a [couple](https://github.com/rust-lang/rust/issues/8937#issuecomment-31661916)
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[different](https://github.com/rust-lang/rfcs/issues/334)
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[threads](https://github.com/rust-num/num/issues/8) to figure out if the library I'm look at is dead
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or just stable.
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And even when the "canonical library" exists, there's no guarantees that it will be well-maintained.
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[Chrono](https://github.com/chronotope/chrono) is the _de facto_ date/time library in Rust, and just
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released version 0.4.4 like two days ago. Meanwhile,
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[chrono-tz](https://github.com/chronotope/chrono-tz) appears to be dead in the water even though
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[there are people happy to help maintain it](https://github.com/chronotope/chrono-tz/issues/19). I
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know relatively little about it, but it appears that most of the release process is automated;
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keeping that up to date should be a no-brainer.
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## Trial Maintenance Policy
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Specifically given "maintenance" being an
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[oft-discussed](https://www.reddit.com/r/rust/comments/48540g/thoughts_on_initiators_vs_maintainers/)
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issue, I'm going to try out the following policy to keep things moving on `dtparse`:
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1. Issues/PRs needing _maintainer_ feedback will be updated at least weekly. I want to make sure
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nobody's blocking on me.
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2. To keep issues/PRs needing _contributor_ feedback moving, I'm going to (kindly) ask the
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contributor to check in after two weeks, and close the issue without resolution if I hear nothing
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back after a month.
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The second point I think has the potential to be a bit controversial, so I'm happy to receive
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feedback on that. And if a contributor responds with "hey, still working on it, had a kid and I'm
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running on 30 seconds of sleep a night," then first: congratulations on sustaining human life. And
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second: I don't mind keeping those requests going indefinitely. I just want to try and balance
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keeping things moving with giving people the necessary time they need.
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I should also note that I'm still getting some best practices in place - CONTRIBUTING and
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CONTRIBUTORS files need to be added, as well as issue/PR templates. In progress. None of us are
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perfect.
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## Roadmap and Conclusion
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So if I've now built a `dateutil`-compatible parser, we're done, right? Of course not! That's not
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nearly ambitious enough.
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Ultimately, I'd love to have a library that's capable of parsing everything the Linux `date` command
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can do (and not `date` on OSX, because seriously, BSD coreutils are the worst). I know Rust has a
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coreutils rewrite going on, and `dtparse` would potentially be an interesting candidate since it
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doesn't bring in a lot of extra dependencies. [`humantime`](https://crates.io/crates/humantime)
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could help pick up some of the (current) slack in dtparse, so maybe we can share and care with each
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other?
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All in all, I'm mostly hoping that nobody's already done this and I haven't spent a bit over a month
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on redundant code. So if it exists, tell me. I need to know, but be nice about it, because I'm going
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to take it hard.
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And in the mean time, I'm looking forward to building more. Onwards.
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