mirror of
https://github.com/bspeice/speice.io
synced 2024-12-23 00:58:09 -05:00
178 lines
10 KiB
Markdown
178 lines
10 KiB
Markdown
---
|
|
layout: post
|
|
title: "What I Learned: Porting Dateutil Parser to Rust"
|
|
description: ""
|
|
category:
|
|
tags: [dtparse, rust]
|
|
---
|
|
|
|
Hi. I'm Bradlee.
|
|
|
|
I've mostly been a lurker in Rust for a while, making a couple small contributions here and there.
|
|
So launching [dtparse](https://github.com/bspeice/dtparse) feels like nice step towards becoming a
|
|
functioning member of society. But not too much, because then you know people start asking you to
|
|
pay bills, and ain't nobody got time for that.
|
|
|
|
But I built dtparse, and you can read about my thoughts on the process. Or don't. I won't tell you
|
|
what to do with your life (but you should totally keep reading).
|
|
|
|
# Slow down, what?
|
|
|
|
OK, fine, I guess I should start with _why_ someone would do this.
|
|
|
|
[Dateutil](https://github.com/dateutil/dateutil) is a Python library for handling dates. The
|
|
standard library support for time in Python is kinda dope, but there are a lot of extras that go
|
|
into making it useful beyond just the [datetime](https://docs.python.org/3.6/library/datetime.html)
|
|
module. `dateutil.parser` specifically is code to take all the super-weird time formats people come
|
|
up with and turn them into something actually useful.
|
|
|
|
Date/time parsing, it turns out, is just like everything else involving
|
|
[computers](https://infiniteundo.com/post/25326999628/falsehoods-programmers-believe-about-time) and
|
|
[time](https://infiniteundo.com/post/25509354022/more-falsehoods-programmers-believe-about-time): it
|
|
feels like it shouldn't be that difficult to do, until you try to do it, and you realize that people
|
|
suck and this is why
|
|
[we can't we have nice things](https://zachholman.com/talk/utc-is-enough-for-everyone-right). But
|
|
alas, we'll try and make contemporary art out of the rubble and give it a pretentious name like
|
|
_Time_.
|
|
|
|
|
|
|
|
> [Time](https://www.goodfreephotos.com/united-states/montana/elkhorn/remains-of-the-mining-operation-elkhorn.jpg.php)
|
|
|
|
What makes `dateutil.parser` great is that there's single function with a single argument that
|
|
drives what programmers interact with:
|
|
[`parse(timestr)`](https://github.com/dateutil/dateutil/blob/6dde5d6298cfb81a4c594a38439462799ed2aef2/dateutil/parser/_parser.py#L1258).
|
|
It takes in the time as a string, and gives you back a reasonable "look, this is the best anyone can
|
|
possibly do to make sense of your input" value. It doesn't expect much of you.
|
|
|
|
[And now it's in Rust.](https://github.com/bspeice/dtparse/blob/7d565d3a78876dbebd9711c9720364fe9eba7915/src/lib.rs#L1332)
|
|
|
|
# Lost in Translation
|
|
|
|
Having worked at a bulge-bracket bank watching Java programmers try to be Python programmers, I'm
|
|
admittedly hesitant to publish Python code that's trying to be Rust. Interestingly, Rust code can
|
|
actually do a great job of mimicking Python. It's certainly not idiomatic Rust, but I've had better
|
|
experiences than
|
|
[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)
|
|
who attempted the same thing for D. These are the actual take-aways:
|
|
|
|
When transcribing code, **stay as close to the original library as possible**. I'm talking about
|
|
using the same variable names, same access patterns, the whole shebang. It's way too easy to make a
|
|
couple of typos, and all of a sudden your code blows up in new and exciting ways. Having a reference
|
|
manual for verbatim what your code should be means that you don't spend that long debugging
|
|
complicated logic, you're more looking for typos.
|
|
|
|
Also, **don't use nice Rust things like enums**. While
|
|
[one time it worked out OK for me](https://github.com/bspeice/dtparse/blob/7d565d3a78876dbebd9711c9720364fe9eba7915/src/lib.rs#L88-L94),
|
|
I also managed to shoot myself in the foot a couple times because `dateutil` stores AM/PM as a
|
|
boolean and I mixed up which was true, and which was false (side note: AM is false, PM is true). In
|
|
general, writing nice code _should not be a first-pass priority_ when you're just trying to recreate
|
|
the same functionality.
|
|
|
|
**Exceptions are a pain.** Make peace with it. Python code is just allowed to skip stack frames. So
|
|
when a co-worker told me "Rust is getting try-catch syntax" I properly freaked out. Turns out
|
|
[he's not quite right](https://github.com/rust-lang/rfcs/pull/243), and I'm OK with that. And while
|
|
`dateutil` is pretty well-behaved about not skipping multiple stack frames,
|
|
[130-line try-catch blocks](https://github.com/dateutil/dateutil/blob/16561fc99361979e88cccbd135393b06b1af7e90/dateutil/parser/_parser.py#L730-L865)
|
|
take a while to verify.
|
|
|
|
As another Python quirk, **be very careful about
|
|
[long nested if-elif-else blocks](https://github.com/dateutil/dateutil/blob/16561fc99361979e88cccbd135393b06b1af7e90/dateutil/parser/_parser.py#L494-L568)**.
|
|
I used to think that Python's whitespace was just there to get you to format your code correctly. I
|
|
think that no longer. It's way too easy to close a block too early and have incredibly weird issues
|
|
in the logic. Make sure you use an editor that displays indentation levels so you can keep things
|
|
straight.
|
|
|
|
**Rust macros are not free.** I originally had the
|
|
[main test body](https://github.com/bspeice/dtparse/blob/b0e737f088eca8e83ab4244c6621a2797d247697/tests/compat.rs#L63-L217)
|
|
wrapped up in a macro using [pyo3](https://github.com/PyO3/PyO3). It took two minutes to compile.
|
|
After
|
|
[moving things to a function](https://github.com/bspeice/dtparse/blob/e017018295c670e4b6c6ee1cfff00dbb233db47d/tests/compat.rs#L76-L205)
|
|
compile times dropped down to ~5 seconds. Turns out 150 lines \* 100 tests = a lot of redundant code
|
|
to be compiled. My new rule of thumb is that any macros longer than 10-15 lines are actually
|
|
functions that need to be liberated, man.
|
|
|
|
Finally, **I really miss list comprehensions and dictionary comprehensions.** As a quick comparison,
|
|
see
|
|
[this dateutil code](https://github.com/dateutil/dateutil/blob/16561fc99361979e88cccbd135393b06b1af7e90/dateutil/parser/_parser.py#L476)
|
|
and
|
|
[the implementation in Rust](https://github.com/bspeice/dtparse/blob/7d565d3a78876dbebd9711c9720364fe9eba7915/src/lib.rs#L619-L629).
|
|
I probably wrote it wrong, and I'm sorry. Ultimately though, I hope that these comprehensions can be
|
|
added through macros or syntax extensions. Either way, they're expressive, save typing, and are
|
|
super-readable. Let's get more of that.
|
|
|
|
# Using a young language
|
|
|
|
Now, Rust is exciting and new, which means that there's opportunity to make a substantive impact. On
|
|
more than one occasion though, I've had issues navigating the Rust ecosystem.
|
|
|
|
What I'll call the "canonical library" is still being built. In Python, if you need datetime
|
|
parsing, you use `dateutil`. If you want `decimal` types, it's already in the
|
|
[standard library](https://docs.python.org/3.6/library/decimal.html). While I might've gotten away
|
|
with `f64`, `dateutil` uses decimals, and I wanted to follow the principle of **staying as close to
|
|
the original library as possible**. Thus began my quest to find a decimal library in Rust. What I
|
|
quickly found was summarized in a comment:
|
|
|
|
> Writing a BigDecimal is easy. Writing a _good_ BigDecimal is hard.
|
|
>
|
|
> [-cmr](https://github.com/rust-lang/rust/issues/8937#issuecomment-34582794)
|
|
|
|
In practice, this means that there are at least [4](https://crates.io/crates/bigdecimal)
|
|
[different](https://crates.io/crates/rust_decimal)
|
|
[implementations](https://crates.io/crates/decimal) [available](https://crates.io/crates/decimate).
|
|
And that's a lot of decisions to worry about when all I'm thinking is "why can't
|
|
[calendar reform](https://en.wikipedia.org/wiki/Calendar_reform) be a thing" and I'm forced to dig
|
|
through a [couple](https://github.com/rust-lang/rust/issues/8937#issuecomment-31661916)
|
|
[different](https://github.com/rust-lang/rfcs/issues/334)
|
|
[threads](https://github.com/rust-num/num/issues/8) to figure out if the library I'm look at is dead
|
|
or just stable.
|
|
|
|
And even when the "canonical library" exists, there's no guarantees that it will be well-maintained.
|
|
[Chrono](https://github.com/chronotope/chrono) is the _de facto_ date/time library in Rust, and just
|
|
released version 0.4.4 like two days ago. Meanwhile,
|
|
[chrono-tz](https://github.com/chronotope/chrono-tz) appears to be dead in the water even though
|
|
[there are people happy to help maintain it](https://github.com/chronotope/chrono-tz/issues/19). I
|
|
know relatively little about it, but it appears that most of the release process is automated;
|
|
keeping that up to date should be a no-brainer.
|
|
|
|
## Trial Maintenance Policy
|
|
|
|
Specifically given "maintenance" being an
|
|
[oft-discussed](https://www.reddit.com/r/rust/comments/48540g/thoughts_on_initiators_vs_maintainers/)
|
|
issue, I'm going to try out the following policy to keep things moving on `dtparse`:
|
|
|
|
1. Issues/PRs needing _maintainer_ feedback will be updated at least weekly. I want to make sure
|
|
nobody's blocking on me.
|
|
|
|
2. To keep issues/PRs needing _contributor_ feedback moving, I'm going to (kindly) ask the
|
|
contributor to check in after two weeks, and close the issue without resolution if I hear nothing
|
|
back after a month.
|
|
|
|
The second point I think has the potential to be a bit controversial, so I'm happy to receive
|
|
feedback on that. And if a contributor responds with "hey, still working on it, had a kid and I'm
|
|
running on 30 seconds of sleep a night," then first: congratulations on sustaining human life. And
|
|
second: I don't mind keeping those requests going indefinitely. I just want to try and balance
|
|
keeping things moving with giving people the necessary time they need.
|
|
|
|
I should also note that I'm still getting some best practices in place - CONTRIBUTING and
|
|
CONTRIBUTORS files need to be added, as well as issue/PR templates. In progress. None of us are
|
|
perfect.
|
|
|
|
# Roadmap and Conclusion
|
|
|
|
So if I've now built a `dateutil`-compatible parser, we're done, right? Of course not! That's not
|
|
nearly ambitious enough.
|
|
|
|
Ultimately, I'd love to have a library that's capable of parsing everything the Linux `date` command
|
|
can do (and not `date` on OSX, because seriously, BSD coreutils are the worst). I know Rust has a
|
|
coreutils rewrite going on, and `dtparse` would potentially be an interesting candidate since it
|
|
doesn't bring in a lot of extra dependencies. [`humantime`](https://crates.io/crates/humantime)
|
|
could help pick up some of the (current) slack in dtparse, so maybe we can share and care with each
|
|
other?
|
|
|
|
All in all, I'm mostly hoping that nobody's already done this and I haven't spent a bit over a month
|
|
on redundant code. So if it exists, tell me. I need to know, but be nice about it, because I'm going
|
|
to take it hard.
|
|
|
|
And in the mean time, I'm looking forward to building more. Onwards.
|