DUdustingetz·Dec 24, 2025Submission
Cats-effect: I/O Integrated Runtime Concept
1 points · Open discussiontypelevel
cats-effect
Scala2017–2025
typelevel / cats-effect
cats-effect
The pure asynchronous runtime for Scala
2Distinct people
4Mentions
1Explicit recommenders
2.3KGitHub stars
What to know before exploring
What it is
The pure asynchronous runtime for Scala
GitHub
Where it is used
No documented product use has been verified yet.
What people are saying
Monads are not directly related to handling side effects. Or to the famous IO that you're refeering here. What a Monad does is to force sequencing by creating a data…
bad_user used a recommendation phrase. One indexed comment matches. This is a phrase match, not a verdict.
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Also in these discussions
No other repository is repeatedly linked in the same discussions.
This week on Hacker News
Nov 7–Nov 14 UTC · Direct repository links and separately labeled related project stories. Indexed through 11/14/2026, 1:49:46 PM UTC.
No direct repository links were found in the indexed HN discussions over the past seven days. Older discussions are available below.
Mentions over time
HN mentions by year4 dated mentions
Observed mentions: 2017-10-11 11:26:19 UTC to 2025-12-24 22:10:54 UTC.
Counts come from the repository’s dated source evidence. Index cutoff: 2026-11-02 08:16:59 UTC.
View mention counts and date ranges
| Period | Start (UTC) | End (exclusive, UTC) | Mentions | Submissions | Comments |
|---|---|---|---|---|---|
| 2017 | 2017-01-01 00:00:00 UTC | 2018-01-01 00:00:00 UTC | 1 | 0 | 1 |
| 2018 | 2018-01-01 00:00:00 UTC | 2019-01-01 00:00:00 UTC | 2 | 0 | 2 |
| 2019 | 2019-01-01 00:00:00 UTC | 2020-01-01 00:00:00 UTC | 0 | 0 | 0 |
| 2020 | 2020-01-01 00:00:00 UTC | 2021-01-01 00:00:00 UTC | 0 | 0 | 0 |
| 2021 | 2021-01-01 00:00:00 UTC | 2022-01-01 00:00:00 UTC | 0 | 0 | 0 |
| 2022 | 2022-01-01 00:00:00 UTC | 2023-01-01 00:00:00 UTC | 0 | 0 | 0 |
| 2023 | 2023-01-01 00:00:00 UTC | 2024-01-01 00:00:00 UTC | 0 | 0 | 0 |
| 2024 | 2024-01-01 00:00:00 UTC | 2025-01-01 00:00:00 UTC | 0 | 0 | 0 |
| 2025 | 2025-01-01 00:00:00 UTC | 2026-01-01 00:00:00 UTC | 1 | 1 | 0 |
What Hacker News says
4 sourcesI've been a Scala developer since 2012.
Not sure what "taking off" means in the context of programming languages, but for me it's been popular enough since 2012.
Several big companies use it and I've introduced Scala (and FP in Scala) at about 3 companies thus far ... it isn't a tough sell if the company is Java-friendly.
We also have a great, active community developing libraries. As a shameless plug here are the projects I've been working on:
1. https://monix.io
2. https://github.com/typelevel/cats-effect
Scala is one of the best languages for FP these days. Note I'm not saying that it's perfect. We can certainly improve it and a lot of improvements are coming in Scala 3.
In OCaml for the Masses (2011)Monads are not directly related to handling side effects. Or to the famous IO that you're refeering here. What a Monad does is to force sequencing by creating a data dependency.
Speaking of IO ...
In an imperative program you get guaranteed ordering of those instructions, but that ordering is thread local, meaning that the compiler, the runtime, the kernel, the CPU are all free to reorder instructions for optimal execution and ordering between threads is forced via memory barriers, which is in fact incredibly error prone.
In an FP language such as Haskell you get the extra problem that the compiler is even more free to rewrite and execute your code in any order it sees fit due to non-strictness and to referential transparency.
What the IO monad does is to "suspend side effects" and to force ordering on evaluation. And guess what, the IO m
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onad is useful even in your average imperative program for describing concurrent logic.
Here are the implementations I contributed to:
1. https://monix.io/docs/2x/eval/task.html (Scala)
2. https://github.com/typelevel/cats-effect (Scala)
3. https://funfix.org (JavaScript)
If you've ever worked with JS Promises, think of IO as a non-broken, pure, awesome alternative.
And this is just IO. There are other monads as well. List is a monad, Option and Either are monads ;-)
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I recommend you let go of your misconceptions and do some learning because this is a freaking useful concept.
> His code is creating an infinite promise chain. Did the author really expect it would not eat all his memory and crash?
Processing an infinite or really long promise chain is what ends up happening when you're working with streams and functional programming in general.
Yes, I expected it to not leak memory, because sane implementations don't leak. One such implementation is Scala's Future [0], using an implementation trick which I then ported in Funfix.org, so it's definitely possible.
> If the author had written the same code with callbacks, a stack overflow would have been thrown.
That's not true. For one, asynchronous calls will not trigger stack overflows. And a callback approach won't leak heap memory either, because you don't have to return a value-wannabe to the caller, so you're not building a stack.
> So misunderstanding of pro
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mises (and/or async coding) is the problem here, not promises.
I'm involved in no less than 3 libraries [1] [2] [3] implementing Promise and Haskell IO alternatives in JavaScript and Scala, so I'm pretty sure that I understand promises by now :-)
I'm also pretty fond of this article I wrote a while back, asynchrony being sort of a fetish of mine: https://alexn.org/blog/2017/01/30/asynchronous-programming-s...
[0] http://www.scala-lang.org/api/2.12.3/scala/concurrent/Future...
[1] https://monix.io
[2] https://github.com/typelevel/cats-effect
[3] https://funfix.org