Freer Monads, More Extensible Effects (2015) [pdf]
okmij.org70 points by todsacerdoti 9 hours ago
70 points by todsacerdoti 9 hours ago
I strongly recommend to check all other papers and articles on https://okmij.org/ftp/, every single one of them is brilliant and insightful. I love the pedagogy, the writing style and clarity. Oleg Kiselyov is one of the best technical writers I've discovered recently.
If you are looking for real-world code for an effect system, not just a PDF paper, you should probably look at the eff library: https://github.com/hasura/eff
The acknowledgement section on that GitHub README mentions this paper.
eff has never been released to Hackage and as far as I know never used in production. I wouldn't call it "real-world code". For effect systems that people do actually use in production I suggest
* Polysemy: https://hackage.haskell.org/package/polysemy
* effectful: https://hackage.haskell.org/package/effectful
* Bluefin: https://hackage-content.haskell.org/package/bluefin/docs/Blu...
[Disclosure: Bluefin in my effect system]
`eff` is a research project that is no longer in active development and never made it to production in any sense. It would be AMAZING if `eff` were completed but I dont think that will happen at this point.
`eff` is based on delimited continuations (which Alexis had to build into GHC), it is not using `Freer`. If you want to look at an effect system in Haskell that actually has been used in production AND is based on this paper then look at `freer-simple`: https://hackage.haskell.org/package/freer-simple
No it is not high performance, but neither are any other Haskell effect systems and performance is relative to your problem domain. It also has the benefit of being implemented very similarly to Oleg's paper making it a lot easier to learn from then most other options.
> No it is not high performance, but neither are any other Haskell effect systems
This is not true. IO-wrapper effect systems (in practice, effectful or Bluefin) have as good performance as Haskell's IO monad, that is to say as good as you can get in Haskell.
Yes but from what I understand at a loss of safety. You can decide if that is worth it but you aren't getting a free lunch.
that said, your library is really cool. : )
As far as I know the shiniest implementations in the effect typing world at the moment are Koka and Effekt, which are both languages in their own right. They each have their own ideas about implementation to make effects (mostly) zero-cost.
https://koka-lang.github.io/ https://effekt-lang.org/
Frank is pretty old now but perhaps a simpler implementation: https://github.com/frank-lang/frank
Also effect-ts in TypeScript world, which is by far the most popular effect system around (quite sure it has overtaken Scala's ZIO from which it is inspired).
The ecosystem is massive.
Cons: TypeScript is a great type system but requires some investment to get the best out of it, it's also very verbose.
Pros: you have access to the entirety of the TypeScript ecosystem.
> by far the most popular effect system around
Crazy claim to make without providing any evidence
What other effect library or language has 6 millions + downloads per month (that's more than angular) and meetups popping all around the world?
I've always loved this paper. Great reading if you're interested in implementing an effect system from scratch. Though rather overkill if you're just interested in using one.
See also: https://scholar.google.com/citations?hl=en&user=0E8zPucAAAAJ...
Particularly (2014): https://scholar.google.com/citations?view_op=view_citation&h...
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I would appreciate a simpler conceptual explanation for someone not steeped in the Haskell / functional programming world :-)
I gave a talk on this topic at Zurihac this year, called A History of Effect systems: https://www.youtube.com/watch?v=RsTuy1jXQ6Y
It was given to a Haskell audience, but not everyone knew Haskell, so I hope it's generally accessible. It describes how Oleg's work fits into the overall history of Haskell effect systems.