servant VS polysemy

> do you really have to understand language extensions?

You do when your code doesn't compile and you're trying to figure out what the error message means, or when the library you want to use makes heavy use of it for even basic functionality.

> These days one just enables GHC2021

My experience was pre-GHC2021. I basically had to enable at a minimum 5-6 language extensions in every single file.

> Mostly they're just about removing unnecessary restrictions from the older standard.

Yeah, those ones are usually fine. I have zero objection to things like FlexibleInstances or DeriveFoldable.

> Could you give an example?

I believe I was trying to implement Central Authentication Service using Servant. However, that required returning a custom HTTP status code. There has been an open Github issue for this since 2017, but it seems to require basically rewriting the entire framework: https://github.com/haskell-servant/servant/issues/732

Looking back at it now Servant does have "ServerError", but that basically requires giving up all the advantages Servant claims to have and I believe it was not a viable option at the time. Looking at the timeline I was probably also on Servant 0.15, and there seems to have been a rewrite since then.

I vaguely recall running into a similar issue trying to interact with a database, but I can't remember the details of that.

a HasSwagger instance for NamedRoutes was added in May 2022 (in this commit) but there hasn't been a package release since March

If you don't like this style, the usual alternative is to change mkDualAuthHandler to take two additional arguments, Proxy tag0 and Proxy tag1 (as e.g. lots of Servant functions do, for historical reasons).

Dunno if this is helpful, but I found this github issue about ResourceT and servant https://github.com/haskell-servant/servant/issues/1345

And what about the cabal repl --with-compiler=doctest, which was added recently, in doctest v0.20? I recently submitted a PR for Servant to use this in place of GHC environment files, because it seems less finicky to me. Was this a bad idea?

I asked a somewhat relevant question recently. Maybe you'll find this discussion somewhat helpful: https://github.com/haskell-servant/servant/issues/1547; two packages were talked about. One of the folks from Well Typed replied, and said they tried it recently (and worked fine).

You are highly encouraged to test this release out and let us know what you think ! For bug reports, features requests or any kind of feedback, just open a ticket on our issue tracker.

Even slower than :<|> quadratic compile time in number of routes?

Check this out https://github.com/haskell-servant/servant/issues/335#issuecomment-172300487

Thirdly, composing arbitrary effects without losing state is really, really difficult. Things are fine when you limit yourself to State and Reader, sure, but once you start with nondeterminism you’ll discover it’s shockingly easy to produce behaviors that are baffling unless you’ve spent a preposterous amount of time thinking about this stuff. (I’ve been bitten in prod by silent state-dropping bugs, and rarely have I been more flummoxed.) Consider this example, which produces silent changes in the semantics of <|> depending on whether you use it inside or outside of a higher-order effect. Every single effect library (besides the still-unreleased eff) gets certain combinations of effects + nondeterminism wrong. You could make the argument that most people don’t use nondeterministic monads, but eDSLs really shine when you have access to them, as you can turn a concrete interpreter to an abstract one fairly easily.

Looking for a few projects that make use of it, I found accelerate, hawk, polysemy and pretty-simple, so I'll be interested to poke around in their code and see how they have things set up.

Right, I think I'll just give it a shot to see. Polysemy is nice but I'm still having trouble getting what I want out of it (which may very well be entirely a fault of my own understanding)

Languages such as Koka only support algebraic effects, not scoping operations such as catch and listen. The Effect Handlers in Scope paper introduces scoping operations, which lead to the Haskell libraries fused-effects and polysemy, but they turned out to have some weird semantics. eff is her effort to fix that.

Looking at the interpreter for Reader might give some clues if this doesn't work. https://github.com/polysemy-research/polysemy/blob/master/src/Polysemy/Reader.hs#L38-L45

*But I'm not terribly well versed in Scala's other DI offerings. I came from Haskell and didn't find anything in Scala that clicked with me until I found ZIO. It reminded me a lot of my favorite way of writing Haskell programs (https://github.com/polysemy-research/polysemy)—albeit with a completely different implementation.