Wartremover VS Scapegoat

By contrast, Java and ECMAScript are essentially what we might call "classical" imperative OOP languages, although ECMAScript reveals much more of its Lisp-inspired "map/filter/reduce" FP roots. IMO ESLint is essentially table stakes for working with ECMAScript, but honestly, I wouldn't stop there and would insist on working in TypeScript, including some of the tooling for ESLint specifically for TypeScript, dialing type-safety up to 11, effectively like using Wart Remover with Scala.

I'm awed by the maturity of the Scala 2 compiler. Every minor version in the 2.13 series adds a new linting improvement. You can see that if you have sbt-tpolecat in your project. I'm always happy to see that some option from Wartremover is no longer used.

I was recently trying to move away from Scapegoat to Wartremover and I got bitten by this bug which is particularly prevalent in codebases using Typelevel libraries.

WartRemover to disable some of Scala’s more questionable features.

There is also wartremover but you cannot run it separately from your compile command.

or `NonUnitStatements` without explicit annotation.

This effectively locks you into writing pure code (you can extend the linter to cover other things like not using `Future` or not using Java libs outside of `MonadError` from cats[4]). The linters operate on typed ASTs at compile time, and have plugins for the most popular scala build tools. Coupled with `-XFatalWarnings', you can guarantee that nothing unexpected happens unless you explicitly pop the escape hatch, for the most part.

You can still bring in external libraries that haven't been compiled with these safties in place, so you aren't completely safe, but if you use ZIO[5]/Typelevel[6] libraries you can be reasonably assured of referentially transparent code in practice.

There are three schools of thought, roughly, in the scala community towards the depth of using the type system and linters to provide guarantees and capabilities, currently:

1) Don't attempt to do this, it makes the barrier to entry to high for Scala juniors. I don't understand this argument - you want to allow runtime footguns you could easily prevent at compile time because the verifiable techniques take time to learn? Why did you even choose to use a typesafe language and pay the compilation time penalty that comes with it?

2) Abstract everything to the smallest possible dependency interface, including effects (code to an effect runtime, F[_] that implements the methods your code needs to run - if you handle errors, F implements MonadError, if you output do concurrent things, F implements Concurrent, etc.) and you extend the effect with your own services using tagless final or free.

3) You still use effect wrappers, but you bind the whole project always to use a concrete effect type, avoiding event abstraction, thus making it easier to code, and limiting footguns to a very particular subset (mainly threadpool providers and unsafeRun or equivalent being called eagerly in the internals of applications).

My opinion is that smallest interface with effect guarantees (#2) is best for very large, long maintenance window apps where thechoice of effect runtime might change(app), or is out of the devs' control (lib); and #3 is best for small apps.

TL/DR; You can go a really, really long way to guaranteeing effects don't run in user code in scala. Not all the way like Haskell, but far enough that it's painful to code without conforming to referential transparency.

1. https://github.com/scalacenter/scalafix

2. https://github.com/scalaz/scalazzi

3. http://www.wartremover.org/

4. https://typelevel.org/cats/api/cats/MonadError.html

5. https://zio.dev/

6. https://typelevel.org/

I've since figured this out (Scapegoat/FinalModifierOnCaseClass and WartRemover/FinalCaseClass). I plan to add it to the next version of the article.

scapegoat, but I am biased because I have contributions to it.

The support for scala 3 has been requested a while ago: https://github.com/scapegoat-scala/scapegoat/issues/521 but the original author has stepped down as a maintainer https://github.com/scapegoat-scala/scapegoat/issues/567 and recently most of the changes are just dependency updates.