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Comment by lmm | Hacker News Reader

So use a language that knows how to be polymorphic over async. Just like you don't want to have to write one version of sort() for each possible array element type, but the solution isn't to make all array elements untyped, the solution is to have a language that can abstract over that.

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Can you give an example of a language which is polymorphic in this way and how that looks?

Haskell or Scala are the immediate examples. The idiomatic way to write async-style functions in those languages tends to be do notation or for/yield style. E.g.

    transformFile = do {
      x <- readFile(...)
      y = process(x)
      z <- writeFile(...)
    }

is the equivalent of something like:

    async transformFile = {
      x = await readFile(...)
      y = process(x)
      z = await writeFile(...)
    }

In Haskell you write exactly the same code when readFile and writeFile are async, or when they're polymorphically async-or-not, your function will just implicitly be as polymorphic as it possibly can be based on the functions it's calling.

In Scala you need an explicit type parameter if you want to be polymorphic:

    def transformFile = for {
      x <- readFile(...)
      y = process(x)
      z <- writeFile(...)
    } yield ()
    def transformFile[F[_]: Monad] = for {
      x <- readFile(...)
      y = process(x)
      z <- writeFile(...)
    } yield ()

The first example is always async, the second example is polymorphic (it's calling readFile[F] and writeFile[F], but those get inferred).

Ah I've got it. Async is a monad and the function is just polymorphic over Monad m. In the sync case m is Identity. Can you point me in the direction of some libraries which offer functions like these?

sttp (HTTP client), http4s (HTTP client/server), quill (database access), fs2 (streaming data processing), I have some vague memory of a gRPC implementation. A lot of stuff that does async in Scala tends to be written in this style because it's almost no extra cost compared to writing it in strictly async fashion.

Yeah, that's the correct solution. I'm not arguing for the solution, I'm arguing for the problem, because a lot of people seem to think that the problem isn't real.

Sure, but if you put it that way it tends to come across as wanting to not track the distinction at all. Like if you said it's good that values are untyped in Ruby because to write a collection type in C you have to reimplement it for every different type of collection you want: yes, but that doesn't mean values having types is bad, it just means you need polymorphism.

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