<https://twitter.com/vergauwen_simon/status/146823...

# arrow

[simon.vergauwen]

https://twitter.com/vergauwen_simon/status/1468234383412891649?s=20

Shameless… 😬

[thanh]

I think you may need to use absolute url here: https://nomisrev.github.io/continuation-monad-in-kotlin/guide/example/example-readme-03.kt it's 404 now.

[thanh]

also can you elaborate more on when to use Cont vs other type like Either. For me now it is kind of interchangeable with Either

[simon.vergauwen]

Thanks @thanh, I fixed the links

[simon.vergauwen]

Oops, guess it deserves more explanation 😄 I plan to write something about a frequently asked question here which is about transactional database operations, and there Cont will also be useful.

[simon.vergauwen]

TL;DR Cont<E, A> <~~> suspend () -> Either<E, A>

[simon.vergauwen]

It’s a bit like IO vs Either. When you receive an Either value the program has already run, but if you receive a Cont you can still control where the operation runs.

I.e.

// Exposed
suspend fun <E, A> Cont<E, A>.transaction(db: Database): Either<E, A> =
  newSuspendedTransaction(db, <http://Dispatchers.IO|Dispatchers.IO>) {
     // run the `suspend () -> E | A` by calling fold
     fold({ e -> rollback(); Either.Left(e) }, { a -> Either.Right(a) })
  }

[simon.vergauwen]

But it’s also useful for other applications, since it also serves as a lightweight runtime

[simon.vergauwen]

So where in Scala where you might use `ApplicativeError`/ MonadError to abstract over things like validation for Either, Option, Validated, ..

[thanh]

thanks @simon.vergauwen for the explanation. So Cont is a function and it is only evaluated to value by using fold function and some other similar functions like toEither .

[thanh]

I'm using similar strategy for the transaction with Either ( copy from your gist about Sqldelight Transaction) which works great, but it seems a little bit better with Cont.

[simon.vergauwen]

That’s 100% correct @thanh! You’re welcome 🙂 I’m probably going to write a bit about this next, and how you can use this for SqlDelight/Exposed (which can be translated to all frameworks suspend/non-suspend).

[thanh]

Thanks for the great work Simon! it seems it opens a lot of possibilities.

[Ivan Dugalic]

Great stuff Simon. It looks like a good abstraction to me! Examples you provide on the blog are also very valuable. Thanks for the great work!

[simon.vergauwen]

Thanks for the kind words @Ivan Dugalic! Very happy you like it, more similar stuff coming soon 🙂