I am learning Arrow in a little command-line proje...

# arrow

[Michael Strasser]

I am learning Arrow in a little command-line project. I want to handle a single argument and, using a context receiver, trying this function:

context(Raise<NoSuchElementException>)
fun onlyArg(args: Array<String>): String = args.first()

How do I call this function and handle the exception? (Does this approach make sense?)

[Michael Strasser]

I tried using effect but am not sure how to use it:

fun main(args: Array<String>) {
    val arg = effect {
        onlyArg(args)
    }.fold(/* what here? */, /* what here? */)
}

Or is this the wrong approach?

[simon.vergauwen]

That doesn’t really make sense since the function doesn’t know how to translate from the exception from first to Raise

[simon.vergauwen]

You could use a custom domain and use ensureNotNull.

context(Raise<String>)
fun onlyArg(args: Array<String>): String =
  ensureNotNull(args.firstOrNull()) { “Expected single argument, but found none” }

[simon.vergauwen]

fold takes 2, or 3 lambdas.

The handler for String or your error type, an optional handler for uncaught exceptions, and finally a handler for your result of the program.

[simon.vergauwen]

You might also want to use recover if you simple want to recover from an error (String).

[Youssef Shoaib [MOD]]

Should there be a bridging function between the throwing world and the raising world for this use case? Something like

inline fun <reified T: Throwable> Raise<T>.catchAndRaise(block: () -> A): A = catch<T>(block) { raise(it) }

Although I can imagine that Raise<Throwable> would be quite a rare sight, but it can help the transition of code from throw to raise

[simon.vergauwen]

I would almost be more in favor of runCatching { }.bind() for that, and take into account fatal / cancellation exceptions

[Youssef Shoaib [MOD]]

Issue is that Result doesn't have a typed Throwable, and so the bind here wouldn't really work for something like Raise<NoSuchElementException> as in the original use-case. I guess catch<NoSuchElementException>({ args.first() }) { raise(it) } is good enough, but it might make the transition harder

[Stylianos Gakis]

Doesn’t runCatching also swallow cancellation exceptions? Or does arrow provide some other runCatching that I am not aware of?

[simon.vergauwen]

You’re absolutely right @Youssef Shoaib [MOD], something to consider indeed 🤔

@Stylianos Gakis my original idea was to resolve that in bind by only putting non-fatal exceptions in Raise

[Michael Strasser]

Thanks, @simon.vergauwen and others for your comments.

There are better ways to check for only one CLI argument than try to catch NoSuchElementException but I am still not clear on the best way to call a function like this:

context(Raise<String>)
fun onlyArg(args: Array<String>): String =
    ensureNotNull(args.firstOrNull()) { "Expected single argument, but found none" }

What is the best way to call it?

[Youssef Shoaib [MOD]]

Depends on how you want to handle the error. One way is:

fold({ /* program goes here */ }, { /* handle success */ }, { /* handle failure */ })

Or

recover({ /* program */ }) { /* handle error */ }

[Youssef Shoaib [MOD]]

There's also the various raise builders like either and effect

[Michael Strasser]

Thanks @Youssef Shoaib [MOD] I will experiment with those.