Hi, I'm new to Kotlin and I just started playing with Arrow and I can't understand something. This i...

# arrow

[Ivan Brko]

Hi, I'm new to Kotlin and I just started playing with Arrow and I can't understand something. This is an example from the documentation:

fun magic(s: String): Either<Exception, String>

val x = magic("2")
val value = when(x) {
    is Either.Left -> when (x.a){
        is NumberFormatException -> "Not a number!"
        is IllegalArgumentException -> "Can't take reciprocal of 0!"
        else -> "Unknown error"
    }
    is Either.Right -> "Got reciprocal: ${x.b}"
}

And above the code, the following is written: You should also notice that we are using SmartCast for accessing Left and Right values.
I expected that to mean that after matching X with Either.Left, the compiler would know that x is of type Exception (and the same for right, but there that x is of type String). Here you still have to call .a or .b to get the value.
So sorry if this is a stupid question, but what exactly is smart casted here and would it be possible to get Kotlin to know that x is of type String after matching it with Right here?

[Mike]

Before the when, the compiler only knows that x is an Either.

After the is in the when it now knows that the x is more specifically a Left or a Right. So you don't have to do something like is Either.Left -> when ((Left) x).a when you want to use x.

[thanerian]

Either type is not part of the kotlin std and cannot do that kind of smartcasting, with arrow-meta and Union Types you could do it

[Mike]

@thanerian Who are you replying to? OP or me?

[thanerian]

I’m replying to @Ivan Brko, you have explained correctly the current smart cast, and I just pointing that in a future with UnionTypes you could “smartcast” to wrapped values out of the box via arrow-meta

[Mike]

Ok, that's what I thought, but wanted to confirm. Thank you.

[pakoito]

what you're trying is deep pattern matching. I believe is Either.Left<Throwable> or similar won't work

[pakoito]

try using fold instead, it's the same as when

[pakoito]

and it pre-casts the value for you

[pakoito]

val value = x.fold({ a ->
    when (a){
        is NumberFormatException -> "Not a number!"
        is IllegalArgumentException -> "Can't take reciprocal of 0!"
        else -> "Unknown error"
    }, { b ->
      "Got reciprocal: $b"
    })

[Ivan Brko]

Thans for the answer, everyone. Fold does seem nicer here so I'll use that.