Starting to get into arrow to improve error handli...

# arrow

[Chris Lee]

Starting to get into arrow to improve error handling. Have methods that may throw an exception (somewhere in their call hierarchy) - struggling with how to turn those into an Either. Have this so far, seems clunky:

private fun someFun() : Either<String,Int> {
        return either.eager {
            Either.catch({ it.message ?: "No message" }) {
                42 // this would be a call hierarchy that may throw exceptions
            }.bind()  // Hmmm.  need to unwrap either created from 'catch' to get a value for 'eager'
        }  
    }

[Sam]

Isn’t either.eager { Either.catch(...).bind() } the same as just Either.catch(...) in this case?

[simon.vergauwen]

Hey @Chris Lee,

Typically you would indeed use Either.catch + mapLeft (+ bind) but in your case you you don't need either.eager nor bind.

private fun sampleFun(): Either<String, Int> =
  Either.catch { 42 }
    .mapLeft { it.message ?: "No message" }

[Chris Lee]

yes, either.eager isn’t needed in this trivial example, but is required if you want to use ensure(), ensureNotNull, or are calling other methods that return eithers to be able to bind() them.

[simon.vergauwen]

Right, I'm currently trying to backport the 2.x.x API to 1.x.x. I think it currently would look like this in 2.x.x

either {
  val res = catch({ 42 }) { 
    raise(it.message ?: "No message")
  }
}

[simon.vergauwen]

This API mimics Flow#catch where you can return a default value or raise another error.

[simon.vergauwen]

The bind happens automatically in the DSL

[Chris Lee]

ah cool. will have a look at 2.x.x stuff to orient there, and patiently await this API 😉

[simon.vergauwen]

I'm currently back-porting it to 1.1.x, so might be here pretty soon 🤞That would also make the migration from 1.x.x to 2.x.x much smaller 🤞

[Chris Lee]

cool. thanks for the info. 🙏