catch

inline fun <E, A> Either<Throwable, A>.catch(catch: Raise<E>.(Throwable) -> A): Either<E, A>

Catch allows for transforming Throwable in the Either.Left side. This API is the same as recover, but offers the same APIs for working over Throwable as Effect&EagerEffect.

This is useful when working with Either.catch since this API offers a reified variant. The reified version allows you to refine Throwable to T : Throwable, where any Throwable not matching the t is T predicate will be rethrown.

import arrow.core.Either
import arrow.core.catch
import io.kotest.assertions.throwables.shouldThrowUnit
import io.kotest.matchers.shouldBe

fun test() {
  val left: Either<Throwable, Int> = Either.catch { throw RuntimeException("Boom!") }

  val caught: Either<Nothing, Int> = left.catch { _: Throwable -> 1 }
  val failure: Either<String, Int> = left.catch { _: Throwable -> shift("failure") }

  shouldThrowUnit<RuntimeException> {
    val caught2: Either<Nothing, Int> = left.catch { _: IllegalStateException -> 1 }
  }

  caught shouldBe Either.Right(1)
  failure shouldBe Either.Left("failure")
}

@JvmName(name = "catchReified")

inline fun <E, T : Throwable, A> Either<Throwable, A>.catch(catch: Raise<E>.(T) -> A): Either<E, A>