A nice curiosity: `fold`, in its full generality, ...

# arrow

[Youssef Shoaib [MOD]]

A nice curiosity: fold, in its full generality, can be implemented thru merge (and an even more-restricted version attempt)

public inline fun <Error> attempt(block: Raise<Error>.() -> Nothing): Error = merge(block)

public inline fun <Error, A, B> fold(
  block: Raise<Error>.() -> A,
  catch: (throwable: Throwable) -> B,
  recover: (error: Error) -> B,
  transform: (value: A) -> B,
): B {
  val success = run success@{
    val error = catch({ attempt { return@success block(this) } }) { return catch(it) }
    return recover(error)
  }
  return transform(success)
}

This works because of the magic of inline!