Using Arrow 1.2.0-RC, what's the most elegant way ...

# arrow

[Emil Kantis]

Using Arrow 1.2.0-RC, what's the most elegant way to handle something like this?

val squares = (1..10).mapOrAccumulate { squareService.square(it).bind() } // EitherNel<E1, Square>
val roots = (1..10).mapOrAccumulate { rootService.root(it).bind() } // EitherNel<E2, Root>

zipOrAccumulate(
  { // check squares? },
  { // check roots? },
  { ensureSomethingElse() },
) { validSquares, validRoots, somethingElse -> 
   // ... 
}

[simon.vergauwen]

So you need to align E1 and E2? Otherwise you can just call bind

[Emil Kantis]

yes, I thought I'd do that

[Emil Kantis]

like bake it into an aggregated problem,

data class InvalidOperation(val problems: List<String>) 

InvalidOperation(listOf(invalidRoots.map { "Invalid root: $it" } + invalidSquares.map { "Invalid square: $it }))

[simon.vergauwen]

With Either I would just use mapLeft + bind.

val squares = (1..10).mapOrAccumulate { squareService.square(it).bind() } // EitherNel<E1, Square>
val roots = (1..10).mapOrAccumulate { rootService.root(it).bind() } // EitherNel<E2, Root>

zipOrAccumulate(
  { squares.mapLeft { }.bind() },
  { roots.mapLeft { }.bind() },
  { ensureSomethingElse() },
) { validSquares, validRoots, somethingElse -> 
   // ... 
}

[simon.vergauwen]

There is also:

Either.zipOrAccumulate(
  squares.mapLeft { },
  roots.mapLeft { },
  either { ensure(...) { } }
) { a, b, c -> }

[Emil Kantis]

Thanks Simon 🙌