//arrow-core/arrow.core/filterOrOther

filterOrOther

common inline fun <A, B> Either<A, B>.filterOrOther(predicate: (B) -> Boolean, default: (B) -> A): Either<A, B>

Returns Right with the existing value of Right if this is a Right and the given predicate holds for the right value.

Returns Left(default({right})) if this is a Right and the given predicate does not hold for the right value. Useful for error handling where ‘default’ returns a message with context on why the value did not pass the filter

Returns Left with the existing value of Left if this is a Left.

Example:

import arrow.core.*

suspend fun main(): Unit {
  //sampleStart
  Either.Right(7).filterOrOther({ it == 10 }, { "Value '$it' is not equal to 10" })
    .let(::println) // Either.Left(Value '7' is not equal to 10")

  Either.Right(10).filterOrOther({ it == 10 }, { "Value '$it' is not equal to 10" })
    .let(::println) // Either.Right(10)

  Either.Left(12).filterOrOther({ str: String -> str.contains("impossible") }, { -1 })
    .let(::println) // Either.Left(12)
  //sampleEnd
}

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