import System.*
import System.Collections.*
import System.Linq.*
import System.Option.*
import System.Result.*
import System.Console.*

func Main() {
    let input: Option<int> = Some(21)
    let answer = input.Map(value => value * 2)
        .Filter(value => value > 0)

    match answer {
        Some(let value) => WriteLine(value)
        None => WriteLine("No answer")
    }

    let result = answer.OkOr("Missing input")
        .Then((value: int) -> Result<int, string> => Ok(value + 1))
    match result {
        Ok(let value) => WriteLine(value)
        Error(let error) => WriteLine(error)
    }

    let absent: Option<int> = None
    let fallback = absent.OrElse(() => Some(7))
    WriteLine(fallback.UnwrapOr(0))

    let failure: Result<int, string> = Error("Unavailable")
    let recovered = failure.MapError(message => String.Concat("Read: ", message))
        .OrElse(message => {
            WriteLine(message)
            return Ok(9)
        })
    WriteLine(recovered.UnwrapOr(0))

    for value in answer.ToIterable() {
        WriteLine(value)
    }
}