WIP : use of the new Arrow Fx DSL - java.io integration

CopyFileFx.kt

import arrow.core.Either
import arrow.fx.coroutines.Resource
import arrow.fx.coroutines.unit
import java.io.*

interface InputStream {
    val s: FileInputStream

    suspend fun read(buf: ByteArray, off: Int, len: Int): Int =
            s.read(buf, off, len)

    suspend fun close() = s.close()

    companion object {
        fun stream(f: File) = object : InputStream {
            override val s = FileInputStream(f)
        }
    }
}

interface OutputStream {
    val s: FileOutputStream

    suspend fun write(buf: ByteArray, off: Int, len: Int) =
            s.write(buf, off, len)

    suspend fun close() = s.close()

    companion object {
        fun stream(f: File) = object : OutputStream {
            override val s = FileOutputStream(f)
        }
    }
}

data class CopyService(private val consumer: InputStream, private val supplier: OutputStream) {
    private tailrec suspend fun copy(origin: InputStream, destination: OutputStream, buffer: ByteArray, acc: Long): Long {
        val amount = origin.read(buffer, 0, buffer.size)
        return if (amount >= 0) {
            destination.write(buffer, 0, amount)
            copy(origin, destination, buffer, acc + amount)
        } else acc
    }

    suspend fun process() = copy(consumer, supplier, ByteArray(4096), 0)
}

val resourceCopy = { src: File, dest: File ->
    Resource({ InputStream.stream(src) }, InputStream::close)
            .zip(Resource({ OutputStream.stream(dest) }, OutputStream::close))
            .flatMap { (consumer, supplier) ->
                Resource({ CopyService(consumer, supplier) }, { _ -> unit() })
            }
}

suspend fun main(args: Array<String>) {
    if (args.size == 2) {

        Either.catch {
            val source = File(args[0])
            val target = File(args[1])

            resourceCopy(source, target).use {
                val copied = it.process()
                println("$copied bytes copied from ${source.path} to ${target.path}")
                Unit
            }
        }.fold(
                { println("Failure: ${it.message}") },
                { println("Success.") }
        )
    } else {
        println("Failure: 2 arguments are required...")
    }
}