import std/sugar

# Declarations

type
  FileDesc* = enum STDIN, STDOUT, STDERR
  IO*[T]    = proc(): T

func flatmap*[T, U](io: IO[T], fn: T -> IO[U]): IO[U]
func lift*(fn: proc(): void): IO[void]
func lift*[T](fn: proc(): T): IO[T]
func join*[T](io: IO[IO[T]]): IO[T]
func map*[T, U](io: IO[seq[T]], fn: T -> U): IO[seq[U]]
func readstr*(stream: FileDesc): IO[string]
func run*[T](io: IO[T]): T
func strput*(str: string): IO[void]
func to_IO*[T](val: T): IO[T]
     
# Definitions

func flatmap*[T, U](io: IO[T], fn: T -> IO[U]): IO[U] = join(io.map(fn))

func lift*(fn: proc(): void): IO[void] =
  proc(): void = fn()
    
func lift*[T](fn: proc(): T): IO[T] =
  proc(): T = fn()

func join*[T](io: IO[IO[T]]): IO[T] = io.map(run)
  
func map*[T, U](io: IO[seq[T]], fn: T -> U): IO[seq[U]] =
  proc(): seq[U] =
    let list: seq[T] = io()

    if list.len == 0:
      @[]
    else:
      let
        head: T          = list[0]
        tail: seq[T]     = list[1..^1]
        new:  IO[seq[T]] = to_IO tail

      @[fn(head)] & run new.map(fn)

func readstr*(stream: FileDesc): IO[string] =
  case stream:
    of STDIN:
      lift proc(): string = readLine(stdin)
    of STDOUT:
      lift proc(): string = readLine(stdout)
    of STDERR:
      lift proc(): string = readLine(stderr)

func run*[T](io: IO[T]): T = io()

func strput*(str: string): IO[void] =
  proc(): void = echo str

func to_IO*[T](val: T): IO[T] =
  proc(): T = val