ReadFromSTDIN (2)

Written by: Hongwei Xi (the creator of ATS)

Intro

If you have not yet read ReadFromSTDIN, please do so first.

The code in this example does essentially the same as the code in ReadFromSTDIN, but it is written in a style that stresses the use of combinators in functional programming.

Prompts

The following function prompts returns a linear stream of integers:

fun
prompts
(
// argless
) : stream_vt(int) =
stream_vt_map<int><int>
(
sint_streamize_gte(0) // HX: generating 0, 1, 2, 3, ...
) where
{
  impltmp
  stream_vt_map$fopr<int><int>(i) =
  (println!("Please input an integer or type Ctrl-D:"); i)
}

For each integer in the stream to be computed, a message (for the purpose of prompting the user) is printed onto the standard output (STDOUT).

Tally

The function tally can be given the following combinator-based implementation:

fun
tally() = let
  val ps = prompts()
  val xs =
  FILEref_streamize_line(the_stdin())
  val xs =
  stream_vt_filter<string>(xs) where
  {
  impltmp
  stream_vt_filter$test<string>(x0) = isneqz(x0)
  }
  val ys =
  (
  stream_vt_z2map<int,string><int>(ps, xs)
  ) where
  {
  impltmp
  stream_vt_z2map$fopr<int,string><int>(p0, x0) = $extfcall(int, "atoi", x0)
  }
in
  stream_vt_foldleft0<int><int>(ys, 0) where
  {
    impltmp
    stream_vt_foldleft0$fopr<int><int>(r0, y0) = r0 + y0
  }
end // end of [tally]

The code for tally is largely self-explanatory: ps refers to a stream for producing prompts and xs to a stream of strings representing non-empty lines read from STDIN; calling z2map on ps and xs builds a stream ys of int-values where each value is converted from a string in xs; calling foldleft0 on ys returns the sum of all of the int-values contained in ys (while consuming the linear stream ys).

Happy programming in ATS!!!