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 different style, which greatly stresses the use of combinators in functional programming.
The following function prompts returns a linear stream of
integers:
fun
prompts
(
// argless
) : stream_vt(int) =
stream_vt_map_cloptr<int><int>
( xs
, lam(i) =>
(println!("Please input an integer or type Ctrl-D:"); i)
) where
{
val xs = intGte_stream_vt(0) // HX: generating 0, 1, 2, 3, ...
}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).
The function tally can be given the following combinator-based
implementation:
fun
tally() = let
val ps = prompts()
val xs =
streamize_fileref_line(stdin_ref)
val xs =
(xs).filter()(lam(x) => isneqz(x))
val ys =
stream_vt_map2_cloptr<int,string><int>(ps, xs, lam(p, x) => g0string2int(x))
in
stream_vt_foldleft_cloptr<int><int>(ys, 0, lam(r, y) => r + y)
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 map2 on
ps and xs (with some closure-function) builds a stream
ys of integers where each integer is converted from a string in
xs; calling foldleft on ys returns the sum of all of
the integers contained in ys.
Happy programming in ATS!!!