Unusual order-of-execution problem with OCaml

Question

I'm new to OCaml. I've written this very trivial program:

open Stdio

let getline = 
  match In_channel.input_line In_channel.stdin with
  | Some x -> x
  | None -> "No input"
;;

let () =
  printf "What's your name? ";
  printf "Hi, %s!" getline;;

and several variations thereof.

You get the idea. I ask for the user's name and print "Hi, [name]!".

It works but the output is in the wrong order. I build the project with dune, run the executable, don't get any prompt for the name, I type it in and then I get all at once "What's your name? Hi, [name]!".

Now, I know the basics of buffering in OCaml. In fact, I remember having the same problem when dabbling in OCaml a while back. I distinctly remember reading this:

The strange order of code execution in OCaml

and I'm almost certain that adding "%!" worked. I've even tried

Out_channel.flush stdout

between the two printf's, and it doesn't work. I've tried adding a "\n" in the first printf. And I've tried this in a couple of terminals.

I'm sure it's something really trivial, possibly just basic syntax.

Answer 1

You defined getline as a variable, not a function.

A function must always accept one parameter, although that parameter can be ():

open Stdio

let getline () = 
  match In_channel.input_line In_channel.stdin with
  | Some x -> x
  | None -> "No input"
;;

let () =
  printf "What's your name? ";
  printf "Hi, %s!" (getline ());;