How to use Readline?

[williamrjribeiro]

Hi there,

I just started learning ReScript and I would like to use it for a NodeJS application. I'm trying to port this piece of NodeJS code to ReScript using rescript-nodejs but I can't find a way to use Readline module:

const fs = require('fs');
const readline = require('readline');

const rl = readline.createInterface({
  input: fs.createReadStream('sample.txt'),
  crlfDelay: Infinity
});

rl.on('line', (line) => {
  console.log(`Line from file: ${line}`);
});

I couldn't find any documentation on Readline module. Can you give me some hints? I just want to read a text file line by line.

If this is not the right place, where should I ask this kind of question?
Thanks.

[TheSpyder]

Hi! This is a fine place to ask. The ReScript forums are also good but people there might just tell you to write your own bindings 🙂

I'll see if I can port your code to use this library and post the results later today (I'm in Australia). I might also enable github discussions for the repo and transfer your question there.

[TheSpyder]

Oh wow, looks like the Readline module was written incorrectly. The make function requires a t, but the only way to create a t is with the make function:

rescript-nodejs/src/Readline.res

Line 17 in 83e9eba

@send external make: (t, interfaceOptions) => t = "createInterface"

It also seems weird that the whole file is inside an Interface sub-module.

I'll look up the nodejs documentation and make sure I get the module right, then push a new release and show you how to use it.

[TheSpyder]

Published as 14.2.0 (it's a breaking change to Readline, but the module wasn't usable before so I figured a minor was ok).

The sample code you requested:

rescript-nodejs/examples/ReadFileByLine.res

Lines 4 to 12 in 6214ab6

	let rl = Readline.make(
	Readline.interfaceOptions(~input=Fs.createReadStream("sample.txt"), ~crlfDelay=infinity, ()),
	)
	rl
	->Readline.Interface.on(Event.fromString("line"), line => {
	Js.log(`Received: ${line}`)
	})
	->ignore

Which generates the correct JS (Pervasives.infinity is just a reference to Infinity):

rescript-nodejs/lib/js/examples/ReadFileByLine.bs.js

Lines 4 to 16 in 6214ab6

	var Fs = require("fs");
	var Readline = require("readline");
	var Pervasives = require("rescript/lib/js/pervasives.js");
	var rl = Readline.createInterface({
	input: Fs.createReadStream("sample.txt"),
	crlfDelay: Pervasives.infinity
	});
	rl.on("line", (function (line) {
	console.log("Received: " + line);
	}));

I'm happy to discuss what each part of the code does if you need help with it.

[williamrjribeiro]

Oh wow! I was not expecting this quickness! You sir deserve at least a nice beer for all this work. Thank you very much! 🍻

The ReScript code is easy to follow (as I was expecting) I just had never seen/heard Pervasives before. 👍

[TheSpyder]

Happy to help 😁 I want this library to be useful, having a broken module is really bad.

Pervasives is one of those OCaml concepts that the ReScript team don't want anyone to know about and may one day be replaced by a module of their own. It's the central (pervasive) module of the OCaml standard library that is available when no open statements are used. Every module has an implicit open Pervasives at the top.

I believe in later OCaml versions the implicit open was changed to Standard, but that isn't really relevant to ReScript.