Reading user input


Here’s a tiny program:

 print_string("Now is the time for all good men to come to the aid of the party\n");
 let s = read_line();
 print_string("You typed: " ++ s);

Running it produces this (before I get the chance to type anything as input):

jfh$ Node src/
Now is the time for all good men to come to the aid of the party

    throw new Error(s + " not polyfilled by BuckleScript yet\n")

Error: caml_ml_input_scan_line not polyfilled by BuckleScript yet

    at Object.resolve (/Users/jfh/.nvm/versions/node/v12.9.0/lib/node_modules/bs-platform/lib/js/caml_external_polyfill.js:16:11)
    at scan (/Users/jfh/.nvm/versions/node/v12.9.0/lib/node_modules/bs-platform/lib/js/pervasives.js:335:38)
    at input_line (/Users/jfh/.nvm/versions/node/v12.9.0/lib/node_modules/bs-platform/lib/js/pervasives.js:367:37)
    at Object.read_line (/Users/jfh/.nvm/versions/node/v12.9.0/lib/node_modules/bs-platform/lib/js/pervasives.js:431:10)
    at Object.<anonymous> (/Users/jfh/foo/repl/src/
    at Module._compile (internal/modules/cjs/loader.js:936:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:947:10)
    at Module.load (internal/modules/cjs/loader.js:790:32)
    at Function.Module._load (internal/modules/cjs/loader.js:703:12)
    at Function.Module.runMain (internal/modules/cjs/loader.js:999:10)

Because read_line is actually in “Pervasives”, I figured it would probably work. Have I done something wrong? Suggestions for alternative ways to read user input?


Short version: since you’re compiling and running the code on NodeJS, you’ll need to write a binding to and use the readline module:

Longer version: the BuckleScript compiler does not ship with a working version of Pervasives.read_line, as you can see from the message ‘not polyfilled by BuckleScript yet’. To get equivalent functionality, you typically bind to a function which is shipped on the platform you’re running on. On NodeJS, that’s readline. In a web browser, it would be an input form or something like that.

To write BuckleScript bindings, please see the BuckleScript website documentation.


There are actually several existing bindings for this already. See


Thanks to both of you; I’ll see how that works out!


OK. I’ve tried a bunch of things without success, so I’m back. I have the feeling that some documentation might be a little out of date, and I know my understanding is limited at best.

Trying to start afresh, I’ve created a new project, so (reading down from my home directory, things look like


where that last file is there because I ran npm run build, and then (from the chat-test directory) tried 'node src/`, which produced the expected one-line output you get from the sample-starter code.

For making something like read_line work, I need to install a “binding” in the form of some module, but there seem to be two choices: bs-readline, and bs-node-readline. As you can see from my description above, I’m using bucklescript to compile ReasonML code to Javascript, and then running it with “node”. I’m unsure which of these I should be using, and the package descriptions don’t seem to disambiguate for me. So:

Question 1: is there a “correct” choice, in my context, for bs-readline, vs bs-node-readilne? All I really need is to be able to read some user-typed text from a terminal.

In my previous attempts, I installed both, figuring that one or the other might be “found” by either bucklescript or Node (I’m not even sure which one needs to find it!). I installed using npm i --save bs-readline, for example, because that’s what the doc at said to do. But wondering if I’d done it wrong because errors persisted — the demo program shown there wouldn’t run! — I tried to read up on the install command, and learned that the --save option is now deprecated, and the the editing of my bsconfig.json file is now done automatically (if I’m reading this correctly). Thus

Question 2: To install one of these modules, should I in fact simply type npm i bs-readline and not have to edit my bsconfig.json file?

Question 3: In what directory should I be when I run the npm i ... (or whatever) command to install these modules? In “reason-tests”? In “chat-test”? In “chat-test/node_modules”?

Any suggestions gratefully received.


I figured it out, after more experimentation.

Q1: I don’t know if there’s a correct choice, but using bs-readline worked for me
Q2: typing npm i bs-readline worked fine. It created a package-lock.json file, which it said I should commit, but I’m not using a version control system yet, and just ignored that message, and things seem fine so far (for my very limited use-goals).
Q3: To make this work, I typed that command in the chat-test directory.

Notes: To test out the module, I used the sample code given there:

Readline.readline((in) => {

which would not compile, because the “Readline” module could not be found. I had to edit the chat-test/src/bsconfig,json file to have a middle section that looked like this:

"suffix": ".bs.js",
"bs-dependencies": [
"warnings": {
"error" : "+101"

where the "bs-readline" was the thing I added.
Even then, it wouldn’t run, because “in” is apparently a reserved word. I edited to make my sample program look like this instead:

Js.log("Hello, BuckleScript and Reason!");

Readline.readline((inp) => {

and that worked better. To actually make it run, in the chat-test directory, I typed

% npm run build
% node src/

The first of these compiles the into javascripts, in a file called, which it places in the “source” directory even though it’s actually the output of some process. The second line runs this Javascript program. The actual execution looked like this

chat-test jfh% node src/
Hello, BuckleScript and Reason!

where the first foobar was what I typed, and the second was ReasonML’s echo of it.


I followed the steps you provided and tried running your code, but it seems that Reason is unable to recognize the Readline module. I installed bs-readline using npm and added it to the bs-dependencies list in my bsconfig.json file, but it’s still not working. Do you have any idea what could be causing this problem?


As you can see, I worked through that particular problem 3.5 years ago, and at this point, I have no idea how to make it work again. I wish I could help, because lots of folks here have helped me, but … I just don’t have the skills. It might be worth posting as a new question…


Ok, Thank you. We have to do a project in REASON for college, are you familiar with the language in other areas?


Yes, I am. In fact, I use it as one of the two languages students see in their first semester in the course I’ve been teaching at Brown University: