How can I generate a Node.js module?

[VictorTaelin]

The --codegen node option generates an executable Node.js program. Suppose, for example, that we try to compile the following Idris file:

module Lib

export
ones : Int -> List Int
ones 0 = []
ones n = 1 :: (ones (n - 1))

It doesn't work because of the following error:

~/dev/idris: idris --codegen node lib.idr -o lib.js
When checking an application of function run__IO:
        No such variable Main.main

Suppose, though, we modify the file to:

module Main

export
ones : Int -> List Int
ones 0 = []
ones n = 1 :: (ones (n - 1))

main : IO ()
main = putStrLn (show (ones 4))

That generates a .js file that outputs [1, 1, 1, 1]. On the last lines of that file, there is:

$_0_runMain();

Which executes Main.main. That is pretty useless, though. Much more desirable would be to export a CommonJS library with the exported values. If we merely replace that line with:

module.exports = {
   ones: Main__ones
};

It works as expected and we can use lib.js from other Node.js modules:

const lib = require("./lib.js");

console.log(JSON.stringify(lib.ones(4)));

Such feature could be useful for many reasons. For one, I could port eth-lib to Idris, compile it to JS and make thin wrappers to publish on npm, where people could use it from their JS codes. Of course, that would require settling a stable format for compiled ADTs. It currently uses: {"type": tag_of_the_constructor, "$1": first_field, "$1": second_field, ...}, which seems fine to me.

Note: it would also be desirable that, when generating a module, the BN.js dependency (which is inlined on Idris's output) would be just require("BN.js"), avoiding importing the same lib twice on JS bundles.

[xaviervia]

@MaiaVictor I'm not an Idris contributor, but I have been playing around with the JS codegen since JS is my day job.

I think it would be good to pull request support for this feature. It could be the default behavior for the target node. My rationale is that:

• Exposing the functions as named entrypoints to the JS module with CommonJS would make it automatically supported in most environments (certainly in Node)
• It could work so that only the code in the "main" function gets built without a wrapper around it so that it gets immediately executed.

[rbarreiro]

There is an --interface option in idris "--interface Generate interface files from ExportLists" I don't know what it is, but sounds what you need. It does not work on the js backend. I will try to figure it out and implement it.

[melted]

@rbarreiro the test ffi006 is an example of the export functionality for the C backend.

[rbarreiro]

I propose a solution on #4073.
@melted thanks, your hint was very helpful :)