Using Javascript compiled Elm function inside nodejs runtime

Question

Okay so I'm using MongoDB Realm functions, a serverless platform that you define your functions like this:

exports = async function(param1, param2){
   var result = {}
   // do stuff 
   return result;
}
if (typeof module === 'object') {
  module.exports = exports;
}

I want to ask if its possible to code in Elm function and run it inside a nodejs runtime? In other words like this:

exports = async function(param1, param2){
   var result = {}
   // do stuff 
   // call elm compiled js
   return elmFunction(param1, param2);
}
var elmFunction = async function(param1, param2) {
  // generator elm code
};

Jakub Hampl · Accepted Answer · 2021-03-09 09:18:56Z

9

Yes, but it can be a little tricky.

First, you need to setup your Elm file using Platform.worker - this basically a headless Elm program.

You would typically pass input that you have available synchronously (param1 and param2 in your example) as flags. You would then define an output port that you would call from your Elm program when it completes. On the JS side you would handle it like this:

exports = async function(param1, param2){
   const elmProgram = Elm.Main.init({flags: {param1, param2}});
   return new Promise((resolve) => {
     elmProgram.ports.outputPort.subscribe((result) => {
       resolve(result);
     });
   });
}

The Elm code might look like this (assuming your code is pure):

port module Main exposing (main)

import Json.Decode exposing (Value)
import Json.Encode

port outputPort : Value -> Cmd msg

main = 
    Platform.worker
        { init = init,
        , subscriptions = always Sub.none
        , update = \msg model -> (model, Cmd.none)
        }

init flags =
    case Json.Decode.decodeValue flagsDecoder flags of
        Ok input ->
            let
                 result = 
                      myFunction input 
            in
            ((), outputPort (resultEncoder result))
        
        Err e ->
            Debug.todo "error handling"

edited Mar 9, 2021 at 9:18

answered Mar 9, 2021 at 8:15

Jakub Hampl

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2 Comments

Fireburn Over a year ago

In my idea, each method would be independent of each other even independent of database access, all methods would be functional and that can be unit tested independently

Jakub Hampl Over a year ago

You can generalise this to call multiple functions in your Elm code. Simply pass the name of the function to call, decode it to the function you want to call, apply it and encode the result.

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