Docs about Elm Runtime

Fabiano_Taioli · January 5, 2023, 8:51am

Hello everybody.

I’m interested on learn more about Elm Runtime. How it works and how is implemented in Javascript with particular focus on the subscription system.

Any documentation page about it?

rupert · January 5, 2023, 9:22am

I have never seen any documentation about it. Whenever I have messed with the runtime, to understand it, debug something or experiment with a change, I just read the code. Most of the time the code is very well written and easy to follow.

Fabiano_Taioli · January 5, 2023, 9:33am

Thank you @rupert , can you point me to the runtime source code on Github?

lydell · January 5, 2023, 11:09am

I find it very hard to follow many times

You’ll find the code in the various elm/* packages. For example:

github.com

elm/browser/blob/master/src/Elm/Kernel/Browser.js

/*

import Basics exposing (never)
import Browser exposing (Internal, External)
import Browser.Dom as Dom exposing (NotFound)
import Elm.Kernel.Debug exposing (crash)
import Elm.Kernel.Debugger exposing (element, document)
import Elm.Kernel.Json exposing (runHelp)
import Elm.Kernel.List exposing (Nil)
import Elm.Kernel.Platform exposing (initialize)
import Elm.Kernel.Scheduler exposing (binding, fail, rawSpawn, succeed, spawn)
import Elm.Kernel.Utils exposing (Tuple0, Tuple2)
import Elm.Kernel.VirtualDom exposing (appendChild, applyPatches, diff, doc, node, passiveSupported, render, divertHrefToApp)
import Json.Decode as Json exposing (map)
import Maybe exposing (Just, Nothing)
import Result exposing (isOk)
import Task exposing (perform)
import Url exposing (fromString)

*/

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github.com

elm/virtual-dom/blob/master/src/Elm/Kernel/VirtualDom.js

/*

import Basics exposing (identity)
import Elm.Kernel.Debug exposing (crash)
import Elm.Kernel.Json exposing (equality, runHelp, unwrap, wrap)
import Elm.Kernel.List exposing (Cons, Nil)
import Elm.Kernel.Utils exposing (Tuple2)
import Elm.Kernel.Platform exposing (export)
import Json.Decode as Json exposing (map, map2, succeed)
import Result exposing (isOk)
import VirtualDom exposing (toHandlerInt)

*/



// HELPERS


var _VirtualDom_divertHrefToApp;

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What I like to do, is opening some compiled Elm app JS in VSCode (or your editor/IDE of choice) and read through the code there. “Go to definition” and “parameter name inlay hints” help tremendously. At the same time, I keep the source code on GitHub open for cross reference, because sometimes things are called just a or b in the compiled code but maybe __callback in the source code.

You might also be interested in this repo:

It’s an attempt at documenting some things about the runtime and experimenting with a different implementation.

gampleman · January 10, 2023, 2:48pm

I’ve written two blog posts that ~~deal with~~ are tangentially related to this subject.

Deals mostly with how rendering/events work from a conceptual perspective.

Might be also interesting, but it’s more about higher level architectural patterns.

Fabiano_Taioli · January 10, 2023, 3:01pm

Very interesting. Thank you.

jerith · January 14, 2023, 10:40pm

The underlying runtime system is based on effects managers. You can google for “elm effects managers” to find some discussion, including:

gist.github.com

https://gist.github.com/harrysarson/6ffeb44925197f961572b61825b97106

effect-managers.md

# The best guide on event managers in elm

Not because this guide is any good, but because it is the only one.
This is (to the best of my knowledge) correct as of version 19.0 of the elm compiler and version 1.0.2 of `elm/core`.

## Types of event managed things in elm

1. `Cmd`s
2. Port `Cmd`s (I believe these to be fundamentally different from normal `Cmd`s).
3. `Sub`s.

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system · January 24, 2023, 10:41pm

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