Today I Learned

Whenever a value is set in localStorage, a storage event will be dispatched on all other windows from the same origin. The event is not fired or catchable in the originator window when it is modified through a script.

This was a problem because I wanted to be able to have an Elm subscription as the only source of truth for my session.

In order to achieve this, I'm mimicking the storage event from within the command port as soon as the message is received through the port.

app.ports.commandPort.subscribe(session => {
  localStorage.session = JSON.stringify(session);
  app.ports.subscriptionPort.send(localStorage.session);
});

window.addEventListener(
    "storage",
    event => {
      if (event.storageArea === localStorage && event.key === "session") {
        app.ports.subscriptionPort.send(event.newValue);
      }
    },
    false
);

And in Elm:

port subscriptionPort : (Encode.Value -> msg) -> Sub msg

port commandPort : Maybe Encode.Value -> Cmd msg

closest: Int -> List Int -> Int
closest size sizes =
  sizes
      |> List.sort
      |> List.filter (\s -> s < size)
      |> List.reverse
      |> List.head
      |> Maybe.withDefault 0

This example is not meant to be exhaustive nor performant.

There is no such thing as !True to get a False value in Elm.

The Elm way to achieve this is:

module Helpers.Bool exposing (flip)

flip: Bool -> Bool
flip bool = 
	if bool == True then
  	False
  else
  	True

And the use it as:

import Helpers.Bool

...

Helpers.Bool.flip True