Elm Ports issue

any port module in Elm has a dependency to Json.Decode as it uses decoders specified in that module for js-elm communications.

So if the project's port module has:

• Name beginning with a letter before J, and
• does not have a declared dependency on Json.Decode

Then the elm compiler will emit the port module code before Json.Decode which causes all decoders to be Undefined. This results in all user defined incoming ports to stop throw exceptions.

SSCCE Summary

• This SSCCE sends a string ("hi Elm! been a long time!") on init.
• renders text "waiting for JS to respond" on page
• JS replies back with the string after a 10 ms (index.js L10)
• Expected response is that the echoed string will show up on the screen. Instead we get an Uncaught TypeError: Cannot read property 'tag' of undefined in the console.

Run Instructions

Install dependencies

npm install && elm package install
npm start

Navigate to http://localhost:8080/ and open the console. you should see the error Uncaught TypeError: Cannot read property 'tag' of undefined show up.

Hackish solution

simply add this line in the imports of port module.

import Json.Decode exposing (..)

structure

AppPorts.elm - contains port defn. Uncomment line 10 to see the expected behaviour