Uncaught TypeError: Cannot read property 'tag' of undefined

[Aditya Vishwakarma]

Hi

I am getting Uncaught TypeError: Cannot read property 'tag' of undefined in a sample code I built to tryout elm. The error happen when I send JSON data across the elm port from JS side. 

Port defn: https://github.com/adityav/elm-reddit-listing/blob/master/src/AppPorts.elm#L14

JS call which causes this issue: https://github.com/adityav/elm-reddit-listing/blob/master/src/index.js#L36

git repo: https://github.com/adityav/elm-reddit-listing

To run. Simply do

npm install && elm package install
npm start

and open http://localhost:8080/. A listing of /r/elm should show up when the app works. 

A screenshot of the exact location where the error occurs:

People on slack recommended me to remove elm-stuff/build-artifacts and try again. It didn't work so I removed node_modules and elm-stuff directories and reinstalled the whole thing. Didn't work as well. 

The code was working before I decided to clean it up a little to show to my teammates. Stopped working the moment I shared the github link with them. :|

anyone got any ideas on this?

[Aditya Vishwakarma]

Hi

So I dug deeper into this issue. turns out that the JS code for my port module was being emitted before the JS of Json.Decode module. 

After some research, I found that 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: https://github.com/adityav/elm-ports-issue

[Duane Johnson]

Cool bug find :) Is it tracked on https://github.com/elm-lang/elm-compiler/issues ?

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[Robert Walter]

It doesn't seem like it is tracked. I ran into the same(?) issue in my project and I tried to create a SSCCE so I could file an issue, but the thing is I cannot get a SSCCE to fail like my project. My projec tis in the middle of a bigger refactoring, which makes it particularly hard for me to boil it down to a SSCCE.

@Aditya: can you share some more information? In my case, my port module is called "Ports". If I do not have an import statement for Json.Decode, I run into the very error you mentioned at the beginning. Adding the import statement fixes the issue.

[Janis Voigtländer]

@Robert, maybe yours is a new bug, that was introduced with core version 4.0.3 or 4.0.4 (released just a few days ago)? See https://github.com/elm-lang/core/issues/683.

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[Aditya Vishwakarma]

@robert: are you using webpack loader? We tracked this bug to an elm-webpack-loader behaviour. here is the issue. https://github.com/rtfeldman/elm-webpack-loader/issues/60

For now, the workaround I am using is to import Json.Decode in my Ports file.

[Robert Walter]

Hi Janis,

yes, this seems to be related. I created an SSCCE to the best of my abilities: https://github.com/RobertWalter83/sscce-elm-ports-uncaughtError

Check out the readme if you are interested.

Will post a comment to the isse you pointed me to.

[Robert Walter]

Hi Aditya, 

thanks for pointing that out. I do not use the webpack-loader. I think it is related to what Janis mentioned.

Robert