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.

 

On Monday, August 1, 2016 at 6:01:09 AM UTC+2, Duane Johnson wrote:
>
> Cool bug find :) Is it tracked on 
> https://github.com/elm-lang/elm-compiler/issues ?
>
> On Sun, Jul 31, 2016 at 3:01 AM, Aditya Vishwakarma <[email protected] 
> <javascript:>> wrote:
>
>> 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
>>
>>
>> On Friday, 29 July 2016 02:27:41 UTC+5:30, Aditya Vishwakarma wrote:
>>>
>>> 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:
>>>
>>>
>>> <https://lh3.googleusercontent.com/-5aH4m9n6gEw/V5pu35oQsEI/AAAAAAAAEbw/QKXJrtSqgcA6-Dfe_BVsDOYPpn9BHl6MACLcB/s1600/Screen%2BShot%2B2016-07-29%2Bat%2B1.39.20%2BAM.png>
>>>
>>> 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?
>>>
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