Json.Decode.decodeString throws runtime for types with cyclic dependencies

[tiziano88]

System details:

Elm Platform 0.17.0
Chrome Version 51.0.2704.103 (64-bit)
Ubuntu Linux 14.04 LTS

Error:

Uncaught TypeError: Cannot read property 'tag' of undefined
runHelp @ test.elm:5165
runHelp @ test.elm:5286
run @ test.elm:5157
runOnString @ test.elm:5152
A2 @ test.elm:92
(anonymous function) @ test.elm:7185
(anonymous function) @ test.elm:7220

Reproducible with the following example (even pasting it in http://elm-lang.org/try):

https://gist.github.com/tiziano88/7beb26152275326eb77bf9f754f14d9d

(I cut it down considerably from the original project, now it seems to be the minimal configuration that reliably causes the error)

I suspect the issue is due to the circular type dependencies, even though only two of the type involved are aliases, and the other two are actual types.

[process-bot]

Thanks for the issue! Make sure it satisfies this checklist. My human colleagues will appreciate it!

Here is what to expect next, and if anyone wants to comment, keep these things in mind.

[process-bot]

Thanks for the issue! Make sure it satisfies this checklist. My human colleagues will appreciate it!

Here is what to expect next, and if anyone wants to comment, keep these things in mind.

[tiziano88]

Actually, I managed to simplify the example even more; now it only contains three actual types, with a cyclic dependency between them. I am actually not sure what the expected outcome should be in this case, but definitely not a runtime error.

[tiziano88]

Actually, I managed to simplify the example even more; now it only contains three actual types, with a cyclic dependency between them. I am actually not sure what the expected outcome should be in this case, but definitely not a runtime error.

[tiziano88]

Turns out, even a single recursive type and associated decoder is enough (see updated Gist).

[tiziano88]

Turns out, even a single recursive type and associated decoder is enough (see updated Gist).

[jvoigtlaender]

See issue 873 in the elm-compiler repo, in particular also all the discussion there and in its linked references about the Json decoding special case.

[jvoigtlaender]

See issue 873 in the elm-compiler repo, in particular also all the discussion there and in its linked references about the Json decoding special case.

[jvoigtlaender]

That is, elm/compiler#873 (comment) (indicating how to work around this for Json decoders specially).

[jvoigtlaender]

That is, elm/compiler#873 (comment) (indicating how to work around this for Json decoders specially).