Stop toString from descending into arbitrary objects

[ThomasWeiser]

The function toString has code to convert Elm-records into a string representation.
This code missed a checked whether the JS object is really an Elm-record.

Problem: If toString is called on an recursively linked JS object (not an Elm-record, e.g. from a native library function), this will lead into an infinite recursion.

The check used to be in the code up to commit 08bf809. I don't know if Evan removed it intentionally for some reason.

[evancz]

Yes, it was removed because records no longer have a _ field that distinguishes them.

Can you give concrete examples of values that mess this up?

[evancz]

Yes, it was removed because records no longer have a _ field that distinguishes them.

Can you give concrete examples of values that mess this up?

[ThomasWeiser]

Ah, didn't know about the dropped _ field.

My example is a test app for elmfire. The native code returns a reference to internal data of the Firebase library. This data happens to have a cyclic structure.

I now have a workaround, where I drop this cyclic Json data from the record before printing.

Does this mean that cyclic structures are disallowed as a Json.Encode.Value? This would be kind of a burden for native wrappers around JS libraries. But it would be a valid argument to not have to do cycle checks in functions like toString (and probably in comparing and equality checking too).

[ThomasWeiser]

Ah, didn't know about the dropped _ field.

My example is a test app for elmfire. The native code returns a reference to internal data of the Firebase library. This data happens to have a cyclic structure.

I now have a workaround, where I drop this cyclic Json data from the record before printing.

Does this mean that cyclic structures are disallowed as a Json.Encode.Value? This would be kind of a burden for native wrappers around JS libraries. But it would be a valid argument to not have to do cycle checks in functions like toString (and probably in comparing and equality checking too).