What input will cause golang's json.Marshal to return an error?

json go marshalling
mdwhatcott picture mdwhatcott · Nov 24, 2015 · Viewed 21.3k times · Source

From the docs:

JSON cannot represent cyclic data structures and Marshal does not handle them. Passing cyclic structures to Marshal will result in an infinite recursion.

I've experienced this situation, which results in a runtime panic.

What I'm wondering is if anyone can provide a working program that demonstrates a non-panic situation where json.Marshal returns a non-nil error. The best answers would clearly include the inputs used.

Answer

hbejgel picture hbejgel · Nov 27, 2015

Just to complement Jonathan's answer, the json.Marshal function can return two types of errors: UnsupportedTypeError or UnsupportedValueError

The first one can be caused, as Jonathan said by trying to Marshal an invalid type:

_, err := json.Marshal(make(chan int))
_, ok := err.(*json.UnsupportedTypeError) // ok == true

On the other hand you can also have the Marshal function return an error by passing an invalid value:

_, err := json.Marshal(math.Inf(1))
_, ok := err.(*json.UnsupportedValueError) // ok == true

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