encoding/json: use the Error method to marshal an error value?

[orofarne]

What steps will reproduce the problem?
http://play.golang.org/p/epXpwk0s4s

What is the expected output?
Something like this:
http://play.golang.org/p/Dk0l6fTj01

What do you see instead?
{"bar":{},"foo":{}}

Which compiler are you using (5g, 6g, 8g, gccgo)?


Which operating system are you using?
Linux (Ubuntu 12.04, 3.2.0-38-generic)

Which version are you using?  (run 'go version')
go version go1.0.3
go version devel +5260abd6df41 Sat Mar 30 19:05:00 2013 +0800 linux/amd64

Please provide any additional information below.
I think it's not intuitive behaviour. fmt prints errors without problems, why json not?

[remyoudompheng]

Comment 1:

See also issue #3353.

[gopherbot]

Comment 2 by pongad:

I believe this is the expected behavior of json.Marshal(interfaceValue). According to
the documentation, if the argument is an interface value, it is encodes the value of
that interface.
The implementation of fmt.Errorf uses errors.errorString type. The type is a struct with
one unexported member 's' which holds the error. Since the member is unexported,
json.Marshal does not care to encode it.
A possible workaround is to create a new error type that both satisfies "error" and
"json.Marshaler" like 
http://play.golang.org/p/NrtoYsGzij

[adg]

Comment 3:

Not sure what the right move is, here. Or even if we can fix it at all. There may be
code that relies on the JSON marhsaller *not* using the Error method to marshal a value.

Labels changed: added priority-later, removed priority-triage.

Status changed to Thinking.

[rsc]

Comment 4:

Labels changed: added go1.3maybe.

[rsc]

Comment 5:

Labels changed: added release-none, removed go1.3maybe.

[rsc]

Comment 6:

Labels changed: added repo-main.