encoding/json: struct field implementing Unmarshaler

[gopherbot]

by akunokuroneko:

What steps will reproduce the problem?
1. Create a custom type that implements Unmarshaller against it's immediate (not
pointer) value.
2. Use type as an exported immediate value in a struct.
3. json.Decode() and watch the unmarshal fail.

What is the expected output?
The Unmarshaller method to be invoked.

What do you see instead?
"json: cannot unmarshal string into Go value of type ..."

(in this case, string is the literal type being used in the JSON stream to fudge the
custom type)

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

Which operating system are you using?
Linux amd64

Which revision are you using?  (hg identify)
r59

Please provide any additional information below.
Patch attached.

Attachments:

1. json-unmarshal-immediate.diff (799 bytes)

[robpike]

Comment 1:

I believe that you shouldn't define Unmarshal that way because it modifies the receiver,
but I'll let rsc respond.

Owner changed to @rsc.

Status changed to Accepted.

[gopherbot]

Comment 2 by akunokuroneko:

I'm only using it this way against single value types (such as those defined ala: "type
Foob uint64") in order to allow me to forcibly alter the way specific fields are
marshaled and unmarshaled without having to reimplement the struct marshaler.
Primarily I'm using this for textual representations in JSON for what are effectively
enums.

[gopherbot]

Comment 3 by akunokuroneko:

After further consideration, You're quite right - this method doesn't quite work the way
I envisaged because the receiver is passed by value.  I'll see if I can fix this
correctly.

[gopherbot]

Comment 4 by akunokuroneko:

Corrected patch to actually do the right thing:
Checks CanAddr() to see if a pointer can be acquired, and then checks to see if an
Unmarshaler is available for pointers to that type.  Failing that, does the usual logic.

Attachments:

1. json-unmarshal-immediate.diff (604 bytes)

[edsrzf]

Comment 5:

Marshal has the same problem.

[gopherbot]

Comment 6 by akunokuroneko:

Marshal does the right thing actually if you define the MarshalJSON message on the type
itself rather than a pointer to the type, and already knows to follow pointers to find a
valid type.

[edsrzf]

Comment 7:

I ran into a case recently where it didn't work, but I didn't test thoroughly. I'll take
another look.

[edsrzf]

Comment 8:

I have a simple case that reproduces the problem I was having. Maybe should be a
separate issue.
package main
import (
    "fmt"
    "json"
    "os"
)
type T int
func (t *T) MarshalJSON() ([]byte, os.Error) {
    return []byte(`"T"`), nil
}
func main() {
    var t struct{T T}
    b, _ := json.Marshal(&t)
    fmt.Printf("%s\n", b)
}
The output is
{"T": 0}
I expect
{"T": "T"}

[rsc]

Comment 9:

Status changed to HelpWanted.

[gopherbot]

Comment 10 by Leo.LVEB:

This is a patch for the second issue, it just checks if *T is a Marshaler.

Attachments:

1. patch.diff (1299 bytes)

[rsc]

Comment 11:

Labels changed: added priority-later.

[rsc]

Comment 12:

Labels changed: added priority-go1.

[robpike]

Comment 13:

Owner changed to builder@golang.org.

[rsc]

Comment 14:

Still broken.

[rsc]

Comment 15:

Labels changed: added go1-must.