proposal: encoding/json: allow unmarshaling singleton into slice

[rogpeppe]

It's common for dynamic languages to be somewhat lax about the distinction
between an array and a single value when encoding and decoding JSON.
For example, a single value may be generated where an array is also acceptable.

Currently this can be worked around by defining a special type
for the slice being unmarshaled to.

For example:

type Val struct {
	S string
}

type Vals []Val

func (r *Vals) UnmarshalJSON(data []byte) error {
	type noUnmarshal Vals
	if data[0] == '[' {
		return json.Unmarshal(data, (*noUnmarshal)(r))
	}
	*r = make(Vals, 1)
	return json.Unmarshal(data, &(*r)[0])
}

This is cumbersome. Instead we could make encoding/json automatically
allow a singleton value when decoding into a slice, or provide an
struct tag option to enable that behaviour.

[rsc]

If it's common I'm a little surprised it hasn't come up already. Can you point to specific JSON implementations that don't care and are causing problems? I mean, JavaScript clearly distinguishes between [1] and 1 for example.

[rsc]

Ping. Any new context here? I'm still skeptical this is a common problem.

[rsc]

Closing for lack of motivation.