proposal: encoding/json: option to clear output before unmarshal

[coco-roll]

What version of Go are you using (go version)?

$ go version
go version go1.16.4 darwin/amd64

Does this issue reproduce with the latest release?

The lastst release is also reproduce

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE="on"
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/Library/Caches/go-build"
GOENV="/Users/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/go"
GOPRIVATE=""
GOPROXY="direct"
GOROOT="/usr/local/Cellar/go/1.16.4/libexec"
GOSUMDB=""
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.16.4/libexec/pkg/tool/darwin_amd64"
GOVCS=""
GOVERSION="go1.16.4"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/go/src/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/7p/syywyq8s4t500nf_fhcw58km0000gn/T/go-build1825247537=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

package main

import (
	"encoding/json"
	"fmt"
)

func main() {
	var testMap map[string]interface{}

	strArr := "{\"11911007\":{\"11911007\":1,\"119110010\":1,\"11911006\":1,\"11911005\":1,\"119110011\":1,\"119110012\":1,\"11911004\":1,\"119110015\":1},\"119110010\":{\"000000\":1}}"
	if err := json.Unmarshal([]byte(strArr), &testMap); err != nil {
		panic(err)
	}
	fmt.Printf("first time Unmarshal map is:%+v\n", testMap)

	if err := json.Unmarshal([]byte("{\"11911007\":{\"000001\":1}}"), &testMap); err != nil {
		panic(err)
	}
	fmt.Printf("second time Unmarshal map is:%+v\n", testMap)

	return
}

What did you expect to see?

See:

first time Unmarshal map is:map[119110010:map[000000:1] 11911007:map[119110010:1 119110011:1 119110012:1 119110015:1 11911004:1 11911005:1 11911006:1 11911007:1]]
second time Unmarshal map is:map[1911007:map[000001:1]]

What did you see instead?

See:

first time Unmarshal map is:map[119110010:map[000000:1] 11911007:map[119110010:1 119110011:1 119110012:1 119110015:1 11911004:1 11911005:1 11911006:1 11911007:1]]
second time Unmarshal map is:map[119110010:map[000000:1] 11911007:map[000001:1]]

[coco-roll]

Can i add a new function like it?

// To unmarshal a JSON object into a map, Unmarshal first establishes a map to
// use. If the map is nil or not, Unmarshal allocates a new map. 
func UnmarshalDeep(data []byte, v interface{}) error {}

[andig]

Imho it‘s simply not how unmarshal works. More importantly, you can use the current behaviour to establish default values.

[coco-roll]

Yes, i change my code for use it. Thanks all.

[rsc]

Sounds like this proposal has been retracted?

[mvdan]

Sounds that way. Closing.

[rsc]

This proposal has been declined as retracted.
— rsc for the proposal review group