encoding/json/v2: format of map values

[rothskeller]

Go version

go version go1.25.0 darwin/amd64

Output of go env in your module/workspace:

AR='ar'
CC='clang'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='clang++'
GCCGO='gccgo'
GO111MODULE=''
GOAMD64='v1'
GOARCH='amd64'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/Users/stever/Library/Caches/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/Users/stever/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT='jsonv2'
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/3y/936sl4d50dd_38rvrs041qn80000gn/T/go-build1330506723=/tmp/go-build -gno-record-gcc-switches -fno-common'
GOHOSTARCH='amd64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMOD='/Users/stever/src/packet/v2/go.mod'
GOMODCACHE='/Users/stever/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/stever/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/local/go'
GOSUMDB='sum.golang.org'
GOTELEMETRY='on'
GOTELEMETRYDIR='/Users/stever/Library/Application Support/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/local/go/pkg/tool/darwin_amd64'
GOVCS=''
GOVERSION='go1.25.0'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

Marshaled a structure including a map of time values with a format string.

package main

import (
	"encoding/json/v2"
	"fmt"
	"os"
	"time"
)

type S struct {
	MT map[string]time.Time `json:"mt,format:'2006-01-02'"`
}

func main() {
	var s = S{MT: map[string]time.Time{"hello": time.Now()}}
	if err := json.MarshalWrite(os.Stdout, s); err != nil {
		fmt.Println(err.Error())
	}
}

What did you see happen?

JSON v2 interpreted the format string as applying to the map, rather than to the map values, and rejected it:

json: cannot marshal from Go map[string]time.Time within "/mt": invalid format flag "2006-01-02"

What did you expect to see?

I expected it to use the format string to format the time values in the map. Attempting to interpret it as a format string for the map itself is pointless since maps don't have format strings. Or, if the 'format' specifier has to apply to the map itself, there should be some other way to specify the format string for the map values.

[gabyhelp]

Related Issues

• encoding/json: custom type marshaling doesn't work for map values #55890
• encoding/json/v2: unexpected support for Inf in map[string]any value #74797 (closed)
• encoding/json: document behaviour of json.Marshal() with non-pointer elements with MarshalJSON() defined for pointers #68919 (closed)
• affected/package: encoding/json #54609 (closed)
• encoding/json: skyrocketing memory allocation in encoding when `jsonv2` experiment enabled #75026
• Adding custom marshaller to value of a map removes the map from the output #48634 (closed)
• encoding/json Inconsistent behaviour with custom MarshalJSON function #46647 (closed)
• encoding/json: allow non-string map keys that implement encoding.TextMarshaler/TextUnmarshaler #12146 (closed)
• encoding/json: MarshalJSON not called in value contexts for integer types #23263 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[dsnet]

Hi, thank you for filing the issue.

Plumbing the format tag option to nested types is a non-trivial problem. Especially with Go map types, we would need some syntactic mechanism to indicate whether the format is intended for the key type or the value type. We discussed this exact problem during the early design of json/v2, and decided not to support format of nested types for the time being. A future proposal could define a syntax for plumbing the format down.

Alternatively, if we had typed struct tags (#74472), we would have a more clear mechanism to indicate whether the format was for the key type versus the value type.