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package toml

import ""

Package toml implements decoding and encoding of TOML files.

This package supports TOML v1.0.0, as specified at

The package implements a TOML validator, and can be used to verify if TOML document is valid. It can also be used to print the type of each key.

Example (StrictDecoding)

Example StrictDecoding shows how to detect if there are keys in the TOML document that weren't decoded into the value given. This is useful for returning an error to the user if they've included extraneous fields in their configuration.


package main

import (


func main() {
	var blob = `
		key1 = "value1"
		key2 = "value2"
		key3 = "value3"

	var conf struct {
		Key1 string
		Key3 string
	md, err := toml.Decode(blob, &conf)
	if err != nil {

	fmt.Printf("Undecoded keys: %q\n", md.Undecoded())


Undecoded keys: ["key2"]
Example (UnmarshalTOML)

Example UnmarshalTOML shows how to implement a struct type that knows how to unmarshal itself. The struct must take full responsibility for mapping the values passed into the struct. The method may be used with interfaces in a struct in cases where the actual type is not known until the data is examined.


package main

import (


type order struct {
	parts parts

type parts []part

type part interface {
	Name() string

type valve struct {
	Type   string
	ID     string
	Size   float32
	Rating int

func (v *valve) Name() string {
	return fmt.Sprintf("VALVE: %s", v.ID)

type pipe struct {
	Type     string
	ID       string
	Length   float32
	Diameter int

func (p *pipe) Name() string {
	return fmt.Sprintf("PIPE: %s", p.ID)

type cable struct {
	Type   string
	ID     string
	Length int
	Rating float32

func (c *cable) Name() string {
	return fmt.Sprintf("CABLE: %s", c.ID)

func (o *order) UnmarshalTOML(data any) error {

	d, _ := data.(map[string]any)
	parts, _ := d["parts"].([]map[string]any)

	for _, p := range parts {

		typ, _ := p["type"].(string)
		id, _ := p["id"].(string)

		switch p["type"] {
		case "valve":

			size := float32(p["size"].(float64))
			rating := int(p["rating"].(int64))

			valve := &valve{
				Type:   typ,
				ID:     id,
				Size:   size,
				Rating: rating,
			} = append(, valve)

		case "pipe":

			length := float32(p["length"].(float64))
			diameter := int(p["diameter"].(int64))

			pipe := &pipe{
				Type:     typ,
				ID:       id,
				Length:   length,
				Diameter: diameter,
			} = append(, pipe)

		case "cable":

			length := int(p["length"].(int64))
			rating := float32(p["rating"].(float64))

			cable := &cable{
				Type:   typ,
				ID:     id,
				Length: length,
				Rating: rating,
			} = append(, cable)


	return nil

func main() {
	blob := `
		type = "valve"
		id = "valve-1"
		size = 1.2
		rating = 4

		type = "valve"
		id = "valve-2"
		size = 2.1
		rating = 5

		type = "pipe"
		id = "pipe-1"
		length = 2.1
		diameter = 12

		type = "cable"
		id = "cable-1"
		length = 12
		rating = 3.1

	// See example_test.go in the source for the implementation of the order
	// type.
	o := &order{}

	err := toml.Unmarshal([]byte(blob), o)
	if err != nil {

	for _, part := range {
Example (Unmarshaler)

Example Unmarshaler shows how to decode TOML strings into your own custom data type.


package main

import (


type address struct{ *mail.Address }

func (a *address) UnmarshalText(text []byte) error {
	var err error
	a.Address, err = mail.ParseAddress(string(text))
	return err

func main() {
	blob := `
		contacts = [
			"Donald Duck <>",
			"Scrooge McDuck <>",

	var contacts struct {
		// Implementation of the address type:
		//     type address struct{ *mail.Address }
		//     func (a *address) UnmarshalText(text []byte) error {
		//         var err error
		//         a.Address, err = mail.ParseAddress(string(text))
		//         return err
		//     }

		Contacts []address

	_, err := toml.Decode(blob, &contacts)
	if err != nil {

	for _, c := range contacts.Contacts {
		fmt.Printf("%#v\n", c.Address)



&mail.Address{Name:"Donald Duck", Address:""}
&mail.Address{Name:"Scrooge McDuck", Address:""}




func Marshal

func Marshal(v any) ([]byte, error)

Marshal returns a TOML representation of the Go value.

See Encoder for a description of the encoding process.

func PrimitiveDecode

func PrimitiveDecode(primValue Primitive, v any) error

PrimitiveDecode is an alias for MetaData.PrimitiveDecode().

Deprecated: use MetaData.PrimitiveDecode.

func Unmarshal

func Unmarshal(data []byte, v any) error

Unmarshal decodes the contents of data in TOML format into a pointer v.

See Decoder for a description of the decoding process.


type Decoder

type Decoder struct {
	// contains filtered or unexported fields

Decoder decodes TOML data.

TOML tables correspond to Go structs or maps; they can be used interchangeably, but structs offer better type safety.

TOML table arrays correspond to either a slice of structs or a slice of maps.

TOML datetimes correspond to time.Time. Local datetimes are parsed in the local timezone.

time.Duration types are treated as nanoseconds if the TOML value is an integer, or they're parsed with time.ParseDuration() if they're strings.

All other TOML types (float, string, int, bool and array) correspond to the obvious Go types.

An exception to the above rules is if a type implements the TextUnmarshaler interface, in which case any primitive TOML value (floats, strings, integers, booleans, datetimes) will be converted to a []byte and given to the value's UnmarshalText method. See the Unmarshaler example for a demonstration with email addresses.

Key mapping

TOML keys can map to either keys in a Go map or field names in a Go struct. The special `toml` struct tag can be used to map TOML keys to struct fields that don't match the key name exactly (see the example). A case insensitive match to struct names will be tried if an exact match can't be found.

The mapping between TOML values and Go values is loose. That is, there may exist TOML values that cannot be placed into your representation, and there may be parts of your representation that do not correspond to TOML values. This loose mapping can be made stricter by using the IsDefined and/or Undecoded methods on the MetaData returned.

This decoder does not handle cyclic types. Decode will not terminate if a cyclic type is passed.

func NewDecoder

func NewDecoder(r io.Reader) *Decoder

NewDecoder creates a new Decoder.

func (*Decoder) Decode

func (dec *Decoder) Decode(v any) (MetaData, error)

Decode TOML data in to the pointer `v`.

type Encoder

type Encoder struct {
	Indent string // string for a single indentation level; default is two spaces.
	// contains filtered or unexported fields

Encoder encodes a Go to a TOML document.

The mapping between Go values and TOML values should be precisely the same as for Decode.

time.Time is encoded as a RFC 3339 string, and time.Duration as its string representation.

The Marshaler and encoding.TextMarshaler interfaces are supported to encoding the value as custom TOML.

If you want to write arbitrary binary data then you will need to use something like base64 since TOML does not have any binary types.

When encoding TOML hashes (Go maps or structs), keys without any sub-hashes are encoded first.

Go maps will be sorted alphabetically by key for deterministic output.

The toml struct tag can be used to provide the key name; if omitted the struct field name will be used. If the "omitempty" option is present the following value will be skipped:

If omitzero is given all int and float types with a value of 0 will be skipped.

Encoding Go values without a corresponding TOML representation will return an error. Examples of this includes maps with non-string keys, slices with nil elements, embedded non-struct types, and nested slices containing maps or structs. (e.g. [][]map[string]string is not allowed but []map[string]string is okay, as is []map[string][]string).

NOTE: only exported keys are encoded due to the use of reflection. Unexported keys are silently discarded.

func NewEncoder

func NewEncoder(w io.Writer) *Encoder

NewEncoder create a new Encoder.

func (*Encoder) Encode

func (enc *Encoder) Encode(v any) error

Encode writes a TOML representation of the Go value to the Encoder's writer.

An error is returned if the value given cannot be encoded to a valid TOML document.



package main

import (


func main() {
	var (
		date, _ = time.Parse(time.RFC822, "14 Mar 10 18:00 UTC")
		buf     = new(bytes.Buffer)
	err := toml.NewEncoder(buf).Encode(map[string]any{
		"date":   date,
		"counts": []int{1, 1, 2, 3, 5, 8},
		"hash": map[string]string{
			"key1": "val1",
			"key2": "val2",
	if err != nil {



counts = [1, 1, 2, 3, 5, 8]
date = 2010-03-14T18:00:00Z

  key1 = "val1"
  key2 = "val2"

type Key

type Key []string

Key represents any TOML key, including key groups. Use MetaData.Keys to get values of this type.

func (Key) String

func (k Key) String() string

type Marshaler

type Marshaler interface {
	MarshalTOML() ([]byte, error)

Marshaler is the interface implemented by types that can marshal themselves into valid TOML.

type MetaData

type MetaData struct {
	// contains filtered or unexported fields

MetaData allows access to meta information about TOML data that's not accessible otherwise.

It allows checking if a key is defined in the TOML data, whether any keys were undecoded, and the TOML type of a key.

func Decode

func Decode(data string, v any) (MetaData, error)

Decode the TOML data in to the pointer v.

See Decoder for a description of the decoding process.



package main

import (


func main() {
	tomlBlob := `
		# Some comments.
		ip = ""

			Ports = [ 8001, 8002 ]
			Location = "Toronto"
			Created = 1987-07-05T05:45:00Z

		ip = ""

			Ports = [ 9001, 9002 ]
			Location = "New Jersey"
			Created = 1887-01-05T05:55:00Z

	type (
		serverConfig struct {
			Ports    []int
			Location string
			Created  time.Time
		server struct {
			IP     string       `toml:"ip,omitempty"`
			Config serverConfig `toml:"config"`
		servers map[string]server

	var config servers
	_, err := toml.Decode(tomlBlob, &config)
	if err != nil {

	for _, name := range []string{"alpha", "beta"} {
		s := config[name]
		fmt.Printf("Server: %s (ip: %s) in %s created on %s\n",
			name, s.IP, s.Config.Location,
		fmt.Printf("Ports: %v\n", s.Config.Ports)



Server: alpha (ip: in Toronto created on 1987-07-05
Ports: [8001 8002]
Server: beta (ip: in New Jersey created on 1887-01-05
Ports: [9001 9002]

func DecodeFS

func DecodeFS(fsys fs.FS, path string, v any) (MetaData, error)

DecodeFS reads the contents of a file from fs.FS and decodes it with Decode.

func DecodeFile

func DecodeFile(path string, v any) (MetaData, error)

DecodeFile reads the contents of a file and decodes it with Decode.

func DecodeReader

func DecodeReader(r io.Reader, v any) (MetaData, error)

DecodeReader is an alias for NewDecoder(r).Decode(v).

Deprecated: use NewDecoder(reader).Decode(&value).

func (*MetaData) IsDefined

func (md *MetaData) IsDefined(key ...string) bool

IsDefined reports if the key exists in the TOML data.

The key should be specified hierarchically, for example to access the TOML key "a.b.c" you would use IsDefined("a", "b", "c"). Keys are case sensitive.

Returns false for an empty key.

func (*MetaData) Keys

func (md *MetaData) Keys() []Key

Keys returns a slice of every key in the TOML data, including key groups.

Each key is itself a slice, where the first element is the top of the hierarchy and the last is the most specific. The list will have the same order as the keys appeared in the TOML data.

All keys returned are non-empty.

func (*MetaData) PrimitiveDecode

func (md *MetaData) PrimitiveDecode(primValue Primitive, v any) error

PrimitiveDecode is just like the other Decode* functions, except it decodes a TOML value that has already been parsed. Valid primitive values can *only* be obtained from values filled by the decoder functions, including this method. (i.e., v may contain more Primitive values.)

Meta data for primitive values is included in the meta data returned by the Decode* functions with one exception: keys returned by the Undecoded method will only reflect keys that were decoded. Namely, any keys hidden behind a Primitive will be considered undecoded. Executing this method will update the undecoded keys in the meta data. (See the example.)



package main

import (


func main() {
	tomlBlob := `
		ranking = ["Springsteen", "J Geils"]

		started = 1973
		albums = ["Greetings", "WIESS", "Born to Run", "Darkness"]

		[bands."J Geils"]
		started = 1970
		albums = ["The J. Geils Band", "Full House", "Blow Your Face Out"]

	type (
		band struct {
			Started int
			Albums  []string
		classics struct {
			Ranking []string
			Bands   map[string]toml.Primitive

	// Do the initial decode; reflection is delayed on Primitive values.
	var music classics
	md, err := toml.Decode(tomlBlob, &music)
	if err != nil {

	// MetaData still includes information on Primitive values.
	fmt.Printf("Is `bands.Springsteen` defined? %v\n",
		md.IsDefined("bands", "Springsteen"))

	// Decode primitive data into Go values.
	for _, artist := range music.Ranking {
		// A band is a primitive value, so we need to decode it to get a real
		// `band` value.
		primValue := music.Bands[artist]

		var aBand band
		err = md.PrimitiveDecode(primValue, &aBand)
		if err != nil {
		fmt.Printf("%s started in %d.\n", artist, aBand.Started)

	// Check to see if there were any fields left undecoded. Note that this
	// won't be empty before decoding the Primitive value!
	fmt.Printf("Undecoded: %q\n", md.Undecoded())



Is `bands.Springsteen` defined? true
Springsteen started in 1973.
J Geils started in 1970.
Undecoded: []

func (*MetaData) Type

func (md *MetaData) Type(key ...string) string

Type returns a string representation of the type of the key specified.

Type will return the empty string if given an empty key or a key that does not exist. Keys are case sensitive.

func (*MetaData) Undecoded

func (md *MetaData) Undecoded() []Key

Undecoded returns all keys that have not been decoded in the order in which they appear in the original TOML document.

This includes keys that haven't been decoded because of a Primitive value. Once the Primitive value is decoded, the keys will be considered decoded.

Also note that decoding into an empty interface will result in no decoding, and so no keys will be considered decoded.

In this sense, the Undecoded keys correspond to keys in the TOML document that do not have a concrete type in your representation.

type ParseError

type ParseError struct {
	Message  string   // Short technical message.
	Usage    string   // Longer message with usage guidance; may be blank.
	Position Position // Position of the error
	LastKey  string   // Last parsed key, may be blank.

	// Line the error occurred.
	// Deprecated: use [Position].
	Line int
	// contains filtered or unexported fields

ParseError is returned when there is an error parsing the TOML syntax such as invalid syntax, duplicate keys, etc.

In addition to the error message itself, you can also print detailed location information with context by using [ErrorWithPosition]:

toml: error: Key 'fruit' was already created and cannot be used as an array.

At line 4, column 2-7:

      2 | fruit = []
      3 |
      4 | [[fruit]] # Not allowed

[ErrorWithUsage] can be used to print the above with some more detailed usage guidance:

toml: error: newlines not allowed within inline tables

At line 1, column 18:

      1 | x = [{ key = 42 #

Error help:

  Inline tables must always be on a single line:

      table = {key = 42, second = 43}

  It is invalid to split them over multiple lines like so:

      # INVALID
      table = {
          key    = 42,
          second = 43

  Use regular for this:

      key    = 42
      second = 43

func (ParseError) Error

func (pe ParseError) Error() string

func (ParseError) ErrorWithPosition

func (pe ParseError) ErrorWithPosition() string

ErrorWithPosition returns the error with detailed location context.

See the documentation on ParseError.

func (ParseError) ErrorWithUsage

func (pe ParseError) ErrorWithUsage() string

ErrorWithUsage returns the error with detailed location context and usage guidance.

See the documentation on ParseError.

type Position

type Position struct {
	Line  int // Line number, starting at 1.
	Start int // Start of error, as byte offset starting at 0.
	Len   int // Lenght in bytes.

Position of an error.

func (Position) String

func (p Position) String() string

type Primitive

type Primitive struct {
	// contains filtered or unexported fields

Primitive is a TOML value that hasn't been decoded into a Go value.

This type can be used for any value, which will cause decoding to be delayed. You can use PrimitiveDecode to "manually" decode these values.

NOTE: The underlying representation of a `Primitive` value is subject to change. Do not rely on it.

NOTE: Primitive values are still parsed, so using them will only avoid the overhead of reflection. They can be useful when you don't know the exact type of TOML data until runtime.

type TextMarshaler

type TextMarshaler encoding.TextMarshaler

TextMarshaler is an alias for encoding.TextMarshaler.

Deprecated: use encoding.TextMarshaler

type TextUnmarshaler

type TextUnmarshaler encoding.TextUnmarshaler

TextUnmarshaler is an alias for encoding.TextUnmarshaler.

Deprecated: use encoding.TextUnmarshaler

type Unmarshaler

type Unmarshaler interface {
	UnmarshalTOML(any) error

Unmarshaler is the interface implemented by objects that can unmarshal a TOML description of themselves.

Source Files

decode.go deprecated.go doc.go encode.go error.go lex.go meta.go parse.go type_fields.go type_toml.go


cmd/toml-test-decoderCommand toml-test-decoder satisfies the toml-test interface for testing TOML decoders.
cmd/toml-test-encoderCommand toml-test-encoder satisfies the toml-test interface for testing TOML encoders.
cmd/tomlvCommand tomlv validates TOML documents and prints each key's type.
v1.4.0 (latest)
May 23, 2024
20 packages
Last checked
2 days ago

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