README.md

# go-toml v2

Go library for the [TOML](https://toml.io/en/) format.

This library supports [TOML v1.0.0](https://toml.io/en/v1.0.0).

[🐞 Bug Reports](https://github.com/pelletier/go-toml/issues)

[💬 Anything else](https://github.com/pelletier/go-toml/discussions)

## Documentation

Full API, examples, and implementation notes are available in the Go
documentation.

[![Go Reference](https://pkg.go.dev/badge/github.com/pelletier/go-toml/v2.svg)](https://pkg.go.dev/github.com/pelletier/go-toml/v2)

## Import

```go
import "github.com/pelletier/go-toml/v2"
```

See [Modules](#Modules).

## Features

### Stdlib behavior

As much as possible, this library is designed to behave similarly as the
standard library's `encoding/json`.

### Performance

While go-toml favors usability, it is written with performance in mind. Most
operations should not be shockingly slow. See [benchmarks](#benchmarks).

### Strict mode

`Decoder` can be set to "strict mode", which makes it error when some parts of
the TOML document was not present in the target structure. This is a great way
to check for typos. [See example in the documentation][strict].

[strict]: https://pkg.go.dev/github.com/pelletier/go-toml/v2#example-Decoder.DisallowUnknownFields

### Contextualized errors

When most decoding errors occur, go-toml returns [`DecodeError`][decode-err],
which contains a human readable contextualized version of the error. For
example:

```
1| [server]
2| path = 100
 |        ~~~ cannot decode TOML integer into struct field toml_test.Server.Path of type string
3| port = 50
```

[decode-err]: https://pkg.go.dev/github.com/pelletier/go-toml/v2#DecodeError

### Local date and time support

TOML supports native [local date/times][ldt]. It allows to represent a given
date, time, or date-time without relation to a timezone or offset. To support
this use-case, go-toml provides [`LocalDate`][tld], [`LocalTime`][tlt], and
[`LocalDateTime`][tldt]. Those types can be transformed to and from `time.Time`,
making them convenient yet unambiguous structures for their respective TOML
representation.

[ldt]: https://toml.io/en/v1.0.0#local-date-time
[tld]: https://pkg.go.dev/github.com/pelletier/go-toml/v2#LocalDate
[tlt]: https://pkg.go.dev/github.com/pelletier/go-toml/v2#LocalTime
[tldt]: https://pkg.go.dev/github.com/pelletier/go-toml/v2#LocalDateTime

### Commented config

Since TOML is often used for configuration files, go-toml can emit documents
annotated with [comments and commented-out values][comments-example]. For
example, it can generate the following file:

```toml
# Host IP to connect to.
host = '127.0.0.1'
# Port of the remote server.
port = 4242

# Encryption parameters (optional)
# [TLS]
# cipher = 'AEAD-AES128-GCM-SHA256'
# version = 'TLS 1.3'
```

[comments-example]: https://pkg.go.dev/github.com/pelletier/go-toml/v2#example-Marshal-Commented

## Getting started

Given the following struct, let's see how to read it and write it as TOML:

```go
type MyConfig struct {
	Version int
	Name    string
	Tags    []string
}
```

### Unmarshaling

[`Unmarshal`][unmarshal] reads a TOML document and fills a Go structure with its
content. For example:

```go
doc := `
version = 2
name = "go-toml"
tags = ["go", "toml"]
`

var cfg MyConfig
err := toml.Unmarshal([]byte(doc), &cfg)
if err != nil {
	panic(err)
}
fmt.Println("version:", cfg.Version)
fmt.Println("name:", cfg.Name)
fmt.Println("tags:", cfg.Tags)

// Output:
// version: 2
// name: go-toml
// tags: [go toml]
```

[unmarshal]: https://pkg.go.dev/github.com/pelletier/go-toml/v2#Unmarshal

### Marshaling

[`Marshal`][marshal] is the opposite of Unmarshal: it represents a Go structure
as a TOML document:

```go
cfg := MyConfig{
	Version: 2,
	Name:    "go-toml",
	Tags:    []string{"go", "toml"},
}

b, err := toml.Marshal(cfg)
if err != nil {
	panic(err)
}
fmt.Println(string(b))

// Output:
// Version = 2
// Name = 'go-toml'
// Tags = ['go', 'toml']
```

[marshal]: https://pkg.go.dev/github.com/pelletier/go-toml/v2#Marshal

## Unstable API

This API does not yet follow the backward compatibility guarantees of this
library. They provide early access to features that may have rough edges or an
API subject to change.

### Parser

Parser is the unstable API that allows iterative parsing of a TOML document at
the AST level. See https://pkg.go.dev/github.com/pelletier/go-toml/v2/unstable.

## Benchmarks

Execution time speedup compared to other Go TOML libraries:

<table>
	<thead>
		<tr><th>Benchmark</th><th>go-toml v1</th><th>BurntSushi/toml</th></tr>
	</thead>
	<tbody>
		<tr><td>Marshal/HugoFrontMatter-2</td><td>1.9x</td><td>2.2x</td></tr>
		<tr><td>Marshal/ReferenceFile/map-2</td><td>1.7x</td><td>2.1x</td></tr>
		<tr><td>Marshal/ReferenceFile/struct-2</td><td>2.2x</td><td>3.0x</td></tr>
		<tr><td>Unmarshal/HugoFrontMatter-2</td><td>2.9x</td><td>2.7x</td></tr>
		<tr><td>Unmarshal/ReferenceFile/map-2</td><td>2.6x</td><td>2.7x</td></tr>
		<tr><td>Unmarshal/ReferenceFile/struct-2</td><td>4.6x</td><td>5.1x</td></tr>
	 </tbody>
</table>
<details><summary>See more</summary>
<p>The table above has the results of the most common use-cases. The table below
contains the results of all benchmarks, including unrealistic ones. It is
provided for completeness.</p>

<table>
	<thead>
		<tr><th>Benchmark</th><th>go-toml v1</th><th>BurntSushi/toml</th></tr>
	</thead>
	<tbody>
		<tr><td>Marshal/SimpleDocument/map-2</td><td>1.8x</td><td>2.7x</td></tr>
		<tr><td>Marshal/SimpleDocument/struct-2</td><td>2.7x</td><td>3.8x</td></tr>
		<tr><td>Unmarshal/SimpleDocument/map-2</td><td>3.8x</td><td>3.0x</td></tr>
		<tr><td>Unmarshal/SimpleDocument/struct-2</td><td>5.6x</td><td>4.1x</td></tr>
		<tr><td>UnmarshalDataset/example-2</td><td>3.0x</td><td>3.2x</td></tr>
		<tr><td>UnmarshalDataset/code-2</td><td>2.3x</td><td>2.9x</td></tr>
		<tr><td>UnmarshalDataset/twitter-2</td><td>2.6x</td><td>2.7x</td></tr>
		<tr><td>UnmarshalDataset/citm_catalog-2</td><td>2.2x</td><td>2.3x</td></tr>
		<tr><td>UnmarshalDataset/canada-2</td><td>1.8x</td><td>1.5x</td></tr>
		<tr><td>UnmarshalDataset/config-2</td><td>4.1x</td><td>2.9x</td></tr>
		<tr><td>geomean</td><td>2.7x</td><td>2.8x</td></tr>
	 </tbody>
</table>
<p>This table can be generated with <code>./ci.sh benchmark -a -html</code>.</p>
</details>

## Modules

go-toml uses Go's standard modules system.

Installation instructions:

- Go ≥ 1.16: Nothing to do. Use the import in your code. The `go` command deals
  with it automatically.
- Go ≥ 1.13: `GO111MODULE=on go get github.com/pelletier/go-toml/v2`.

In case of trouble: [Go Modules FAQ][mod-faq].

[mod-faq]: https://github.com/golang/go/wiki/Modules#why-does-installing-a-tool-via-go-get-fail-with-error-cannot-find-main-module

## Tools

Go-toml provides three handy command line tools:

 * `tomljson`: Reads a TOML file and outputs its JSON representation.

	```
	$ go install github.com/pelletier/go-toml/v2/cmd/tomljson@latest
	$ tomljson --help
	```

 * `jsontoml`: Reads a JSON file and outputs a TOML representation.

	```
	$ go install github.com/pelletier/go-toml/v2/cmd/jsontoml@latest
	$ jsontoml --help
	```

 * `tomll`: Lints and reformats a TOML file.

	```
	$ go install github.com/pelletier/go-toml/v2/cmd/tomll@latest
	$ tomll --help
	```

### Docker image

Those tools are also available as a [Docker image][docker]. For example, to use
`tomljson`:

```
docker run -i ghcr.io/pelletier/go-toml:v2 tomljson < example.toml
```

Multiple versions are available on [ghcr.io][docker].

[docker]: https://github.com/pelletier/go-toml/pkgs/container/go-toml

## Migrating from v1

This section describes the differences between v1 and v2, with some pointers on
how to get the original behavior when possible.

### Decoding / Unmarshal

#### Automatic field name guessing

When unmarshaling to a struct, if a key in the TOML document does not exactly
match the name of a struct field or any of the `toml`-tagged field, v1 tries
multiple variations of the key ([code][v1-keys]).

V2 instead does a case-insensitive matching, like `encoding/json`.

This could impact you if you are relying on casing to differentiate two fields,
and one of them is a not using the `toml` struct tag. The recommended solution
is to be specific about tag names for those fields using the `toml` struct tag.

[v1-keys]: https://github.com/pelletier/go-toml/blob/a2e52561804c6cd9392ebf0048ca64fe4af67a43/marshal.go#L775-L781

#### Ignore preexisting value in interface

When decoding into a non-nil `interface{}`, go-toml v1 uses the type of the
element in the interface to decode the object. For example:

```go
type inner struct {
	B interface{}
}
type doc struct {
	A interface{}
}

d := doc{
	A: inner{
		B: "Before",
	},
}

data := `
[A]
B = "After"
`

toml.Unmarshal([]byte(data), &d)
fmt.Printf("toml v1: %#v\n", d)

// toml v1: main.doc{A:main.inner{B:"After"}}
```

In this case, field `A` is of type `interface{}`, containing a `inner` struct.
V1 sees that type and uses it when decoding the object.

When decoding an object into an `interface{}`, V2 instead disregards whatever
value the `interface{}` may contain and replaces it with a
`map[string]interface{}`. With the same data structure as above, here is what
the result looks like:

```go
toml.Unmarshal([]byte(data), &d)
fmt.Printf("toml v2: %#v\n", d)

// toml v2: main.doc{A:map[string]interface {}{"B":"After"}}
```

This is to match `encoding/json`'s behavior. There is no way to make the v2
decoder behave like v1.

#### Values out of array bounds ignored

When decoding into an array, v1 returns an error when the number of elements
contained in the doc is superior to the capacity of the array. For example:

```go
type doc struct {
	A [2]string
}
d := doc{}
err := toml.Unmarshal([]byte(`A = ["one", "two", "many"]`), &d)
fmt.Println(err)

// (1, 1): unmarshal: TOML array length (3) exceeds destination array length (2)
```

In the same situation, v2 ignores the last value:

```go
err := toml.Unmarshal([]byte(`A = ["one", "two", "many"]`), &d)
fmt.Println("err:", err, "d:", d)
// err: <nil> d: {[one two]}
```

This is to match `encoding/json`'s behavior. There is no way to make the v2
decoder behave like v1.

#### Support for `toml.Unmarshaler` has been dropped

This method was not widely used, poorly defined, and added a lot of complexity.
A similar effect can be achieved by implementing the `encoding.TextUnmarshaler`
interface and use strings.

#### Support for `default` struct tag has been dropped

This feature adds complexity and a poorly defined API for an effect that can be
accomplished outside of the library.

It does not seem like other format parsers in Go support that feature (the
project referenced in the original ticket #202 has not been updated since 2017).
Given that go-toml v2 should not touch values not in the document, the same
effect can be achieved by pre-filling the struct with defaults (libraries like
[go-defaults][go-defaults] can help). Also, string representation is not well
defined for all types: it creates issues like #278.

The recommended replacement is pre-filling the struct before unmarshaling.

[go-defaults]: https://github.com/mcuadros/go-defaults

#### `toml.Tree` replacement

This structure was the initial attempt at providing a document model for
go-toml. It allows manipulating the structure of any document, encoding and
decoding from their TOML representation. While a more robust feature was
initially planned in go-toml v2, this has been ultimately [removed from
scope][nodoc] of this library, with no plan to add it back at the moment. The
closest equivalent at the moment would be to unmarshal into an `interface{}` and
use type assertions and/or reflection to manipulate the arbitrary
structure. However this would fall short of providing all of the TOML features
such as adding comments and be specific about whitespace.


#### `toml.Position` are not retrievable anymore

The API for retrieving the position (line, column) of a specific TOML element do
not exist anymore. This was done to minimize the amount of concepts introduced
by the library (query path), and avoid the performance hit related to storing
positions in the absence of a document model, for a feature that seemed to have
little use. Errors however have gained more detailed position
information. Position retrieval seems better fitted for a document model, which
has been [removed from the scope][nodoc] of go-toml v2 at the moment.

### Encoding / Marshal

#### Default struct fields order

V1 emits struct fields order alphabetically by default. V2 struct fields are
emitted in order they are defined. For example:

```go
type S struct {
	B string
	A string
}

data := S{
	B: "B",
	A: "A",
}

b, _ := tomlv1.Marshal(data)
fmt.Println("v1:\n" + string(b))

b, _ = tomlv2.Marshal(data)
fmt.Println("v2:\n" + string(b))

// Output:
// v1:
// A = "A"
// B = "B"

// v2:
// B = 'B'
// A = 'A'
```

There is no way to make v2 encoder behave like v1. A workaround could be to
manually sort the fields alphabetically in the struct definition, or generate
struct types using `reflect.StructOf`.

#### No indentation by default

V1 automatically indents content of tables by default. V2 does not. However the
same behavior can be obtained using [`Encoder.SetIndentTables`][sit]. For example:

```go
data := map[string]interface{}{
	"table": map[string]string{
		"key": "value",
	},
}

b, _ := tomlv1.Marshal(data)
fmt.Println("v1:\n" + string(b))

b, _ = tomlv2.Marshal(data)
fmt.Println("v2:\n" + string(b))

buf := bytes.Buffer{}
enc := tomlv2.NewEncoder(&buf)
enc.SetIndentTables(true)
enc.Encode(data)
fmt.Println("v2 Encoder:\n" + string(buf.Bytes()))

// Output:
// v1:
//
// [table]
//   key = "value"
//
// v2:
// [table]
// key = 'value'
//
//
// v2 Encoder:
// [table]
//   key = 'value'
```

[sit]: https://pkg.go.dev/github.com/pelletier/go-toml/v2#Encoder.SetIndentTables

#### Keys and strings are single quoted

V1 always uses double quotes (`"`) around strings and keys that cannot be
represented bare (unquoted). V2 uses single quotes instead by default (`'`),
unless a character cannot be represented, then falls back to double quotes. As a
result of this change, `Encoder.QuoteMapKeys` has been removed, as it is not
useful anymore.

There is no way to make v2 encoder behave like v1.

#### `TextMarshaler` emits as a string, not TOML

Types that implement [`encoding.TextMarshaler`][tm] can emit arbitrary TOML in
v1. The encoder would append the result to the output directly. In v2 the result
is wrapped in a string. As a result, this interface cannot be implemented by the
root object.

There is no way to make v2 encoder behave like v1.

[tm]: https://golang.org/pkg/encoding/#TextMarshaler

#### `Encoder.CompactComments` has been removed

Emitting compact comments is now the default behavior of go-toml. This option
is not necessary anymore.

#### Struct tags have been merged

V1 used to provide multiple struct tags: `comment`, `commented`, `multiline`,
`toml`, and `omitempty`. To behave more like the standard library, v2 has merged
`toml`, `multiline`, `commented`, and `omitempty`. For example:

```go
type doc struct {
	// v1
	F string `toml:"field" multiline:"true" omitempty:"true" commented:"true"`
	// v2
	F string `toml:"field,multiline,omitempty,commented"`
}
```

Has a result, the `Encoder.SetTag*` methods have been removed, as there is just
one tag now.

#### `Encoder.ArraysWithOneElementPerLine` has been renamed

The new name is `Encoder.SetArraysMultiline`. The behavior should be the same.

#### `Encoder.Indentation` has been renamed

The new name is `Encoder.SetIndentSymbol`. The behavior should be the same.


#### Embedded structs behave like stdlib

V1 defaults to merging embedded struct fields into the embedding struct. This
behavior was unexpected because it does not follow the standard library. To
avoid breaking backward compatibility, the `Encoder.PromoteAnonymous` method was
added to make the encoder behave correctly. Given backward compatibility is not
a problem anymore, v2 does the right thing by default: it follows the behavior
of `encoding/json`. `Encoder.PromoteAnonymous` has been removed.

[nodoc]: https://github.com/pelletier/go-toml/discussions/506#discussioncomment-1526038

### `query`

go-toml v1 provided the [`go-toml/query`][query] package. It allowed to run
JSONPath-style queries on TOML files. This feature is not available in v2. For a
replacement, check out [dasel][dasel].

This package has been removed because it was essentially not supported anymore
(last commit May 2020), increased the complexity of the code base, and more
complete solutions exist out there.

[query]: https://github.com/pelletier/go-toml/tree/f99d6bbca119636aeafcf351ee52b3d202782627/query
[dasel]: https://github.com/TomWright/dasel

## Versioning

Expect for parts explicitely marked otherwise, go-toml follows [Semantic
Versioning](https://semver.org). The supported version of
[TOML](https://github.com/toml-lang/toml) is indicated at the beginning of this
document. The last two major versions of Go are supported (see [Go Release
Policy](https://golang.org/doc/devel/release.html#policy)).

## License

The MIT License (MIT). Read [LICENSE](LICENSE).