# interpres

A TOML 1.1 parser and encoder for Go, written with the standard library
alone. `interpres` (Latin for *interpreter*) gives zero-dependency
programs an `encoding/json`-style API for reading and writing TOML, and passes
the entire official [toml-test](https://github.com/toml-lang/toml-test) suite:
214 valid, 467 invalid and 214 encoder cases, zero failures.

## Features

- **Full TOML 1.1**: bare, quoted and dotted keys; tables and arrays of
  tables; basic and literal strings including multiline, with the 1.1 `\e` and
  `\xHH` escapes; integers in the four radixes with `_` separators; floats with
  exponents, `inf` and `nan`; booleans; the four date-time kinds, seconds
  optional as of 1.1; arrays and inline tables, multi-line as of 1.1.
- **Decoding and encoding**: `Parse` for an untyped tree, `Unmarshal` and
  `Marshal` for structs and maps, mirroring `encoding/json`.
- **Strict decoding**: `RejectUnknownFields(true)` rejects keys that
  match no destination field, at every struct depth.
- **Custom types**: `Marshaler` and `Unmarshaler` let a type control its own
  TOML representation in both directions, and `encoding.TextMarshaler` and
  `TextUnmarshaler` are honoured by default, so `net.IP`, `time.Duration` and
  user types with text methods need no configuration.
- **Cancellation**: every entry point has a `*Context` sibling that honours a
  `context.Context`.
- **Ordered documents**: `Parse` gives a `*Document` that keeps the key order,
tells an inline table from a header one, and carries the comments; `ParseMap`
gives the plain `map[string]any` tree.
- **Configurable emission**: `Marshal` options for declaration-order output,
  omitting empty arrays, literal multiline strings, and inlining small
  sub-tables.

## Install

As a library:

```sh
go get sourcedock.dev/petrbalvin/interpres/v2
```

Requires Go 1.27.1 or newer. The module imports only the standard library.

The module is public and resolves through proxy.golang.org and sum.golang.org
like any other; no GOPROXY or GOPRIVATE setup is needed to fetch it. A machine
that sets `GOPRIVATE=sourcedock.dev` fetches directly from the forge instead,
which skips the proxy and the checksum database.

## Quick start

```sh
git clone https://sourcedock.dev/petrbalvin/interpres.git
cd interpres
just example
```

`just example` runs the tour in `examples/basic`: it decodes an embedded
document into a struct, prints it, and re-encodes it under both layouts.

## Usage

### Decode into a struct

```go
type Config struct {
	Title  string `toml:"title"`
	Server struct {
		Host string `toml:"host"`
		Port int    `toml:"port"`
	} `toml:"server"`
}

var cfg Config
err := interpres.Unmarshal(data, &cfg)
```

Fields match by the `toml:"name"` tag, or by the lower-cased field name when no
tag is present; `toml:"-"` skips a field. `Parse` returns a `*Document` that
also carries the key order and the comments, `ParseMap` returns the plain
`map[string]any` tree, and `UnmarshalContext` accepts a context.

### Encode from a struct

```go
out, err := interpres.Marshal(cfg)
```

writes:

```toml
title = "example"

[server]
host = "127.0.0.1"
port = 9090
```

Tables are laid out scalars first, then sub-tables, then arrays of tables,
which is the layout that re-parses to the same tree.

### Strict decoding

```go
err := interpres.Unmarshal(data, &cfg, interpres.RejectUnknownFields(true))
```

A key with no matching field becomes an error instead of a silent drop.

### Custom types

```go
type Port int

func (p Port) MarshalTOML() (any, error) {
	return int64(p), nil
}

type IP struct{ net.IP }

func (ip *IP) UnmarshalTOML(data any) error {
	s, ok := data.(string)
	if !ok {
		return fmt.Errorf("ip: not a string")
	}
	ip.IP = net.ParseIP(s)
	return nil
}
```

The value `MarshalTOML` returns is encoded in place of the receiver;
`UnmarshalTOML` receives the parsed value verbatim.

### Options

```go
out, err := interpres.Marshal(cfg,
	interpres.Layout(interpres.LayoutKindDeclaration), // preserve declaration order
	interpres.OmitEmptyArrays(true),       // skip empty scalar arrays
	interpres.LiteralMultiline(80),        // long multi-line strings as literal blocks
)
```

The decode and encode calls take variadic options, the shape
encoding/json/v2 uses for its own. `UnmarshalRead(r, v, opts...)` and
`MarshalWrite(w, v, opts...)` are the streaming forms.

### Cancellation

```go
ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
defer cancel()

out, err := interpres.MarshalContext(ctx, cfg)
```

`ParseContext`, `UnmarshalContext` and `MarshalContext` follow the same pattern.

The full rules for field matching, numeric conversion and emission live in
[docs/API.md](docs/API.md).

## Development

```sh
just build       # compile bin/interpres-decode
just test        # the suite, with the 80 percent coverage floor
just toml-test   # the official compliance suite, needs toml-test on PATH
just fmt         # gofmt in place
```

See [docs/DEVELOPMENT.md](docs/DEVELOPMENT.md) for the full workflow, and
[CONTRIBUTING.md](CONTRIBUTING.md) for how to contribute.

## Documentation

- [docs/ARCHITECTURE.md](docs/ARCHITECTURE.md): components and data flow
- [docs/API.md](docs/API.md): the API reference, decoding and encoding rules
- [docs/CLI.md](docs/CLI.md): the interpres-decode adapter and validator

## Licence

MIT. See [LICENSE](LICENSE).

Copyright © 2026 [Petr Balvín](https://petrbalvin.org)