interpres/README.md

# interpres

A TOML 1.0 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: 185 valid
and 371 invalid cases, zero failures.

## Features

- **Full TOML 1.0**: bare, quoted and dotted keys; tables and arrays of tables;
  basic and literal strings including multiline; integers in the four radixes
  with `_` separators; floats with exponents, `inf` and `nan`; booleans; the
  four date-time kinds; arrays and inline tables.
- **Decoding and encoding**: `Parse` for an untyped tree, `Unmarshal` and
  `Marshal` for structs and maps, mirroring `encoding/json`.
- **Strict decoding**: `NewDecoder().DisallowUnknownFields()` 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.
- **Cancellation**: every entry point has a `*Context` sibling that honours a
  `context.Context`.
- **Configurable emission**: `Encoder` options for declaration-order output,
  omitting empty arrays, and literal multiline strings.

## Install

As a library:

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

Requires Go 1.27.0 or newer. The module imports only the standard library.

## 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 `Encoder`
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 the untyped
`map[string]any` tree instead, 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.NewDecoder().
	DisallowUnknownFields().
	Decode(data, &cfg)
```

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.

### Encoder options

```go
out, err := interpres.NewEncoder().
	GroupByKind(false).      // preserve declaration order
	OmitEmptyArrays().       // skip empty scalar arrays
	UseLiteralMultiline(80). // long multi-line strings as literal blocks
	Marshal(cfg)
```

### Cancellation

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

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

`ParseContext`, `UnmarshalContext`, `(*Decoder).DecodeContext` and
`(*Encoder).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 toml-test adapter

## Licence

MIT. See [LICENSE](LICENSE).

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