# hocon-parser — HOCON Parser for Rust
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[](https://docs.rs/hocon-parser)
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[](https://codecov.io/gh/o3co/rs.hocon)
[](LICENSE)
Full [Lightbend HOCON specification](https://github.com/lightbend/config/blob/main/HOCON.md)-compliant
parser for Rust. Hand-written lexer, recursive-descent parser, and a typed `Config` API
with optional Serde integration.
[日本語](README.ja.md)
## Quick Start
### 1. Install
```sh
cargo add hocon-parser
```
To enable Serde support:
```sh
cargo add hocon-parser --features serde
```
### 2. Use
```rust
use hocon;
fn main() -> Result<(), Box<dyn std::error::Error>> {
let config = hocon::parse(r#"
server {
host = "localhost"
port = 8080
}
database {
url = "jdbc:postgresql://localhost/mydb"
pool-size = 10
}
"#)?;
let host = config.get_string("server.host")?;
let port = config.get_i64("server.port")?;
println!("Server: {}:{}", host, port);
Ok(())
}
```
## Why HOCON?
| Comments | No | No | Yes | Yes |
| Nesting | No | Yes | Yes | Yes |
| References / Substitution | No | No | No | Yes (`${var}`) |
| File inclusion | No | No | No | Yes (`include`) |
| Object merging | No | No | Anchors (fragile) | Yes (deep merge) |
| Optional values | No | No | No | Yes (`${?var}`) |
| Trailing commas | N/A | No | N/A | Yes |
| Unquoted strings | Yes | No | Yes | Yes |
HOCON gives you the readability of YAML, the structure of JSON, and features that neither has — substitutions, includes, and deep merge. If your config is more than a few flat key-value pairs, HOCON is worth considering.
## Features
- Complete HOCON syntax: objects, arrays, comments, multi-line strings, unquoted strings
- Substitutions (`${foo}`, `${?foo}`) with cycle detection
- `include` directives (file, classpath, URL) with relative path resolution
- Object merging and array concatenation per spec
- String, array, and object value concatenation
- Duration and byte-size parsing (`10 seconds`, `512 MB`)
- Environment variable substitution (`${HOME}`)
- Dot-separated path expressions (`server.host`)
- Fallback configuration merging (`with_fallback`)
- Optional Serde deserialization support
- Passes Lightbend equivalence tests (equiv01 through equiv05)
## API Reference
### Parsing
```rust
// Parse a HOCON string
let config = hocon::parse(input)?;
// Parse a HOCON file (resolves include directives relative to file location)
let config = hocon::parse_file("application.conf")?;
// Parse with custom environment variables
use std::collections::HashMap;
let env: HashMap<String, String> = HashMap::new();
let config = hocon::parse_with_env(input, &env)?;
let config = hocon::parse_file_with_env("application.conf", &env)?;
```
### Typed Getters
All typed getters return `Result<T, ConfigError>`. Paths use dot notation.
```rust
let host: String = config.get_string("server.host")?;
let port: i64 = config.get_i64("server.port")?;
let rate: f64 = config.get_f64("rate")?;
let debug: bool = config.get_bool("debug")?; // also accepts "yes"/"no", "on"/"off"
let sub: Config = config.get_config("database")?; // sub-object as Config
let items: Vec<HoconValue> = config.get_list("items")?;
```
### Option Variants
Return `Option<T>` instead of `Result` -- return `None` for missing keys or type mismatches.
```rust
let host: Option<String> = config.get_string_option("server.host");
let port: Option<i64> = config.get_i64_option("server.port");
let rate: Option<f64> = config.get_f64_option("rate");
let debug: Option<bool> = config.get_bool_option("debug");
```
### Duration and Byte-Size Values
```rust
use std::time::Duration;
// Supports: ns, us, ms, s/seconds, m/minutes, h/hours, d/days
let timeout: Duration = config.get_duration("server.timeout")?;
// Supports: B, KB, KiB, MB, MiB, GB, GiB, TB, TiB (and long forms)
let max_size: i64 = config.get_bytes("upload.max-size")?;
```
### Inspection
```rust
let exists: bool = config.has("server.host");
let keys: Vec<&str> = config.keys(); // top-level keys in insertion order
let raw: Option<&HoconValue> = config.get("server.host");
```
### Fallback Merge
```rust
// Receiver wins; fallback fills missing keys. Objects are deep-merged.
let merged = app_config.with_fallback(&defaults);
```
### Serde Deserialization
Requires the `serde` feature.
```rust
use serde::Deserialize;
#[derive(Deserialize)]
struct ServerConfig {
host: String,
port: u16,
}
let config = hocon::parse(input)?;
let server: ServerConfig = config
.get_config("server")?
.deserialize()?;
```
## Error Types
| `ParseError` | Syntax errors during lexing/parsing (includes line and column) |
| `ResolveError` | Substitution failures, cyclic references, missing required variables |
| `ConfigError` | Missing keys or type mismatches during value access |
| `DeserializeError` | Serde deserialization failures (with `serde` feature) |
## HOCON Examples
```hocon
# Comments start with // or #
server {
host = "0.0.0.0"
port = 8080
timeout = 30 seconds
max-upload = 512 MB
}
# Substitutions
app {
name = "my-app"
title = "Welcome to "${app.name}
}
# Array concatenation
base-tags = ["production"]
tags = ${base-tags} ["v2"]
# Include other files
include "defaults.conf"
# Unquoted strings
path = /usr/local/bin
# Multi-line strings
description = """
This is a multi-line
string value.
"""
# Object merging
defaults { color = "blue", size = 10 }
defaults { size = 20 } # merges: color stays, size updated
```
## Performance
Measured with [Criterion](https://github.com/bheisler/criterion.rs). Each iteration includes parsing and a `get_string` lookup. Run `cargo bench` to reproduce.
| Small config (10 keys) | ~62,000 | ~16 µs |
| Medium config (100 keys) | ~19,000 | ~52 µs |
| Large config (1,000 keys) | ~2,400 | ~408 µs |
| 10 substitutions | ~37,000 | ~27 µs |
| 50 substitutions | ~12,000 | ~86 µs |
| 100 substitutions | ~6,400 | ~156 µs |
| Depth 5 nesting | ~58,000 | ~17 µs |
| Depth 10 nesting | ~50,000 | ~20 µs |
| Depth 20 nesting | ~39,000 | ~26 µs |
For typical application configs (loaded once at startup), the parsing cost is negligible — even a 1,000-key config parses in under 0.5 ms.
## Comparison
✅ Full support / ⚠️ Partial / ❌ Not supported
### HOCON Implementation
| Substitutions (`${path}`) | ✅ | ✅ |
| Optional substitutions (`${?path}`) | ✅ | ✅ |
| Include | ✅ | ✅ |
| `include required(file(...))` | ✅ | ❌ |
| Object/Array concatenation | ✅ | ✅ |
| Type coercion | ✅ | ⚠️ |
| Duration parsing | ✅ | ✅ |
| Byte size parsing | ✅ | ✅ |
| `+=` append | ✅ | ❌ |
| Serde deserialization | ✅ | ✅ |
| Env variable fallback | ✅ | ❌ |
| Circular include detection | ✅ | ❌ |
### Config Framework
| **Formats** | | |
| HOCON | ✅ | ❌ |
| JSON | ✅ | ✅ |
| YAML | ❌ | ✅ |
| TOML | ❌ | ✅ |
| Env vars | ✅ (fallback) | ✅ |
| .properties | ✅ (via include) | ❌ |
| **Features** | | |
| Substitutions | ✅ | ❌ |
| File includes | ✅ | ❌ |
| Type coercion | ✅ | ✅ |
| Serde support | ✅ | ✅ |
| Watch/reload | ❌ | ❌ |
| Layered config | ❌ | ✅ |
## Spec Compliance
This library targets full compliance with the
[Lightbend HOCON specification](https://github.com/lightbend/config/blob/main/HOCON.md).
The test suite includes the Lightbend equivalence tests (equiv01 through equiv05),
verifying correct handling of object merging, array concatenation, substitutions,
and all other spec-defined behaviors.
## Minimum Supported Rust Version
The MSRV is **1.82**.
## Related Projects
| [ts.hocon](https://github.com/o3co/ts.hocon) | TypeScript | [npm](https://www.npmjs.com/package/@o3co/ts.hocon) | HOCON parser for TypeScript/Node.js |
| [go.hocon](https://github.com/o3co/go.hocon) | Go | [pkg.go.dev](https://pkg.go.dev/github.com/o3co/go.hocon) | HOCON parser for Go |
| [hocon2](https://github.com/o3co/hocon2) | Go | [pkg.go.dev](https://pkg.go.dev/github.com/o3co/hocon2) | HOCON → JSON/YAML/TOML/Properties CLI |
All implementations are full Lightbend HOCON spec compliant.
## Best Practices
### Config Structure
- **Split by domain**: Separate configuration into logical units (`database.conf`, `server.conf`, `logging.conf`)
- **Use `include` for composition**: Compose a full config from domain-specific files
- **Avoid logic in config**: HOCON is for declarative data, not conditionals or computation
### Environment Variables
- **Minimize `${ENV}` usage**: Prefer `${?ENV}` (optional) with sensible defaults defined in the config itself
- **Never require env vars for local development**: Defaults should work out of the box
- **Document required env vars**: List them in your project's README or a `.env.example`
### Dev / Prod Separation
```text
config/
├── application.conf # shared defaults
├── dev.conf # include "application.conf" + dev overrides
└── prod.conf # include "application.conf" + prod overrides
```
### Validation
- Always validate config at application startup, not at point-of-use
- Use schema validation (Zod for TypeScript, struct unmarshaling for Go, Serde for Rust) to catch errors early
```rust
use serde::Deserialize;
#[derive(Deserialize)]
struct ServerConfig {
host: String,
port: u16,
}
#[derive(Deserialize)]
struct AppConfig {
server: ServerConfig,
debug: bool,
}
// requires the `serde` feature
let cfg: AppConfig = config.deserialize()?; // fails fast on startup
```
## Known Limitations
- **`include url(...)`** is not supported. Fetching remote configuration is outside the scope of this parser. Use your application's HTTP client to fetch the content, then pass it to `parse()`.
- **`include classpath(...)`** is not supported. This is a JVM-specific include form with no equivalent outside Java runtimes.
- **No watch/reload** — the library parses config at load time. For live-reloading, call `parse()` / `parse_file()` again on change.
- **No streaming parser** — the entire input is loaded into memory.
- **`.properties` include** — supports basic `key=value` syntax. Does not support multiline values (backslash continuation), unicode escapes, or key escaping from the full Java .properties specification.
For full API documentation, see [docs.rs](https://docs.rs/hocon-parser) (available after crate publication).
## Security Considerations
When parsing untrusted HOCON input, be aware of:
- **Path traversal in includes:** `include "../../../etc/passwd"` will resolve relative to `base_dir`. Validate include paths if parsing untrusted input.
- **Input size:** The parser has no built-in input size limit. For untrusted input, validate size before calling `parse()`.
## License
Licensed under the [Apache License, Version 2.0](LICENSE).
## Attribution
Designed and built end-to-end with [Claude Code](https://claude.ai/claude-code).
Reviewed by [GitHub Copilot](https://github.com/features/copilot) and [OpenAI Codex](https://openai.com/index/openai-codex/).