pklr

A pure Rust parser and evaluator for Apple's Pkl configuration language. No external binary or CLI required.

Features

• Lexer, parser, and evaluator written entirely in Rust
• Evaluates .pkl files to serde_json::Value
• Import and amends resolution for local files
• Glob imports, as import* "dir/*.pkl" as Mods declarations and import*("dir/*.pkl") expressions
• Persistent caching, cache preloading, and offline evaluation for package:// imports
• Concurrent prefetching of remote http(s):// and package:// imports
• String interpolation, lambdas, higher-order methods
• Rich error diagnostics via miette

Usage

use pklr::eval_to_json;

let json = eval_to_json(std::path::Path::new("config.pkl"))?;
println!("{}", json);

The API is synchronous. A default build uses the standard library for files and ureq for HTTP, and creates no Tokio runtime. Use EvaluatorBuilder::http_agent with a custom pklr::ureq::Agent for proxy, certificate, or timeout configuration. Applications that already configure reqwest can instead select the reqwest feature and pass that client to EvaluatorBuilder::http_client.

Before evaluating a module, pklr prefetches its remote http(s):// and package:// imports, and theirs in turn, fetching each level in one batch. The native capabilities run up to eight requests at once. Prefetching only fills caches: a failed prefetch is retried when evaluation needs the module, so results and errors are the same as fetching one module at a time.

Cargo features

An embedder with its own IO can depend on the evaluator alone:

pklr = { version = "5", default-features = false, features = ["eval-core"] }

let mut evaluator = pklr::Evaluator::with_capabilities(MyCapabilities::new());
let value = evaluator.eval_file(std::path::Path::new("config.pkl"))?;

EvalCapabilities methods are synchronous. A host with asynchronous IO can run the evaluator on a blocking thread and block on its own futures inside them. Override fetch_text_many and fetch_bytes_many to fetch prefetch batches concurrently; the defaults fetch one URL after another. Both get a FetchBudget (prefetching allows 64 MiB per evaluation) to charge response bytes to: start no request once it is spent, and drop a body that no longer fits.

Reqwest and optional async support

To use a configured reqwest client without linking ureq, select reqwest:

pklr = { version = "5", default-features = false, features = ["reqwest", "package-zip"] }

let client = pklr::reqwest::Client::builder()
    .proxy(pklr::reqwest::Proxy::all("http://proxy.internal:8080")?)
    .build()?;
let json = pklr::EvaluatorBuilder::new()
    .http_client(client)
    .eval_to_json(std::path::Path::new("config.pkl"))?;

The reqwest feature enables synchronous evaluation with the supplied client; the default http behavior is unchanged. Select async when you also need the async evaluation entry points. It remains additive for compatibility.

The async feature fetches prefetch batches concurrently on Tokio:

pklr = { version = "5", features = ["async"] }

let json = pklr::EvaluatorBuilder::new()
    .http_client(pklr::reqwest::Client::new())
    .eval_to_json_async(std::path::Path::new("config.pkl"))
    .await?;

eval_async, eval_to_json_async and the free pklr::eval_to_json_async run the synchronous evaluation on Tokio's blocking thread pool. With a reqwest client, requests run on the caller's runtime under block_in_place when called from a multi-threaded runtime, and on a private runtime otherwise, so the synchronous methods also work with a reqwest client.

Package downloads can be shared across evaluator instances and reused without network access:

fn load_config() -> pklr::Result<serde_json::Value> {
    pklr::EvaluatorBuilder::new()
        .package_cache_dir(".pklr-cache")
        .offline(true)
        .eval_to_json(std::path::Path::new("config.pkl"))
}

A host that already ships a copy of a package can seed the cache with it, so a config importing that package evaluates without any network round trip:

static PACKAGE: &[u8] = include_bytes!("pkg@1.0.0.zip");

fn load_bundled_config() -> pklr::Result<serde_json::Value> {
    pklr::EvaluatorBuilder::new()
        .package_cache_dir(".pklr-cache")
        .preload_package("https://example.com/pkg@1.0.0.zip", "zip", PACKAGE)
        .eval_to_json(std::path::Path::new("config.pkl"))
}

Cached content already on disk wins, so preloading never overrides a package fetched from the network, and a config pinning a different version still resolves that version normally.

License

MIT