libero - v1.0.1

Libero

Libero is the defensive specialist in volleyball, who handles passes and digs so the rest of the team can focus on hits. This library handles the wire plumbing (serialization, dispatch, error envelopes, panic recovery) so your app can focus on its domain logic.

Wiring a Gleam server to a Lustre SPA usually means, for every interaction: define a REST route, write a JSON encoder for the request, write a JSON decoder for the response, write a fetch wrapper on the client, and keep the type definitions on both sides in sync by hand. Every new endpoint is the same boilerplate, and every mistake (typo, missing field, drifted shape) waits for runtime to bite.

Libero replaces the whole loop. You write a normal server function, annotate it with /// @rpc, and call it from the client as if it were a local function. Routes, encoders, decoders, dispatch, and error envelopes are all generated from the function’s signature, and the compiler catches drift between the two sides at build time. Server and client talk to each other over WebSocket.

Quick tour

The server function

// server/src/server/records.gleam

import shared/record.{type Record, type SaveError}

/// @rpc
pub fn save(
  name name: String,
  email email: String,
) -> Result(Record, SaveError) {
  // ... persist the record, return a Record or a SaveError ...
}

Labelled parameters are wire-exposed, so the client stub takes them. The return type can be a bare T or Result(T, E), and both shapes are handled.

The client call

import client/generated/libero/rpc/records as rpc_records
import shared/record.{type Record, type SaveError}
import libero/error.{type RpcError, AppError, InternalError, MalformedRequest, UnknownFunction}

pub type Msg {
  RecordSaved(Result(Record, RpcError(SaveError)))
  // ...
}

// In your update:
FormSubmitted -> #(
  Model(..model, saving: True),
  rpc_records.save(
    name: model.form.name,
    email: model.form.email,
    on_response: RecordSaved,
  ),
)

RecordSaved(Ok(record)) -> { /* merge into list, clear form */ }
RecordSaved(Error(AppError(DuplicateEmail))) -> { /* show form error */ }
RecordSaved(Error(InternalError(trace_id))) -> { /* show generic error */ }
RecordSaved(Error(_)) -> { /* framework fallthrough */ }

The compiler statically checks that you handle every RpcError variant.

Install

In your server package:

gleam add libero

And the same in your client package. Libero is cross-target (Erlang + JavaScript), so server and client both depend on it.

Initial setup

The two snippets above are the day-to-day surface of libero. Everything in this section is one-time setup: shared state injection, the generated dispatch files, and the WebSocket handler that connects them.

Skip to a working example. The examples/fizzbuzz/ directory is a complete, runnable libero app with four RPC functions, a Session, an @inject function, and a Lustre client. Every file is annotated with whether it’s SETUP (write once) or DAY-TO-DAY (where you add features), so you can copy it as a starting point and replace the fizzbuzz logic with your own. The walkthrough below covers the same pieces in prose.

The inject function

If your RPC functions need shared state (a database connection, an authenticated user, a tenant ID), declare a /// @inject function for each value.

// server/src/server/rpc_inject.gleam

import server/session.{type Session}
import sqlight

/// @inject
pub fn conn(session: Session) -> sqlight.Connection {
  session.db
}

Then add a matching labelled parameter to any @rpc function that needs it:

/// @rpc
pub fn save(
  conn conn: sqlight.Connection,
  name name: String,
  email email: String,
) -> Result(Record, SaveError) {
  // ... use conn to persist ...
}

The first labelled parameter whose label matches an @inject function’s name gets injected at dispatch time. Inject fns take your Session type as input. The Session type is inferred from the first inject fn found, and all inject fns in a namespace must share the same Session type.

If you have zero inject fns, libero uses Session = Nil and your WebSocket handler passes Nil to the dispatch entry point.

The generated dispatch

After gleam run -m libero -- --ws-url=wss://your.host/ws/rpc, libero writes:

server/src/server/generated/libero/rpc_dispatch.gleam    # pub fn handle(session:, text:)
client/src/client/generated/libero/rpc/records.gleam     # pub fn save(..., on_response:)
client/src/client/generated/libero/rpc_config.gleam      # pub const ws_url

Your WebSocket handler

Hand-written, stays tiny:

// server/src/server/websocket.gleam

import libero/error.{type PanicInfo, PanicInfo}
import server/generated/libero/rpc_dispatch

pub fn handle_message(state, message, conn) {
  case message {
    mist.Text(text) -> {
      let #(response, maybe_panic) =
        rpc_dispatch.handle(session: state.session, text: text)
      log_panic(maybe_panic)
      let _ = mist.send_text_frame(conn, response)
      mist.continue(state)
    }
    _ -> mist.continue(state)
  }
}

handle returns both the wire response and an Option(PanicInfo). If a server fn panicked, PanicInfo carries the trace id, function name, and stringified reason. Route that to wisp.log_error, Sentry, Datadog, or wherever you want. Libero itself has no logging dependency.

CLI flags

Libero’s generator is driven by three flags:

All other paths are derived by convention.

Without a namespace:

With --namespace=admin:

Invoke from your server package directory:

cd server
gleam run -m libero -- --ws-url=wss://your.host/ws/rpc

Multi-SPA

When your project has multiple SPAs sharing a server (admin + public, say), invoke libero once per namespace:

gleam run -m libero -- --ws-url=wss://your.host/admin/ws/rpc --namespace=admin
gleam run -m libero -- --ws-url=wss://your.host/public/ws/rpc --namespace=public

Each namespace scans src/server/<ns>/** recursively and gets its own /// @inject functions, Session type, and handle_<ns> entry point, so admin can carry User + DB while public carries CartCookie. Wire names are prefixed with the namespace, so admin.items.save is distinct from public.items.save even if the function names collide. Your server router mounts one WebSocket endpoint per namespace and calls the matching dispatch.

Error envelope

Every RPC response follows this shape:

pub type RpcError(e) {
  /// Server function returned a Result(T, E) with Error(value).
  /// Only present when the function's return type is Result(T, E).
  AppError(e)

  /// The server couldn't decode the incoming envelope. Usually a
  /// client-side bug or deployment skew.
  MalformedRequest

  /// The wire `fn` name doesn't match any dispatch case. Usually
  /// deployment skew (client built against a newer server).
  UnknownFunction(name: String)

  /// Server function panicked. The real panic is logged server-side
  /// under this opaque trace_id.
  InternalError(trace_id: String)
}

Bare-return functions are exposed as Result(T, RpcError(Never)). Never is an uninhabited type, so the AppError(_) arm is statically unreachable and you can omit it from your pattern match. Functions that return Result(T, E) use RpcError(E) and require the full match.

Wire format

The wire format is reflective. Custom types, tuples, Options, Results, and primitives all serialize and rebuild automatically without Decoders or encode functions. The on-wire shape is {"fn": "...", "args": [...]} for the call and {"@": "ok", "v": [...]} for the response: simple enough to tcpdump and read.

Runnable example

See examples/fizzbuzz/ for a self-contained FizzBuzz calculator with three RPC functions that exercise different parts of the generator:

Run it:

cd examples/fizzbuzz
./bin/dev

Then open http://localhost:4000.

License

MIT. See LICENSE.

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