Sky

sky-lang.org · Docs · Examples

Status: v0.17.x release candidate. Public APIs are stable for the v1.0 line; minor versions ship features additively. Internals can still change between minor versions. v0.17.0 closes the typed-emit soundness floor under the reframed "rock solid + ~100% sound with documented surface for remaining rt.Coerce" goal — see docs/v0.17/release-plan.md.

Sky is a fullstack functional language that compiles to typed Go. You write Elm-style syntax — explicit types, exhaustive pattern matching, no runtime exceptions — and ship a single static binary with a batteries-included stdlib, observability built in, and any Go package just an import away.

module Main exposing (main)

import Std.Log exposing (println)

main =
    println "Hello from Sky!"

sky init hello && cd hello && sky run src/Main.sky

Why Sky

• If it compiles, it works. Every side effect returns Task Error a; every fallible value returns Result Error a; sky check invokes go build on the emitted Go so any shape mismatch surfaces at type-check time. There is no runtime null, no uncaught exception, no silent numeric coercion.
• One language, every shape. The same init / update / view / subscriptions source compiles to a server-rendered web app (Sky.Live), a terminal UI (Sky.Tui), or a native desktop window (Sky.Webview).
• Batteries included. Auth, database, HTTP client + server, WebSocket, JSON, JWT, CSV, email, encryption, observability — every primitive a real app needs is in the stdlib (Std.Db, Std.Auth, Std.Ui, Std.Cache, Std.Email, …) and documented with sky doc --serve.
• Go's whole ecosystem. sky add github.com/some/package — the compiler introspects the Go package and generates strict, typed Sky bindings. No hand-written FFI glue. Stripe SDK (~76k FFI symbols) compiles and tree-shakes to a 4k-line main.go.
• AI-friendly by design. Explicit annotations, exhaustive pattern matching, no implicit coercions, no exceptions. LLMs generate code that compiles the first time. The shipped CLAUDE.md and sky init's starter CLAUDE.md give any AI assistant the load-bearing context to scaffold production apps directly.
• One binary out the back. Every project compiles to a static Go binary. Deploy with scp, with Docker, or as a CLI you brew install.

Hello, Sky

A counter web app — type-checked, server-driven, no JavaScript.

module Main exposing (main)

import Std.Cmd as Cmd
import Std.Live exposing (app, route)
import Std.Sub as Sub
import Std.Ui as Ui
import Std.Ui.Font as Font


type Msg
    = Increment
    | Decrement


type alias Model = { count : Int }


init : a -> ( Model, Cmd Msg )
init _ = ( { count = 0 }, Cmd.none )


update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case msg of
        Increment -> ( { model | count = model.count + 1 }, Cmd.none )
        Decrement -> ( { model | count = model.count - 1 }, Cmd.none )


view : Model -> Ui.Element Msg
view model =
    Ui.layout []
        (Ui.row [ Ui.spacing 16, Ui.padding 24 ]
            [ Ui.button [] { onPress = Just Decrement, label = Ui.text "−" }
            , Ui.el [ Font.size 24 ] (Ui.text (String.fromInt model.count))
            , Ui.button [] { onPress = Just Increment, label = Ui.text "+" }
            ])


main =
    app
        { init = init
        , update = update
        , view = view
        , subscriptions = \_ -> Sub.none
        , routes = [ route "/" () ]
        , notFound = ()
        }

sky run src/Main.sky    # http://localhost:8000

Add Std.Tui.app cfg to ship the same view to a terminal canvas, or Std.Webview.app cfg for a native desktop window.

Install

# macOS / Linux — single-binary install
curl -fsSL https://sky-lang.org/install | bash

# or build from source (Haskell GHC 9.4+ required to build the compiler)
git clone https://github.com/anzellai/sky
cd sky && cabal install --installdir=$HOME/.local/bin exe:sky

The sky binary embeds the runtime, stdlib, and Sky Console. End users only need sky on PATH and Go 1.21+ available for codegen.

Pick your shape

Match the application to the right surface — every shape uses the same TEA-style init / update / view / subscriptions.

Every backend shares Std.Ui for layout, Std.Auth for sessions, Std.Db for persistence, Std.Log / Std.Trace for observability, and Sky.Core.* for pure primitives.

What ships with Sky

A short tour. Full reference at sky doc --serve or docs/stdlib.md.

Observability — built in

Every Sky.Live and Sky.Http.Server app auto-mounts:

• /_sky/console — Std.Ui dashboard with overview, logs, metrics, traces, errors (production-gated via SKY_CONSOLE_AUTH).
• /_sky/metrics — Prometheus scrape endpoint (sky_live_requests_total{route,status}, latency histograms, drop counters).
• /_sky/healthz / /_sky/readyz — liveness + readiness probes.
• /_sky/buildinfo — commit, build timestamp, Sky version.

Run sky console serve to stand up a central hub that multiple Sky apps push telemetry to via the HubExporter (OTLP/HTTP). See docs/v0.16.x-console/HUB.md for the multi-service dashboard, tenant isolation, and the 3-layer auth defense-in-depth model.

OTEL_EXPORTER_OTLP_ENDPOINT is honoured for the standard OpenTelemetry collector — point at Honeycomb, Grafana Tempo, Datadog, etc.

Going to production

# sky.toml
name = "myapp"
version = "1.0.0"
entry = "src/Main.sky"

[live]
port = 8000
store = "sqlite"          # memory / sqlite / redis / postgres / firestore
storePath = "sessions.db"
ttl = "30m"

[database]
driver = "sqlite"         # sqlite / postgres
url = "DATABASE_URL"

[auth]
cookie = "sky_sid"
ttl = "24h"
# tokenSecret read from SKY_AUTH_TOKEN_SECRET (never put secrets in sky.toml)

[log]
format = "json"           # plain / json
level  = "info"

ENV=production \
SKY_AUTH_TOKEN_SECRET="$(openssl rand -base64 48)" \
SKY_CONSOLE_AUTH=app SKY_CONSOLE_TOKEN="$(openssl rand -base64 48)" \
sky build src/Main.sky && ./sky-out/app

The production gate is ENV (then SKY_ENV fallback). Unset or dev / development / local → dev mode. Anything else locks down the dev console, banner, and metrics endpoint.

Deploy with scp + your favourite supervisor, drop the binary into a Docker FROM scratch image, or run it directly on any platform with a Go 1.22+ runtime.

Documentation

• Getting started — install + your first app in 5 minutes.
• Stdlib reference — every module, every function, indexed by tier (Pure / Fallible-pure / Task / Diverging).
• Sky.Live — server-driven UI
• SSE patches + sessions + routing.
• Std.Ui — typed no-CSS layout DSL.
• Sky.Tui — terminal backend for Std.Ui.
• Sky.Webview — native desktop window.
• Std.Auth — sessions + JWT + roles.
• Std.Db — SQLite + PostgreSQL.
• sky.toml — every config key.
• CLI / LSP / Testing.
• Known limitations — current-version constraints + workarounds.
• Compiler journey — how Sky got here (historical context, kept for contributors).

Examples

39 examples ship in examples/. Each builds clean from a wiped slate (rm -rf sky-out .skycache .skydeps && sky build).

Contributing

Issues and PRs welcome at github.com/anzellai/sky. The compiler architecture write-up and pipeline doc are the right starting points for compiler work. Run cabal test (cap with timeout 3600) before any PR; scripts/example-sweep.sh validates every example builds.

Licence

Apache 2.0 — © 2025–2026 Anzel Lai. Includes patent grant + trademark clause. Prior-art attribution for derivative files (parts of the type-inference core adapted from elm/compiler under BSD-3-Clause) lives in NOTICE.md. Contributions accepted under the same Apache 2.0 terms — see CONTRIBUTING.md.

Sky was previously distributed under the MIT licence (releases up to and including v0.10.0). Those releases remain available under their original MIT terms; v0.10.1 onwards ships under Apache 2.0.