Crepuscularity

Stability: This project is unstable and in active development. APIs, CLI flags, and template semantics may change without a semver-major release until 1.0. Pin exact dependency versions and expect occasional breakage.

The first GPUI component and runtime system with hot reloading, and the first non-web-language, plug-and-play browser extension framework.

Write UI in a concise, indentation-based template DSL (.crepus files). Templates compile at build time via the view! macro or render at runtime with full hot-reload support. The same .crepus syntax drives native desktop (GPUI), browser extensions (MV3), HTML output, and React/JSX — and is the foundation for native mobile backends targeting SwiftUI and Jetpack Compose.

Why Crepuscularity

• First GPUI component system with hot reload — live template updates without recompiling; no other GPUI framework offers this
• First plug-and-play browser extension framework in Rust — write your popup/background/content scripts in .crepus, get a MV3-compliant extension bundle out; no JavaScript framework or bundler required
• One syntax, multiple backends — the same template works across GPUI (native desktop), HTML, React/JSX, and browser extensions today; crepuscularity-native lowers .crepus to JSON View IR for SwiftUI / Jetpack Compose shells (see examples/native-shells)
• Compile-time and runtime paths — view! macro for zero-overhead AOT compilation; parse_template / render_nodes for full runtime flexibility and hot reload

Quick Start

# Install the CLI
cargo install --path crates/crepuscularity-cli

# If `crepus` is not found, Cargo still installed the binary under ~/.cargo/bin — add it to PATH,
# e.g. for zsh:  export PATH="$HOME/.cargo/bin:$PATH"
# (Rustup normally prepends this for login shells; some terminals omit it.)

# Create a new GPUI app
crepus new my-app
cd my-app
SDKROOT=$(xcrun --show-sdk-path) cargo run

# Or create a browser extension
crepus webext new my-extension
cd my-extension
crepus webext build
# Load dist/unpacked/ in chrome://extensions

Template Syntax

div w-full h-full bg-zinc-950 text-white flex flex-col p-8
  div text-2xl font-bold mb-4
    "Hello {name}"
  if {score > 50}
    div text-green-400
      "High score!"
  else
    div text-red-400
      "Keep going"

Features

• .crepus syntax — indentation-based, Tailwind-style classes
• Control flow — if/else, match, for
• String interpolation — "Hello {name}"
• Expressions — arithmetic, comparison, logical operators, property access
• Components — single-file and multi-component files with slot support
• Hot reload — live template updates via the runtime renderer
• Browser extensions — crepus webext commands for MV3 extensions; popup pre-rendered at build time; no JS bundler needed
• IDE integration — structured JSON events with --emit-events

Output Targets

The .crepus DSL is the primary language. Each output target is a renderer that consumes the same parsed template — not a different framework.

JSX/HTML tag syntax is supported as an input format in the core parser — the same .crepus templates can be written in either indentation style or <tag> style, and both compile to the same AST.

CLI Commands

crepus new <name>                    # Scaffold GPUI app
crepus dev [--emit-events]           # Hot-reload dev loop
crepus build [--release]             # Build wrapper
crepus preview <file.crepus>         # Live preview

crepus webext new <name>             # Scaffold browser extension
crepus webext build [--app PATH]     # Build to dist/unpacked/ (WASM + manifest + popup pre-render)
crepus webext manifest               # Print manifest.json

crepus ios new <name>                # XcodeGen + SwiftPM NativeShell (View IR) host app
crepus ios generate [--dir]          # Run xcodegen (finds crepus.toml [ios] upward)
crepus ios build [--dir] [...]       # Simulator build via xcodebuild

Documentation

• Rendered site (GitHub Pages): here — WASM landing page plus HTML generated from the Markdown in docs/. Built in CI with crepus web build --site docs-site.
• Sources: docs/README.md indexes DSL, components, CLI, and extensions. Compiler-focused detail stays in-repo as CREPUS_WEB_IMPLEMENTATION_SPEC.md but is not shipped on the public docs site.
• Native shells: examples/native-shells — SwiftPM (ios/), Gradle (android/), and shared View IR fixture.json next to crepuscularity-native (replaces the old separate crepuscularity-native-ui checkout).
• Contributors & coding agents: root AGENTS.md is the canonical instructions (macOS SDKROOT, cargo fmt / clippy / test before push, workspace layout, DSL notes). CLAUDE.md is a symlink to AGENTS.md so duplicate context files cannot drift.

GPUI — Tailwind Class Support

The GPUI renderer maps Tailwind-style class strings to native GPUI style calls. This is a native desktop renderer, not a browser, so the mapping is intentionally selective.

Supported

Not Supported (GPUI hard limits)

These cannot be added without forking GPUI itself:

Project Structure

crates/
  crepuscularity/           Facade re-exporting prelude
  crepuscularity-core/      AST, parser, evaluator
  crepuscularity-web/       HTML backend
  crepuscularity-gpui/      GPUI prelude + view! macro
  crepuscularity_macros/    Compile-time view! proc-macro
  crepuscularity-runtime/   Hot-reload renderer (Tailwind → GPUI styler)
  crepuscularity-cli/       crepus CLI
  crepuscularity-webext/    Browser extension support
examples/
  weather/                  Weather app example
  quicknote/                Browser extension example

Building

Install the CLI:

cargo install --path crates/crepuscularity-cli

Pre-built binaries for Linux and Windows are in the repo root (crepus-linux-x86_64, crepus-windows-x86_64.exe).

On macOS, GPUI requires the Xcode SDK path:

SDKROOT=$(xcrun --show-sdk-path) cargo build

When Xcode uses the downloadable Metal Toolchain component, let Cargo inherit the exact Xcode environment GPUI's build script needs:

eval "$(scripts/metal-env.sh)"
cargo build
scripts/metal-env.sh -- cargo check -p crepuscularity-gpui
scripts/metal-env.sh --check

The required variables are SDKROOT for SDK headers, DEVELOPER_DIR for the active Xcode, and TOOLCHAINS=Metal for xcrun toolchain selection. scripts/metal-env.sh reads the downloaded Metal toolchain search path from xcodebuild -showComponent MetalToolchain -json, exports the short selector that xcrun -sdk macosx metal accepts, and prepends Metal.xctoolchain/usr/bin to PATH for direct metal / metallib probes. If --check reports xcrun_metal=failed, run xcodebuild -downloadComponent MetalToolchain or install the component from Xcode Settings > Components before debugging Cargo code.

License

Mozilla Public License 2.0 — see LICENSE.