lasm

lasm Main.lean [--release] [-- args...]lasm build Main.lean [--output dir] [--target name] [--dev] [--rebuild] [--verbose]lasm dev [Main.lean] [--target name] [--release] [-- args...]lasm init directory [--template name] [--deploy name]lasm package [dist] --target namelasm editor [project-directory]lasm cache repair

lasm compiles ordinary Lean 4 programs ahead of time to WebAssembly. One codebase can become a Node (or Bun or Deno) server, a serverless or edge function, a WASI or Fastly module, or a browser app. The CLI is the lasm binary from npm package `@lasm/cli` (also invokable as `npx @lasm/cli`). It installs Lean, Lake, and the link toolchain itself; you need Node 24 or newer (Bun 1.4+ and Deno 2.x are also tested).`lasm Main.lean` is a dev build (faster, not for deployment) that then runs the program. `lasm build` defaults to a release build (optimized and meant to be reproducible) and writes `dist/` unless --output is set. Function-platform targets package that output the way `lasm package` would. Application arguments are accepted only by run/dev and must follow `--`.Lean at run time is restricted: elaboration, `#eval`-style evaluation, loading `.olean` files, and Lean's kernel are build-time only. Node builds keep Lean's full runtime (threads, files, processes, sockets). Edge, Bun/Deno, WASI, and browser builds are single-threaded and drop sockets and processes (and files, except on WASI).

--target name

Build (and, for function platforms, package) for that platform. Aliases include cloudflare for workers, aws for lambda, web for browser, spin for wasi-http. Only with lasm build.

Output directory (default dist). Only with lasm build.

Choose the profile. Mutually exclusive. build defaults to release; run defaults to dev.

Rebuild even when the cache would reuse a previous result.

Extra compiler and linker progress.

Browser builds only. none (default) means blocking waits stop the instance; jspi suspends in browsers that have JSPI.

Browser builds only: leave npm imports as imports so Vite, esbuild, or webpack can bundle them.

Package a dev build for a non-Node target anyway. Without it, `lasm build --dev --target` for a function platform is refused.

Print usage.

Print package version, Lean version, JS runtime, and host.

run (default)

`lasm Main.lean` builds (dev profile unless --release) and runs the program with Node, or with Bun/Deno when the CLI itself is running there.

Compile without running. Default target is node (or bun / deno when the CLI runs on those). Other targets: lambda, lambda-edge, vercel, netlify, azure, gcf, workers, vercel-edge, netlify-edge, deno-deploy, supabase-edge, wasi, fastly, wasi-http, browser.

Watch sources, rebuild, and restart. Optional --target among node, bun, deno, workers, vercel-edge, netlify-edge, deno-deploy, supabase-edge, browser.

Create a project directory. Templates: plain, express, hono, fastify, std-http, web, fetch, fullstack. `npm create @lasm@latest` is the same flow.

Package an existing `dist/` for lambda, lambda-edge, vercel, netlify, azure, gcf, or browser.

Point the Lean editor at this project's toolchain so `import Lasm.*` resolves.

Verify managed tools, remove invalid ones, and clean leftover staging directories.

Node 24 or newer is required; older Node exits before the compiler loads.`--target`, `--output`, `--suspend`, and `--bundler` apply only to the commands noted above. Putting program flags before `--` is an error.A `.json` argument to build is a binding configuration, not a Lean application; `--target` / `--dev` / `--release` do not apply to it.

node(1), npm(1), bun(1), deno(1), wasm-opt(1), wasmtime(1), emcc(1), zig(1)

Source code · Documentation