Clojure and WebAssembly, joined at the interface.
ClojureWit makes Clojure a first-class citizen of the WebAssembly component ecosystem — in both directions:
- Call Wasm from Clojure.
requirea component written in any language as an ordinary Clojure namespace. - Call Clojure from anything. Compile a Clojure namespace into a
self-contained
.wasmcomponent that Rust, Go, JavaScript, Python — or another Clojure program — can call without knowing Clojure exists.
The name is from WIT, the language-neutral interface definition language of the Component Model. WIT is the seam; ClojureWit is what sits on the Clojure side of it.
Status: pre-alpha. There is no compiler yet. The feasibility benchmarks are measured and the design survived them — the verdict is
doc/design/0010.cljwit.hostworks today; its API is not yet stable. Seedoc/roadmap.md.
| What | State | |
|---|---|---|
cljwit.host | A plain Clojure library: call Wasm components from JVM Clojure. No new dialect, no compiler. | works, pre-alpha |
cljwit | The compiler: Clojure → Wasm component (WasmGC, tail calls, Wasm 3.0). | design |
cljwit.host ships first and stands on its own. It also settles the WIT ↔ Clojure
type mapping that the compiler needs, so the hard part gets solved once.
Requirements: Java 22+ (java.lang.foreign; developed on 25) and
libwasmtime ≥ 43 — brew install wasmtime provides it, or unpack a
*-c-api release tarball
and point CLJWIT_WASMTIME_LIB at the shared library
(doc/design/0019 has the full
resolution order).
;; deps.edn
{:deps {io.github.clojurewasm/ClojureWit
{:git/sha"..."}} ;; pre-alpha: pin a sha, expect breakage:aliases {:dev {:jvm-opts ["--enable-native-access=ALL-UNNAMED"]}}}(require '[cljwit.host :as host])
;; Three lifetimes, three orders of magnitude apart (0014):
(with-open [e (host/engine) ; process-wide, share it
a (host/compile e "component.wasm") ; milliseconds — cache it
i (host/instantiate a)] ; ~0.06 ms — one per request is fine;; Exports are discovered from the component itself. Names are the exact;; WIT strings; keywords work where the name reads back as itself.
((i"greet") "world") ; => "hello, world"
((i"wasi:cli/run@0.2.3#run"))) ; interface functions too;; The guest can call *you*: imports are ordinary Clojure functions,;; and WASI capabilities are named explicitly or absent (deny-by-default).
(host/instantiate a {:imports {"acme:log/sink@1.0.0#write" println}
:wasi {:inherit-stdouttrue:args ["-v"]}})WIT values arrive as the Clojure data you would guess: record → map,
variant → [:tag payload], enum → keyword, flags → set, option →
value-or-nil, result → [:ok v]/[:err e] (with host/unwrap as the
opt-in throw-on-err sugar), resources → AutoCloseable handles. The full
mapping — and why it is not negotiable — is
doc/design/0012.
Prefer names an editor can complete and clj-kondo can arity-check? Generate
an ordinary namespace and check it in
(doc/design/0020):
(require '[cljwit.host.gen :as gen])
(gen/write-ns!"component.wasm" {:ns 'acme.component})
;; => "src/acme/component.clj" — then: (acme.component/greet i "world")With more than one component, declare them once in cljwit.edn at the
project root and let cljwit.project reconcile
(doc/design/0021):
;; cljwit.edn
{:components {acme.resize {:wasm"components/resize.wasm"}}}(require '[cljwit.project :as project])
(project/sync!) ; regenerate what changed
(project/check) ; CI: throw if anything drifted — stale, edited,; missing, or orphanedThree things landed that were not true a year ago:
| When | What changed | |
|---|---|---|
| Wasm 3.0 | 2025-09 | GC, tail calls, exception handling became a W3C standard |
| WasmGC everywhere | 2024-12 | Chrome 119+, Firefox 120+, Safari 18.2+ |
| WASI 0.3.0 | 2026-06 | Native async in the Component Model (async func, stream<T>, future<T>) |
A garbage-collected, higher-order language can now target Wasm without shipping its own GC — and can be consumed as a component by any other language.
;; Use a component written in another language, as a namespace
(nsapp.core
(:require ["./resize.wasm":as resize]))
(resize/thumbnail bytes 128128);; Publish a namespace as a component other languages can call
(nsacme.handler
{:wit/world"acme:api/handler"})
(defn ^{:wit/export"handle"} handle [req]
{:status200:body"hello"})Design decisions live in doc/design/, the plan in
doc/roadmap.md, and the current state in
doc/status.md. Start with
doc/design/0001-scope-and-shape.md.
With Nix:
nix develop # exact toolchain
bb tasks # what you can runWithout Nix, install the tools in tools.json and check them:
bb check-tools- ClojureWasm — a sibling project in this org: a Clojure runtime in Zig that executes Wasm. ClojureWit is a compiler that emits Wasm. They compose: a component built with ClojureWit can be required by ClojureWasm.
- clj.wasm — earlier exploration of the same question, and the source of several ideas here.
EPL-2.0. See LICENSE.