Aver

Every effect in the standard library has a typed signature on the source side and a runtime implementation on each backend. Some backends host every effect natively, others stub or short-circuit them when the underlying platform can't satisfy the contract. This document catalogues which backend supports which effect, and what "supported" actually means in each cell.

Backends

Compilation path	Output	Where it runs
VM (aver run)	bytecode interpreter	local CLI, dev loop
Rust codegen (aver compile)	Cargo project + native binary	server-side Rust deployments
wasm-gc (--target wasm-gc)	self-contained .wasm with engine GC + tail calls; per-instantiation helpers DCE'd to what the program calls. --handler <fn> synthesises a fetch-style HTTP wrapper; --preset cloudflare --handler <fn> packages it for Workers	Cloudflare Workers, modern browsers (Chrome 119+, Firefox 120+, Safari 18.2+), wasmtime 25+, Node 22+, Deno, Bun
wasip2 (--target wasip2)	.component.wasm + sibling .wit. wasm-gc core module wrapped via wit-component; Aver effects lower directly to canonical-ABI WASI imports — no preview-1 adapter	wasmtime, Spin, NGINX Unit, wasmCloud, every other Component Model host
Lean / Dafny proof export (aver proof)	.lean / .dfy projects	offline verification
Self-host (aver run --self-host)	Aver-in-Aver bootstrap	development sanity, replay coverage

The two WASM rows are independent compilation paths. --target wasm-gc covers JS hosts and embedded wasmtime via aver/* host imports; --target wasip2 covers Component Model hosts via canonical-ABI WIT imports. The pre-2024 NaN-boxed --target wasm backend was dropped in 0.18 (Phase 1.8 of "Span") — modern hosts run the wasm-gc pipeline, standalone runtimes use wasip2.

Lean / Dafny columns describe the proof export treatment, not the runtime. Effects render as Oracle-style stubs with effect-list contracts and invariant lemmas; user-side theorems carry the per-effect bounds (Random.int in [min, max], Time.unixMs ≥ 0, …) as hypotheses. See docs/oracle.md for the full Oracle model.

Legend

Symbol	Meaning
✅	Real implementation; the effect does what its source-side signature promises
⚠️	Partial / convention-based; documented caveat on the cell
❌	Stubbed; the call typechecks and runs but returns a documented sentinel (Result.Err, Option.None, Unit) — programs branch through the failure shape, not crash
n/a	Concept doesn't apply on this host (e.g. HttpServer.listen under a fetch-style host — the worker IS the server, the handler shape is the API)

Matrix

The wasm-gc column covers the default invocation (--target wasm-gc, host wires aver/* imports). The HTTP-handler shape (--handler <fn>, --preset cloudflare) is the same column with Request.* / Response.* host imports replacing the corresponding effect cells when aver_http_handle() runs — see Notes per backend below. The wasip2 column is what --target wasip2 produces today (0.18 "Span" landed all 17 effect call sites listed); cells marked n/a indicate the effect can't structurally land on WASI 0.2 and is rejected at compile time by wasip2::effect_check.

Effect	VM	Rust	wasm-gc	wasip2	Lean	Dafny
Args.get	✅	✅	✅ wasmtime / host wires	✅ wasi:cli/environment.get-arguments	Oracle	Oracle
Console.print	✅	✅	✅ wasmtime / console.log	✅ wasi:cli/stdout + blocking-write-and-flush	Oracle	Oracle
Console.error	✅	✅	✅ wasmtime / console.error	✅ wasi:cli/stderr + blocking-write-and-flush	Oracle	Oracle
Console.warn	✅	✅	✅ wasmtime / console.warn	✅ wasi:cli/stderr (warn → stderr)	Oracle	Oracle
Console.readLine	✅	✅	✅ wasmtime / host stdin	✅ wasi:cli/stdin + blocking-read line loop	Oracle	Oracle
Disk.readText / writeText / appendText	✅	✅	✅ wasmtime / ❌ in JS hosts	✅ wasi:filesystem/preopens + open-at + via-stream	Oracle	Oracle
Disk.exists / delete / deleteDir / listDir / makeDir	✅	✅	✅ wasmtime / ❌ in JS hosts	✅ wasi:filesystem/types (stat-at / unlink-file-at / etc.)	Oracle	Oracle
Env.get	✅	✅	✅ wasmtime / Workers env	✅ wasi:cli/environment.get-environment + linear search	Oracle	Oracle
Env.set	✅	✅	⚠️ wasmtime / no-op in JS	n/a — WASI 0.2 environment is read-only by design	Oracle	Oracle
Http.get / head / delete / post / put / patch	✅	✅	✅ wasmtime / ✅ JSPI-suspending fetch()	❌ deferred to 0.19+ (lowering planned for wasi:http)	Oracle	Oracle
HttpServer.listen / listenWith	✅ (runtime-net)	✅ (runtime-net)	n/a — --handler <fn> shape	❌ deferred to 0.19+ (lowers via wasi:http/proxy)	Oracle	Oracle
Random.int	✅	✅	✅ wasmtime / Math.random	✅ wasi:random/random.get-random-u64 + range scale	Oracle ([min, max] lemma)	Oracle
Random.float	✅	✅	✅ wasmtime / Math.random	✅ wasi:random/random.get-random-u64 → [0.0, 1.0)	Oracle ([0.0, 1.0) lemma)	Oracle
Tcp.connect / send / ping / writeLine / readLine / close	✅	✅	✅ wasmtime / ❌ in JS hosts	❌ deferred to 0.19+ (lowers via wasi:sockets)	Oracle	Oracle
Terminal.* (12 methods)	✅ via crossterm (terminal feature)	✅ via crossterm	✅ wasmtime / ❌ in JS hosts	n/a — WASI 0.2 has no terminal interface	Oracle	Oracle
Time.now (ISO string)	✅	✅	✅ wasmtime / new Date().toISOString()	✅ wasi:clocks/wall-clock.now + guest-side civil_from_days	Oracle	Oracle
Time.unixMs	✅	✅	✅ wasmtime / Date.now()	✅ wasi:clocks/wall-clock.now → ms	Oracle (≥ 0 lemma)	Oracle
Time.sleep	✅	✅	✅ wasmtime / ⚠️ blocks worker isolate	✅ wasi:clocks/monotonic-clock.subscribe-duration + wasi:io/poll.poll	Oracle	Oracle

Print.value / Format.value are no longer needed — Console.print / error / warn take String since 0.16, so stringification happens at the call site (interpolation "{x}" for primitives, a per-type render fn for compound shapes).

Notes per backend

wasm-gc (--target wasm-gc)

The recommended target. Same aver/* import surface across every host that runs the binary — the difference is who supplies the implementation, and that's reflected in cells that read "wasmtime / <JS thing>":

• aver run --wasm-gc <file> — embedded wasmtime executor with the full effect surface (Args, Console incl. readLine, Time, Random, Float math, Terminal, Disk, Env, Tcp, Http) wired against aver_rt::*. This is the cell on the left of the slash.
• JS hosts (Cloudflare Workers, browsers, Deno, Bun, Node 22+) — playground / worker.js template / custom embedder satisfies the aver/* imports. JS-host effects available are the cell on the right of the slash. Disk / raw TCP / Terminal don't have native JS equivalents and stub to Result.Err / Option.None / Unit.

--handler <fn> (and the bundled --preset cloudflare --handler <fn>) generates an aver_http_handle() synthesised wrapper that consumes Request fields via dedicated host imports (request_method, request_url, request_query, request_body, request_headers_load) and writes the response via response_text / response_set_header. Inside the handler body, Http.* calls still go through the standard effect surface (✅ JSPI-suspending fetch() on Workers, ✅ wasmtime if you ever ran the same handler under aver run --wasm-gc).

HttpServer.listen is n/a on wasm-gc — the deployment shape is "the host calls into your handler", which is exactly what --handler <fn> declares. There's no listening loop to write.

wasip2 (--target wasip2)

Component Model output landed in 0.18 "Span". Aver effects lower directly to canonical-ABI WASI imports — the wasm-gc backend emits a core module shaped to canonical-ABI conventions, the wrapper embeds a component-type:wasi:cli/command custom section via wit-component::metadata, and the resulting .component.wasm runs on every Component Model host (wasmtime, Spin, NGINX Unit, wasmCloud, …) without a preview-1 adapter. See docs/wasip2.md for the full contract.

aver compile app.av --target wasip2 -o out
aver run app.av --wasip2  -- alpha beta   # embedded wasmtime + wasmtime-wasi

What lands today (0.18) vs. deferred:

• ✅ Console (print/error/warn/readLine), Args.get, Env.get, Time (unixMs/now/sleep), Random (int/float), Disk (all 7: exists/readText/writeText/appendText/delete/deleteDir/makeDir/listDir).
• ❌ Http., Tcp., HttpServer.* — deferred to 0.19+ (Phase 2/3 of the Span follow-up); will lower via wasi:http and wasi:sockets respectively.
• n/a Env.set, Terminal.* — structurally absent from WASI 0.2 (read-only environment by design; no terminal interface). Rejected at compile time.

Effect calls > 4 KB on Console.* / Disk.write* chunk through blocking-write-and-flush (wasmtime-wasi enforces a 4096-byte limit per call); the chunked-write loop lives in emit_chunked_blocking_write and is shared by both call sites. Time.sleep uses subscribe-duration + poll + [resource-drop]pollable (real wait, not busy-loop).