Make /try run the homepage sample and capture io.print

crates/echo_wasm/src/lib.rs

@@ -8,7 +8,9 @@
 
 mod run;
 
-pub use run::{RunResult, SAMPLE_RESULT, SAMPLE_STRUCT, SAMPLE_SUM, run_json, run_source};
+pub use run::{
+    HOMEPAGE_SUM, RunResult, SAMPLE_RESULT, SAMPLE_STRUCT, SAMPLE_SUM, run_json, run_source,
+};
 
 use std::path::{Path, PathBuf};
 
@@ -299,6 +301,14 @@ io.print("sum={sum}")
         assert!(result.host_error.is_none(), "{result:?}");
     }
 
+    #[test]
+    fn run_homepage_sum_echo_prints_sum() {
+        let result = run_source(HOMEPAGE_SUM);
+        assert!(result.ok, "{result:?}");
+        assert_eq!(result.printed.as_deref(), Some("sum=6\n"), "{result:?}");
+        assert!(result.host_error.is_none(), "{result:?}");
+    }
+
     #[test]
     fn run_result_sample_prints_ok_arm() {
         let result = run_source(SAMPLE_RESULT);
@@ -330,7 +340,26 @@ io.print("sum={sum}")
 
     #[test]
     fn run_refuses_fs_as_playground_host() {
-        let result = run_source("/ std/fs\n/ std/io\n$ ok = fs.exists(\"x\")\n");
+        assert_playground_host("/ std/fs\n/ std/io\n$ ok = fs.exists(\"x\")\n");
+    }
+
+    #[test]
+    fn run_refuses_net_as_playground_host() {
+        assert_playground_host("/ std/net/tcp\n$ c = tcp.connect(\"127.0.0.1:1\")\n");
+    }
+
+    #[test]
+    fn run_refuses_process_as_playground_host() {
+        assert_playground_host("/ std/process\n$ xs = process.args()\n");
+    }
+
+    #[test]
+    fn run_refuses_tasks_as_playground_host() {
+        assert_playground_host("+ job = {\n    ^ 1\n}\n- job\n");
+    }
+
+    fn assert_playground_host(source: &str) {
+        let result = run_source(source);
         assert!(!result.ok, "{result:?}");
         assert!(result.printed.is_none(), "{result:?}");
         let err = result.host_error.as_deref().unwrap_or("");

crates/echo_wasm/src/run.rs

@@ -31,7 +31,7 @@ use crate::{CheckDiagnostic, PLAYGROUND_PATH, check_source, playground_workspace
 const MAX_STEPS: u32 = 2_000_000;
 const MAX_CALL_DEPTH: u32 = 256;
 
-/// `/try` Sum sample (must stay in lockstep with `www/src/try.tsx`).
+/// `/try` Sum sample (must stay in lockstep with `www/src/docs/site.ts` `homePage.sample`).
 pub const SAMPLE_SUM: &str = r#"/ std/io
 
 $ xs = [1, 2, 3]
@@ -42,6 +42,16 @@ $ xs = [1, 2, 3]
 io.print("sum={sum}")
 "#;
 
+/// Homepage `sum.echo` figure: same program as [`SAMPLE_SUM`], no trailing newline.
+pub const HOMEPAGE_SUM: &str = r#"/ std/io
+
+$ xs = [1, 2, 3]
+~ sum = 0
+* x : xs {
+    ~ sum = sum + x
+}
+io.print("sum={sum}")"#;
+
 /// `/try` Result sample.
 pub const SAMPLE_RESULT: &str = r#"/ std/io
 / std/str

docs/development-speed.md

@@ -175,7 +175,8 @@ just fmt-check
 The site playground (`/try`) runs the shared frontend (lex → parse → resolve →
 semantics) as `wasm32-unknown-unknown`, then a playground run executes checked
 MIR and captures `io.print`. It does not ship LLVM. Playground run is a host
-demo; native compile and run stay on `xo`.
+demo; native compile and run stay on `xo`. Filesystem, net, process, and tasks
+fail with a playground-host error. Bindings stay in `www/public/echo-wasm/`.
 
 ```bash
 just wasm                 # echo_wasm + wasm-bindgen → www/public/echo-wasm/

scripts/build-wasm.sh

@@ -87,7 +87,7 @@ test -f "$OUT_DIR/echo_wasm_bg.wasm"
 # Tiny loader note used by the site when someone opens the directory.
 cat > "$OUT_DIR/README.md" <<'EOF'
 Generated by `just wasm` / `scripts/build-wasm.sh`. Do not edit.
-Browser check host (frontend only). Reload `/try` after rebuilding.
+Browser check + playground run host. Reload `/try` after rebuilding.
 EOF
 
 # Fingerprint the bindings so www can cache-bust the unhashed public URLs.

www/SITE.md

@@ -137,13 +137,13 @@ maturity.
 
 The logo is the only Home control. Book stays at `/book` and in the footer.
 
-| Item              | Path        | Notes                                      |
-| ----------------- | ----------- | ------------------------------------------ |
-| Documents         | `/docs`     | Language reference hub                     |
-| Packages          | `/docs/std` | Standard library                           |
-| Echo 2026         | `/e26`      | Edition + Spec TOC + suite                 |
-| Try               | `/try`      | In-browser `xo check` (frontend wasm host) |
-| **Install** (CTA) | `/install`  | Solid button; get `xo`                     |
+| Item              | Path        | Notes                                         |
+| ----------------- | ----------- | --------------------------------------------- |
+| Documents         | `/docs`     | Language reference hub                        |
+| Packages          | `/docs/std` | Standard library                              |
+| Echo 2026         | `/e26`      | Edition + Spec TOC + suite                    |
+| Try               | `/try`      | In-browser check + playground run (wasm host) |
+| **Install** (CTA) | `/install`  | Solid button; get `xo`                        |
 
 ## Docs left rail
 
@@ -187,8 +187,9 @@ Cross-links: Reference ↔ Spec ↔ suite pages keep the triangle explicit.
 
 `/try` runs the shared compiler frontend in WebAssembly (`just wasm`). It
 checks source the same way `xo check` does, including bundled `std`. A
-playground run then executes the checked MIR and captures `io.print`. Compile
-and native run stay on `xo` (LLVM).
+playground run then executes the checked MIR and captures `io.print`.
+Filesystem, net, process, and tasks fail with a playground-host error.
+Compile and native run stay on `xo` (LLVM).
 
 ## Out of scope (later)
 

www/package.json

@@ -10,10 +10,11 @@
     "format": "oxfmt --check .",
     "lint": "oxlint . --ignore-pattern public/echo-wasm",
     "preview": "vite preview",
-    "test": "npm run test:docs && npm run test:prose && npm run test:std-ref",
+    "test": "npm run test:docs && npm run test:prose && npm run test:std-ref && npm run test:try",
     "test:docs": "node scripts/verify-docs-pages.mjs",
     "test:std-ref": "node scripts/verify-std-reference.mjs",
     "test:prose": "node scripts/verify-prose.mjs",
+    "test:try": "node scripts/verify-try.mjs",
     "sync:tree-sitter": "node scripts/sync-tree-sitter.mjs",
     "postinstall": "npm run sync:tree-sitter"
   },

www/public/_headers

@@ -1,3 +1,7 @@
 # Bindings are cache-busted with ?v= from ECHO_WASM_REV; do not pin them long.
 /echo-wasm/*
   Cache-Control: public, max-age=0, must-revalidate
+
+/echo-wasm/*.wasm
+  Content-Type: application/wasm
+  Cache-Control: public, max-age=0, must-revalidate

www/public/echo-wasm/README.md

@@ -1,2 +1,2 @@
 Generated by `just wasm` / `scripts/build-wasm.sh`. Do not edit.
-Browser check host (frontend only). Reload `/try` after rebuilding.
+Browser check + playground run host. Reload `/try` after rebuilding.

www/scripts/verify-try.mjs

@@ -0,0 +1,120 @@
+/**
+ * Verifies the /try playground contract:
+ * - default Sum sample is the homepage sum.echo figure
+ * - page copy names host limits and does not claim `xo run` or native LLVM
+ * - shipped www/public/echo-wasm bindings check the sample and capture io.print
+ *
+ * Loads site/playground modules through Vite SSR. Instantiates the committed
+ * wasm bindings in-process so Pages can ship without a Rust toolchain.
+ */
+import { createServer } from "vite";
+import { existsSync, readFileSync } from "node:fs";
+import { readFile } from "node:fs/promises";
+import { pathToFileURL } from "node:url";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+
+const root = path.resolve(path.dirname(fileURLToPath(import.meta.url)), "..");
+const wasmDir = path.join(root, "public/echo-wasm");
+const trySourcePath = path.join(root, "src/try.tsx");
+
+const server = await createServer({
+  root,
+  logLevel: "error",
+  server: { middlewareMode: true },
+  appType: "custom",
+});
+
+const failures = [];
+
+function fail(message) {
+  failures.push(message);
+}
+
+try {
+  const site = await server.ssrLoadModule("/src/docs/site.ts");
+  const playground = await server.ssrLoadModule("/src/lib/playground.ts");
+  const tryPage = await server.ssrLoadModule("/src/try.tsx");
+
+  const homepage = site.homePage.sample.trim();
+  const sum = playground.playgroundSumSource().trim();
+  if (sum !== homepage) {
+    fail("playgroundSumSource must match homePage.sample");
+  }
+
+  const samples = tryPage.PLAYGROUND_SAMPLES;
+  const sumSample = samples?.find((sample) => sample.id === "sum");
+  if (sumSample == null) {
+    fail("PLAYGROUND_SAMPLES must include a sum sample");
+  } else if (sumSample.source.trim() !== homepage) {
+    fail("PLAYGROUND_SAMPLES sum source must match homePage.sample");
+  }
+
+  const limits = playground.PLAYGROUND_HOST_LIMITS;
+  for (const limit of ["filesystem", "net", "process", "tasks"]) {
+    if (!limits?.includes(limit)) {
+      fail(`PLAYGROUND_HOST_LIMITS missing ${limit}`);
+    }
+  }
+
+  const trySource = readFileSync(trySourcePath, "utf8");
+  if (/\bxo run\b/.test(trySource)) {
+    fail("/try must not describe itself as xo run");
+  }
+  if (/native LLVM/i.test(trySource)) {
+    fail("/try must not claim native LLVM in the browser");
+  }
+  for (const limit of ["filesystem", "net", "process", "tasks"]) {
+    if (!trySource.toLowerCase().includes(limit)) {
+      fail(`/try page must name host limit ${limit}`);
+    }
+  }
+  if (!trySource.includes("io.print")) {
+    fail("/try page must mention captured io.print");
+  }
+
+  const requiredWasm = ["echo_wasm.js", "echo_wasm_bg.wasm", "echo_wasm.d.ts"];
+  for (const name of requiredWasm) {
+    if (!existsSync(path.join(wasmDir, name))) {
+      fail(`missing shipped binding www/public/echo-wasm/${name}`);
+    }
+  }
+
+  if (failures.length === 0) {
+    const jsUrl = pathToFileURL(path.join(wasmDir, "echo_wasm.js")).href;
+    const wasmBytes = await readFile(path.join(wasmDir, "echo_wasm_bg.wasm"));
+    const wasm = await import(jsUrl);
+    await wasm.default({ module_or_path: wasmBytes });
+
+    if (typeof wasm.check !== "function" || typeof wasm.playgroundRun !== "function") {
+      fail("echo_wasm bindings must export check and playgroundRun");
+    } else {
+      const checked = JSON.parse(wasm.check(site.homePage.sample));
+      if (!checked.ok) {
+        fail(`homepage sample must check: ${JSON.stringify(checked)}`);
+      }
+      const ran = JSON.parse(wasm.playgroundRun(site.homePage.sample));
+      if (!ran.ok || ran.printed !== "sum=6\n") {
+        fail(`homepage sample must print sum=6, got ${JSON.stringify(ran)}`);
+      }
+    }
+  }
+} catch (error) {
+  fail(error instanceof Error ? error.message : String(error));
+} finally {
+  await server.close();
+}
+
+if (failures.length) {
+  console.error(JSON.stringify({ ok: false, failures }, null, 2));
+  process.exitCode = 1;
+} else {
+  console.log(
+    JSON.stringify({
+      ok: true,
+      sample: "sum.echo",
+      printed: "sum=6",
+      wasm: "www/public/echo-wasm",
+    }),
+  );
+}

www/src/docs/content.ts

@@ -463,7 +463,9 @@ io.print("sum={sum}")`,
             text: [
               "Open ",
               { code: "/try" },
-              " to check this program in the browser. Build ",
+              " to check this program and capture ",
+              { code: "io.print" },
+              ". Build ",
               { code: "xo" },
               " via ",
               { code: "/install" },

www/src/lib/playground.ts

@@ -0,0 +1,14 @@
+/**
+ * /try contract: homepage sample, host limits, and copy that must not
+ * claim `xo run` or native LLVM in the browser.
+ */
+
+import { homePage } from "../docs/site";
+
+/** Sum buffer on /try. Same program as the homepage `sum.echo` figure. */
+export function playgroundSumSource(): string {
+  return homePage.sample.endsWith("\n") ? homePage.sample : `${homePage.sample}\n`;
+}
+
+/** Host services the playground refuses. Keep this list on the page. */
+export const PLAYGROUND_HOST_LIMITS = ["filesystem", "net", "process", "tasks"] as const;

www/src/try.tsx

@@ -8,26 +8,19 @@ import {
   type EchoCheckApi,
   type RunResult,
 } from "./lib/echo-check";
+import { playgroundSumSource } from "./lib/playground";
 
 type Sample = {
   id: string;
   label: string;
   source: string;
 };
 
-const SAMPLES: Sample[] = [
+export const PLAYGROUND_SAMPLES: Sample[] = [
   {
     id: "sum",
     label: "Sum",
-    source: `/ std/io
-
-$ xs = [1, 2, 3]
-~ sum = 0
-* x : xs {
-    ~ sum = sum + x
-}
-io.print("sum={sum}")
-`,
+    source: playgroundSumSource(),
   },
   {
     id: "result",
@@ -120,11 +113,12 @@ function diagnosticLabel(diag: CheckDiagnostic) {
 }
 
 /**
- * In-browser xo check. LLVM run stays on a native xo install.
+ * In-browser check plus a playground run that captures io.print.
+ * Native compile stays on an xo install.
  */
 export function TryPage() {
-  const [source, setSource] = useState(SAMPLES[0].source);
-  const [activeSample, setActiveSample] = useState(SAMPLES[0].id);
+  const [source, setSource] = useState(PLAYGROUND_SAMPLES[0].source);
+  const [activeSample, setActiveSample] = useState(PLAYGROUND_SAMPLES[0].id);
   const [api, setApi] = useState<EchoCheckApi | null>(null);
   const [loadError, setLoadError] = useState<string | null>(null);
   const [result, setResult] = useState<CheckResult | null>(null);
@@ -237,9 +231,10 @@ export function TryPage() {
           Try Echo
         </h1>
         <p className="mt-4 max-w-3xl text-pretty text-lg leading-8 text-slate-600">
-          This page checks with the shared compiler frontend, then a playground run executes the
+          This page checks with the shared compiler frontend. A playground run then executes the
           checked program and captures{" "}
-          <span className="font-mono font-semibold text-slate-800">io.print</span>. Install{" "}
+          <span className="font-mono font-semibold text-slate-800">io.print</span>. Filesystem, net,
+          process, and tasks stay unavailable here. Install{" "}
           <span className="font-mono font-semibold text-slate-800">xo</span> to compile through
           LLVM.
         </p>
@@ -264,7 +259,7 @@ export function TryPage() {
           <section className="min-w-0">
             <div className="flex flex-wrap items-center justify-between gap-3">
               <div className="flex flex-wrap gap-2" role="tablist" aria-label="Sample programs">
-                {SAMPLES.map((sample) => {
+                {PLAYGROUND_SAMPLES.map((sample) => {
                   const selected = sample.id === activeSample;
                   return (
                     <button