WebAssembly

echoc build --target-os wasi already cross-compiles to wasm32-unknown-wasip1. That used to mean one thing: a command. _start runs the file's top-level statements and exits. Fine for a CLI. Useless for a page that wants to call into Echo, or for Echo that wants to call the page.

A lib target is the other shape. No entry. The surface is the functions you export. The page instantiates the module, calls _initialize, then calls you.

A lib, not a command

#[module: "plot"]
#[sources: "src/*.eco"]
#[target: lib { name: "plot" }]

echoc build --target-os wasi writes ecobuild/plot.wasm. run refuses it: there is nothing to start. On the host the same #[target: lib] is a .dylib / .so / .dll. That recipe lives on Libraries and plugins. This page is the wasm one: a reactor, _initialize, then the exports.

name is required. entry is refused. Top-level statements in every file are refused. Put work in functions. Statics still initialize on first use.

What a reactor does not do: static teardown, and the --track-allocations report. There is no main to hang them off. std::env::argc() is 0. echo of a number still prints. die still calls exit(1) in the host.

#[export]

The function's symbol is its own unqualified name, unmangled, C ABI. On wasm it is also in the module's export section.

#[export]
function add(int32 $a, int32 $b) : int32
{
    return $a + $b;
}

echo add(2, 3);     // 5

#[export: "name"] picks a different symbol. Same function, different thing the host looks up.

What may cross is the C ABI, the same rule C interop already uses: scalars and ptr<T>. int64 and uint64 are i64 in the wasm type. JavaScript sees them as BigInt. int32 stays a Number.

A #[export] on a dependency is in the final module's export section. If dep exports helper and your lib calls it, instance.exports.helper is there next to your own exports.

Two exports of one name are a compile error. So is exporting as main or __main_argc_argv.

#[wasm: import "env"]

The other direction. An extern function whose body lives in the host. The value is the wasm module name. The import name is the extern's C symbol, including as.

#[wasm: import "env"]
extern {
    function host_now_ms() : float64;
}

echo host_now_ms() > 0.0;

That is legal only on an extern. A function with a body is exported with #[export].

The attribute on the block applies to every function in it. A function's own #[wasm: import] of the same name wins.

A misspelled libc extern is still a link error. The explicit import module is what lets env.host_now_ms through without --allow-undefined.

#[wasm: export "memory"]

Linear memory is a wasm object, not an Echo function. This is how a page reads the bytes a ptr<uint8> return names.

#[wasm: export "memory"]

#[export]
function pixels() : ptr<uint8>
{
    return null;
}

It sits at file scope. Written above a function, it does not attach to that function. The payload is closed: memory. A function is still #[export].

A WASI toolchain may export memory anyway; the attribute is how Echo asks.

Calling it from a page

The working example is examples/mandelbulb_web/. Build it with echoc build --target-os wasi, then serve the directory (python3 -m http.server). Browsers refuse fetch of a .wasm from file://.

JS has to re-wrap memory.buffer after every call. memory.grow detaches the old ArrayBuffer, and an ImageData built from the detached one is a blank canvas and a quiet bug.

And one export call is one uninterrupted stretch of the page's main thread. A frame that takes a few hundred milliseconds, asked for in one call, freezes the page for that long. Asked for a few rows at a time, it doesn't.

Features

--target-features is the last word: a list you pass replaces the wasm baseline, it does not add to it. The baseline is +simd128, so --target-features +bulk-memory compiles without SIMD and LLVM scalarizes every simd<T, N>. Write both when you want both: --target-features +simd128,+bulk-memory.

Next

• Libraries and plugins for the same #[target: lib] on the host.
• C interop for extern and the types that may cross.
• Modules for #[target:].
• Attributes for export and import.