funcref_callbacks.rs

use eyre::Result;
use tinywasm::{types::FuncRef, Extern, FuncContext, Imports, Module, Store};

fn main() -> Result<()> {
    by_func_ref_passed()?;
    by_func_ref_returned()?;
    Ok(())
}

/// Example of passing Wasm functions (as `funcref`) to an imported host function
/// and the imported host function calling them.
fn by_func_ref_passed() -> Result<()> {
    // A module with:
    // - Imported function "host.call_this" that accepts a callback.
    // - Exported Wasm function "tell_host_to_call" that calls "host.call_this" with Wasm functions $add and $sub.
    // - Wasm functions $add and $sub and an imported function $mul used as callbacks
    //   (just to show that imported functions can be referenced too).
    // - Exported Wasm function "call_binop_by_ref", a proxy used by the host to call func-references of type (i32, i32) -> i32.
    const WASM: &str = r#"
    (module
        (import "host" "call_this" (func $host_callback_caller (param funcref)))
        (import "host" "mul" (func $host_mul (param $x i32) (param $y i32) (result i32)))
      
        (func $tell_host_to_call (export "tell_host_to_call")
            (call $host_callback_caller (ref.func $add))
            (call $host_callback_caller (ref.func $sub))
            (call $host_callback_caller (ref.func $host_mul))
        )
      
        (type $binop (func (param i32 i32) (result i32)))
        
        (table 3 funcref)
        (elem (i32.const 0) $add $sub $host_mul) ;; Function can only be referenced if added to a table.
        (func $add (param $x i32) (param $y i32) (result i32)
            local.get $x
            local.get $y
            i32.add
        )
        (func $sub (param $x i32) (param $y i32) (result i32)
            local.get $x
            local.get $y
            i32.sub
        )

        (table $callback_register 1 funcref)
        (func (export "call_binop_by_ref") (param funcref i32 i32) (result i32)
            (table.set $callback_register (i32.const 0) (local.get 0))
            (call_indirect $callback_register (type $binop) (local.get 1)(local.get 2)(i32.const 0))
        )
    )
    "#;

    let wasm = wat::parse_str(WASM).expect("Failed to parse WAT");
    let module = Module::parse_bytes(&wasm)?;
    let mut store = Store::default();
    let mut imports = Imports::new();

    // Import host function that takes callbacks and calls them.
    imports.define(
        "host",
        "call_this",
        Extern::typed_func(|mut ctx: FuncContext<'_>, fn_ref: FuncRef| -> tinywasm::Result<()> {
            let proxy_caller =
                ctx.module().exported_func::<(FuncRef, i32, i32), i32>(ctx.store(), "call_binop_by_ref")?;
            // Call the callback we got as an argument using "call_binop_by_ref".
            let res = proxy_caller.call(ctx.store_mut(), (fn_ref, 5, 3))?;
            println!("(funcref {fn_ref:?})(5,3) results in {res}");

            Ok(())
        }),
    )?;

    // Import host.mul function (one of the functions whose references are taken).
    imports.define(
        "host",
        "mul",
        Extern::typed_func(|_, args: (i32, i32)| -> tinywasm::Result<i32> { Ok(args.0 * args.1) }),
    )?;

    let instance = module.instantiate(&mut store, Some(imports))?;
    let caller = instance.exported_func::<(), ()>(&store, "tell_host_to_call")?;

    // Call "tell_host_to_call".
    caller.call(&mut store, ())?;
    // An interesting detail is that neither $add, $sub, nor $mul were exported,
    // but with a little help from the proxy "call_binop_by_ref", references to them are callable by the host.
    Ok(())
}

/// Example of returning a Wasm function as a callback to a host function
/// and the host function calling it.
fn by_func_ref_returned() -> Result<()> {
    // A module with:
    // - An exported function "what_should_host_call" that returns 3 `funcref`s.
    // - Wasm functions $add and $sub and an imported function $mul used as callbacks
    //   (just to show that imported functions can be referenced too).
    // - Another exported Wasm function "call_binop_by_ref", a proxy used by the host to call func-references of type (i32, i32) -> i32
    const WASM: &str = r#"
    (module
        (import "host" "mul" (func $host_mul (param $x i32) (param $y i32) (result i32)))
        (type $binop (func (param i32 i32) (result i32)))
        (table 3 funcref)
        (elem (i32.const 0) $add $sub $host_mul)
        (func $add (param $x i32) (param $y i32) (result i32)
            local.get $x
            local.get $y
            i32.add
        )
        (func $sub (param $x i32) (param $y i32) (result i32)
            local.get $x
            local.get $y
            i32.sub
        )
        (func $ref_to_funcs (export "what_should_host_call")  (result funcref funcref funcref)
            (ref.func $add)
            (ref.func $sub)
            (ref.func $host_mul)
        )

        (table $callback_register 1 funcref)
        (func $call (export "call_binop_by_ref") (param funcref i32 i32) (result i32)
            (table.set $callback_register (i32.const 0) (local.get 0))
            (call_indirect $callback_register (type $binop) (local.get 1)(local.get 2)(i32.const 0))
        )
    )
    "#;

    let wasm = wat::parse_str(WASM).expect("Failed to parse WAT");
    let module = Module::parse_bytes(&wasm)?;
    let mut store = Store::default();
    let mut imports = Imports::new();

    // Import host.mul function (one of the possible operations).
    imports.define(
        "host",
        "mul",
        Extern::typed_func(|_, args: (i32, i32)| -> tinywasm::Result<i32> { Ok(args.0 * args.1) }),
    )?;

    let instance = module.instantiate(&mut store, Some(imports))?;

    // Ask the module what to call.
    let funcrefs = {
        let address_getter =
            instance.exported_func::<(), (FuncRef, FuncRef, FuncRef)>(&store, "what_should_host_call")?;
        address_getter.call(&mut store, ())?
    };

    let proxy_caller = instance.exported_func::<(FuncRef, i32, i32), i32>(&store, "call_binop_by_ref")?;

    for (idx, func_ref) in [funcrefs.0, funcrefs.1, funcrefs.2].iter().enumerate() {
        // Call those `funcref`s via "call_binop_by_ref".
        let res = proxy_caller.call(&mut store, (*func_ref, 5, 3))?;
        println!("At idx: {idx}, funcref {func_ref:?}(5,3) results in {res}");
    }
    Ok(())
}