Any plans for first-class function support in sound type mode?

[lee-sihun]

Hi, I have a question about treating functions as values in Static Hermes' sound type mode.

Currently in sound type mode:

• Assigning a function to a variable → LLVM unreachable
• Passing a function as a parameter → no way to express function types
• Storing a function as an object property → same limitation as above

Because of this, using callback patterns or object method patterns in typed code requires relying on any casts and "use unsafe".

In #1269, you mentioned that callbacks are not yet supported but are planned, and in #1688, you mentioned WIP code for compile-time native callback wrappers. Are these related to function type support at the sound type level, or are they separate efforts?

Are there any plans or a roadmap for first-class function support in sound type mode?

[tmikov]

Functions work as values perfectly fine, as can be seen from this example:

$ cat t.js
function foo(a: number): number {
    return a + 1;
}
function callcb(cb: (number)=>number): number {
    return cb(10);
}
let t = foo;
print(callcb(t));

$ shermes -typed -exec t.js
11

[lee-sihun]

Thank you! This cleared up a big misconception on my end.

The reason I got confused was that when I first tried passing callbacks, I declared the parameter as callback: number and then cast it with any, instead of using a proper function type. That caused LLVM unreachable, so I jumped to the conclusion that functions couldn't be used as values in sound type mode. but the real issue was just the wrong type declaration.

After seeing your example, I retested with proper function types like (cb: () => void) => void and everything worked perfectly. Really appreciate it!

[tmikov]

Glad to help! Can you please post the example where you got the LLVM unreachable? I want to make sure it is not a new problem.

[lee-sihun]

Sorry, I used "LLVM unreachable" inaccurately in my previous comment.

The actual runtime error is Checked cast failed, not an LLVM unreachable. I initially suspected that the AOT compiler was emitting unreachable, but after checking the generated C code, I found that it does not in this case. That part of my previous comment was incorrect.

Reproduction:

function onEvent(event: string, callback: number): void {
  "use unsafe";
  (callback as any)();
}

(function(): void {
  "use unsafe";
  (onEvent as any)("test", function(): void {
    print("callback fired");
  });
})(); 

Passing a function to a parameter declared as callback: number causes Checked cast failed at runtime. This isn't a compiler bug — just a runtime type-mismatch caused by my incorrect type declaration. I used number here only as a workaround because I didn't know how to declare a function type at the time.