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Luka 04 - Don't `panic!`

teckert profile image Thomas Eckert ・5 min read

This post is part of a series on the development of a reverse polish notation calculator web-app using Rust/Wasm, Vue, and TypeScript. You can read more and follow along with development at the GitHub Repo.

At the end of Luka 03, I wasn't sure whether to work on connecting the Rust and TypeScript using Wasm or organize how state is passed in the Vue application. I decided to take on the former first. I think I'm going to hit some interesting walls that might affect how I structure the Vue application.

Finding References

I've connected Rust and JavaScript once in a tutorial, but I don't have a great understanding of how that connection works and how to set it up properly. That's part of the motivation for this project -- pushing myself to learn.

Here are some resources I'm reading through to understand the structure:

I don't think I'll need wasm-pack as I am adding Wasm to an existing project, but I downloaded the latest version anyway (0.8.1). I know I will need wasm-bindgen as a dependency.

wasm-bindgen = "0.2.50"
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I think we're off to a great start.

I see references to using wee_alloc to minimize the amount of Wasm that is generated. I think that sounds nice, but I won't start with that yet.

To the, I'm adding the import for wasm-bindgen decorators. All the tests still pass so that's good. It turns out I do need wasm-pack to build the .wasm file (probably another way to do it, but this is what's showing up in the tutorial).

First attempt at building the .wasm gives a good, informational error message.

Error: crate-type must be cdylib to compile to wasm32-unknown-unknown. Add the following to your Cargo.toml file:

crate-type = ["cdylib", "rlib"]
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Ok. I added that bit to the cargo.toml.

It... worked. I mean, I didn't decorate any functions so it didn't generate any functions on the wasm side, but still. Wow.

Now to decorate the functions. I'm not sure if I have to decorate every function or if I can get away with just decorating the struct and impl...

Well it failed, but it failed because the std::vec::Vec<f64> doesn't implement std::marker::Copy. Looking at the Rust docs, this is true for all Vec<T> because it's managing some other resource as evidenced by its implementation of Drop.

I can't pass a Vec<T>? That's confusing because how do they do that in the Universe struct here? I'm definitely not understanding something here.

If I truly can't pass a Vec<T>, I really only need to worry about the last two values of the stack in the Rust code at any given time if I have the TypeScript manage the full stack state. A workable solution could look like this:

  1. TypeScript holds state [4.5, 3, 99, 0.5].
  2. The "+" button is pressed.
  3. The last two values in the stack are passed to Rust (99, 0.5).
  4. Rust returns (99.5).
  5. TypeScript changes the stack to read [4.5, 3, 99.5].

This actually seems like a better design. It minimizes the amount of state passing and gives the stack a clear owner.

Have you tried rewriting this in Rust?

Instead of a State struct, everything in this new implementation is going to be a function. Every function will take in two numbers and return a Result type that will give the result of the numbers undergoing the operation or an error. For operations (+, -, *, /), there will be an operate function that takes in the two numbers and a string matching one of those operations then handing the state back to the TypeScript.

There is only one big problem. And it's one I would have had in the earlier implementation anyway.

The +, -, and * functions in Rust all return primitive types as if their input types are okay, there are no cases in which a primitive number cannot be returned. However, / has one horrific edge case: division by zero. If you are designing Rust, you have to make a choice. Either the / function is the only function that returns a Result or Option type, or the function panics if division by zero is attempted and the return type is primitive which matches the other functions ergonomically.

In the former case, handling division would be a whole mess:

//                                 Type
let a = 2                       // i32
let b = 0                       // i32

let added = 2 + 0               // i32
let subtracted = 2 - 0          // i32
let multiplied = 2 * 0          // i32
let divided = 2 / 0             // Err

match divided {
    Ok(quotient) => quotient,   // i32 (or some float maybe)
    Err(message) => message,    // String
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I mean, what? You want to unpack the quotient every time you divide? I think they made the right decision that if you try to divide by zero, division panics!. However, I don't know what this will do across the Wasm boundary. Here is one way I could solve it:

/// Perform the operation
pub fn operate(a: f64, b: f64, operation: String) -> Result<f64, String> {
    match &operation[..] {
        "+" => Ok(a + b),
        "-" => Ok(a - b),
        "*" => Ok(a * b),
        "/" => divide(a, b),
        _ => Err(format!(
            "Operation string `{}` does not match any allowed operation (+, -, *, /)",

fn divide(a: f64, b: f64) -> Result<f64, String> {
    // Preemptively catch the division by zero panic case.
    // See documentation for [Div](
    match b {
        0.0 => Err(String::from("Division by zero is not ok.")),
        _ => Ok(a / b),
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Except matching on a float won't be possible in future versions of Rust. This code, on compilation gives the error:

warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
note: for more information, see issue #41620 <>
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Wrap Up

I don't want to write code that won't work one day, so I'm not going to. Instead, I will need to figure out how to handle the panic! and pass back an Err to Vue. For now, I opened up this topic on the Rust-lang forum and am hoping someone is nice enough to help me out.

Starting Commit: dfa42833e39afbf33fc202581dc0564321dff15f

Ending Commit: 8da99f691e0b0bb50486b7bdeb348b1a8df8b1c4


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