[Question] Try catch support

Is there any support for using try/catch in AS?
Currently the compiler says "Operation not supported".
I've found this old issue: https://github.com/AssemblyScript/prototype/issues/103
Though when looking at the webassembly link: https://github.com/WebAssembly/exception-handling/blob/master/proposals/Exceptions.md
it is talking about try/catch blocks. What's the state for it in Assemblyscript?

Unfortunately, the exception-handling spec hasn't landed yet and we are waiting for it as well. What AS does currently is converting throw statements to a an abort(...); unreachable(); (simply put), which cannot be caught.

Thanks for the clarification :)

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

Does that mean we cannot detect whether an exception occurred? For example, if I want to write a test and assert that an exception has occurred, is there anyway to do that?

You could throw exceptions but can't catch or rethrow it currently without interrupt module's execution. Current WebAssembly MVP cupport only global traps but this feature extension will allow support fully featured try/catch/finally in near future

Background: Doing throw new Error("something") currently compiles to an unreachable instruction that cannot be safely recovered from. You can catch an unreachable on the JS side of course, but it's not safe because the internal state might be broken as soon as that occurs. Or, short: Don't use exceptions yet unless the exception is fatal and not meant to be caught (but to abort the program).

I see. So what's the recommended way of handling non-fatal exceptions?

So far I've used return values to indicate the outcome of an operation that might fail. Fanciest one can do probably is

enum Status {
  OK,
  ERROR_A,
  ERROR_B
}

function doSomething(): Status {
  ...
}

Languages like Go and Rust haven't support exceptions and usually use Optional types or tuples / objects during return result. You could follow same approach:

function parseArray<T>(str: string): T[] | null {
   if (isValid(str)) return parseArr(str)
   return null;
}

// or
class Result<T> {
   value: T | null = null;
   error: string | null = null;
}

function getRequestResult(url: string): Result {
    let body = ...
    if (body == null) {
      return { value: null, error: "can't request from url" };
    } else {
      return { value: body, error: null };
    }
}

Unfortunately, the exception-handling spec hasn't landed yet and we are waiting for it as well. What AS does currently is converting throw statements to a an abort(...); unreachable(); (simply put), which cannot be caught.

It's so funny that I'm stuck because the language doesn't have the most basic exception handling. Do you get that? your language can't do try-catch..why work on anything else other than this?

Reopen this issue, there's still no try/catch support.

Essentially, throw in AssemblyScript is not at all like throw in TypeScript or JavaScript. It's more like panic in Go, and there is no equivalent of Go's recover.

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[Question] Try catch support #302

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[Question] Try catch support #302

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dcodeIO commented on Oct 16, 2018

t57ser commented on Oct 16, 2018

stale commented on Feb 8, 2019

bowenwang1996 commented on May 13, 2019

MaxGraey commented on May 13, 2019

dcodeIO commented on May 13, 2019

bowenwang1996 commented on May 13, 2019

dcodeIO commented on May 13, 2019

MaxGraey commented on May 13, 2019

thebakersteve commented on Feb 13, 2022

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mattjohnsonpint commented on Mar 2, 2025

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