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0 pointsrandomdata4y ago0 comments

Your code is wrong. It would normally be written something like this:

  func read_file(filename string) string {
    panic(FileNotFound{filename})
  }

  func foo() {
    a := read_file("a.txt")
    b := read_file("b.txt")
    // do stuff with a and b
  }

  func main() {
    defer func() {
      if err, ok := recover().(FileNotFound); ok {
        fmt.Fprintf(os.Stderr, "file not found: %s\n%s\n", err.filename, err.Stacktrace())
      }
    }

    foo()
  }

However, exceptions are meant for exceptional circumstances (hence the name), not errors. A file error is not exceptional in the slightest. It is very much expected.

While you can overload exceptions to pass errors (or any other value), that does not mean you should. Your use of exceptions for flow control (i.e. goto) is considered harmful.

0 comments

linkdd4y ago

Ok I was wrongly assuming that panic was expecting an error type, in fact it's an interface{}.

> Your use of exceptions for flow control (i.e. goto) is considered harmful

Exceptions are a way to delegate error handling to the caller by giving them informations about the unexpected behavior. It implies that the expected behavior is the "happy path" (everything went well) and any deviations (errors) is unexpected.

This is far from a goto because you can have `try/finally` blocks without catch (or defer in golang).

Also, exceptions are just a kind of algebraic effects that do not resume. There was a proposal to JS for this: https://github.com/macabeus/js-proposal-algebraic-effects

This is also easier to test. assertRaises(ErrorType, func() { code... })

Almost every Go library I've seen just return an error (which is just a string), you'd need to parse it to assert that the correct error is returned in special conditions.

randomdataOP4y ago

> It implies that the expected behavior is the "happy path" (everything went well) and any deviations (errors) is unexpected.

Errors are the "happy path", though. Your network connection was lost for the data you were trying to transmit so you saved it to your hard drive instead means that everything went well! Throwing your hands up in the air and crashing your program because you had to make a decision is not something you would normally want to do. If statements are present in most happy paths for good reason. That the inputs presented you with a choice does not remove you from the happy path.

Now, if you made a programming mistake and tried to access an array index that is out of bounds, then there isn't much else you can do but crash. Exceptions are appropriate for that kind of problem. They are exceptional in that they should never happen. Errors, on the other hand, are expected to happen and you can happily deal with them.

linkdd4y ago

> Throwing your hands up in the air and crashing your program because you had to make a decision is not something you would normally want to do

And that's not something you do thanks to try/catch. You just handle the error where it's meaningful to handle it.

The happy path of "make a request" is that there is no network error.

The happy path of "make sure this request is sent" is that you handle the unexpected network error to save the request to disk for further retry.

If the disk is full, you're not on the happy path anymore.

In Erlang/Elixir, there is a philosophy of "let it crash" which is basically "delegate the error handling/recovery to where it's meaningful to do so". For example:

  start a supervised process to send a request
  if there is an unexpected network failure, let it crash
  the supervisor retries on its own

Or:

  start a process to send a request
  if there is an unexpected network failure, let it crash
  monitor the process to be notified when it crash
  do something if it happens, like saving the request to disk

A "write_file" function can return many kind of errors:

  - file not found (some folder in the path does not exist)
  - permission denied
  - disk full

When you call write_file, you might want to handle some of those errors, and delegate the handling of others to your caller.

You're still not addressing my main point. How do you check which errors you want to handle and which one you want to propagate with just a string describing your error ?

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linkdd4y ago

Ok I was wrongly assuming that panic was expecting an error type, in fact it's an interface{}.

> Your use of exceptions for flow control (i.e. goto) is considered harmful

This is far from a goto because you can have `try/finally` blocks without catch (or defer in golang).

Also, exceptions are just a kind of algebraic effects that do not resume. There was a proposal to JS for this: https://github.com/macabeus/js-proposal-algebraic-effects

This is also easier to test. assertRaises(ErrorType, func() { code... })

Almost every Go library I've seen just return an error (which is just a string), you'd need to parse it to assert that the correct error is returned in special conditions.

randomdataOP4y ago

> It implies that the expected behavior is the "happy path" (everything went well) and any deviations (errors) is unexpected.

linkdd4y ago

> Throwing your hands up in the air and crashing your program because you had to make a decision is not something you would normally want to do

And that's not something you do thanks to try/catch. You just handle the error where it's meaningful to handle it.

The happy path of "make a request" is that there is no network error.

The happy path of "make sure this request is sent" is that you handle the unexpected network error to save the request to disk for further retry.

If the disk is full, you're not on the happy path anymore.

In Erlang/Elixir, there is a philosophy of "let it crash" which is basically "delegate the error handling/recovery to where it's meaningful to do so". For example:

  start a supervised process to send a request
  if there is an unexpected network failure, let it crash
  the supervisor retries on its own

Or:

  start a process to send a request
  if there is an unexpected network failure, let it crash
  monitor the process to be notified when it crash
  do something if it happens, like saving the request to disk

A "write_file" function can return many kind of errors:

  - file not found (some folder in the path does not exist)
  - permission denied
  - disk full

When you call write_file, you might want to handle some of those errors, and delegate the handling of others to your caller.

You're still not addressing my main point. How do you check which errors you want to handle and which one you want to propagate with just a string describing your error ?

2 more replies

j / k navigate · click thread line to collapse