Proposal: A built-in Go error check function, "try"

[griesemer]

Proposal: A built-in Go error check function, try

This proposal has been closed. Thanks, everybody, for your input.

Before commenting, please read the detailed design doc and see the discussion summary as of June 6, the summary as of June 10, and most importantly the advice on staying focussed. Your question or suggestion may have already been answered or made. Thanks.

We propose a new built-in function called try, designed specifically to eliminate the boilerplate if statements typically associated with error handling in Go. No other language changes are suggested. We advocate using the existing defer statement and standard library functions to help with augmenting or wrapping of errors. This minimal approach addresses most common scenarios while adding very little complexity to the language. The try built-in is easy to explain, straightforward to implement, orthogonal to other language constructs, and fully backward-compatible. It also leaves open a path to extending the mechanism, should we wish to do so in the future.

[The text below has been edited to reflect the design doc more accurately.]

The try built-in function takes a single expression as argument. The expression must evaluate to n+1 values (where n may be zero) where the last value must be of type error. It returns the first n values (if any) if the (final) error argument is nil, otherwise it returns from the enclosing function with that error. For instance, code such as

f, err := os.Open(filename)
if err != nil {
	return …, err  // zero values for other results, if any
}

can be simplified to

f := try(os.Open(filename))

try can only be used in a function which itself returns an error result, and that result must be the last result parameter of the enclosing function.

This proposal reduces the original draft design presented at last year's GopherCon to its essence. If error augmentation or wrapping is desired there are two approaches: Stick with the tried-and-true if statement, or, alternatively, “declare” an error handler with a defer statement:

defer func() {
	if err != nil {	// no error may have occurred - check for it
		err = …	// wrap/augment error
	}
}()

Here, err is the name of the error result of the enclosing function. In practice, suitable helper functions will reduce the declaration of an error handler to a one-liner. For instance

defer fmt.HandleErrorf(&err, "copy %s %s", src, dst)

(where fmt.HandleErrorf decorates *err) reads well and can be implemented without the need for new language features.

The main drawback of this approach is that the error result parameter needs to be named, possibly leading to less pretty APIs. Ultimately this is a matter of style, and we believe we will adapt to expecting the new style, much as we adapted to not having semicolons.

In summary, try may seem unusual at first, but it is simply syntactic sugar tailor-made for one specific task, error handling with less boilerplate, and to handle that task well enough. As such it fits nicely into the philosophy of Go. try is not designed to address all error handling situations; it is designed to handle the most common case well, to keep the design simple and clear.

Credits

This proposal is strongly influenced by the feedback we have received so far. Specifically, it borrows ideas from:

• Key Parts of Error Handling,
• issue #31442
• and, related, issue #32219.

Detailed design doc

https://github.com/golang/proposal/blob/master/design/32437-try-builtin.md

tryhard tool for exploring impact of try

https://github.com/griesemer/tryhard

[rasky]

I agree this is the best way forward: fixing the most common issue with a simple design.

I don't want to bikeshed (feel free to postpone this conversation), but Rust went there and eventually settled with the ? postfix operator rather than a builtin function, for increased readability.

The gophercon proposal cites ? in the considered ideas and gives three reason why it was discarded: the first ("control flow transfers are as a general rule accompanied by keywords") and the third ("handlers are more naturally defined with a keyword, so checks should too") do not apply anymore. The second is stylistic: it says that, even if the postfix operator works better for chaining, it can still read worse in some cases like:

check io.Copy(w, check newReader(foo))

rather than:

io.Copy(w, newReader(foo)?)?

but now we would have:

try(io.Copy(w, try(newReader(foo))))

which I think it's clearly the worse of the three, as it's not even obvious anymore which is the main function being called.

So the gist of my comment is that all three reasons cited in the gophercon proposal for not using ? do not apply to this try proposal; ? is concise, very readable, it does not obscure the statement structure (with its internal function call hierarchy), and it is chainable. It removes even more clutter from the view, while not obscuring the control flow more than the proposed try() already does.

[jimmyfrasche]

To clarify:

Does

func f() (n int, err error) {
  n = 7
  try(errors.New("x"))
  // ...
}

return (0, "x") or (7, "x")? I'd assume the latter.

Does the error return have to be named in the case where there's no decoration or handling (like in an internal helper function)? I'd assume not.

[ianlancetaylor]

Your example returns 7, errors.New("x"). This should be clear in the full doc that will soon be submitted (https://golang.org/cl/180557).

The error result parameter does not need to be named in order to use try. It only needs to be named if the function needs to refer to it in a deferred function or elsewhere.

[dominikh]

I am really unhappy with a built-in function affecting control flow of the caller. This is very unintuitive and a first for Go. I appreciate the impossibility of adding new keywords in Go 1, but working around that issue with magic built-in functions just seems wrong to me. It's worsened by the fact that built-ins can be shadowed, which drastically changes the way try(foo) behaves. Shadowing of other built-ins doesn't have results as unpredictable as control flow changing. It makes reading snippets of code without all of the context much harder.

I don't like the way postfix ? looks, but I think it still beats try(). As such, I agree with @rasky .

Edit: Well, I managed to completely forget that panic exists and isn't a keyword.

[griesemer]

The detailed proposal is now here (pending formatting improvements, to come shortly) and will hopefully answer a lot of questions.

[griesemer]

@dominikh The detailed proposal discusses this at length, but please note that panic and recover are two built-ins that affect control flow as well.

[nictuku]

One clarification / suggestion for improvement:

if the last argument supplied to try, of type error, is not nil, the enclosing function’s error result variable (...) is set to that non-nil error value before the enclosing function returns

Could this instead say is set to that non-nil error value and the enclosing function returns? (s/before/and)

On first reading, before the enclosing function returns seemed like it would eventually set the error value at some point in the future right before the function returned - possibly in a later line. The correct interpretation is that try may cause the current function to return. That's a surprising behavior for the current language, so a clearer text would be welcomed.

[purpleidea]

I think this is just sugar, and a small number of vocal opponents teased golang about the repeated use of typing if err != nil ... and someone took it seriously. I don't think it's a problem. The only missing things are these two built-ins:

https://github.com/purpleidea/mgmt/blob/a235b760dc3047a0d66bb0b9d63c25bc746ed274/util/errwrap/errwrap.go#L26

[webermaster]

Not sure why anyone ever would write a function like this but what would be the envisioned output for

try(foobar())

If foobar returned (error, error)

[dominikh]

I retract my previous concerns about control flow and I no longer suggest using ?. I apologize for the knee-jerk response (though I'd like to point out this wouldn't have happened had the issue been filed after the full proposal was available).

I disagree with the necessity for simplified error handling, but I'm sure that is a losing battle. try as laid out in the proposal seems to be the least bad way of doing it.

[ianlancetaylor]

@webermaster Only the last error result is special for the expression passed to try, as described in the proposal doc.

[akyoto]

Like @dominikh, I also disagree with the necessity of simplified error handling.

It moves vertical complexity into horizontal complexity which is rarely a good idea.

If I absolutely had to choose between simplifying error handling proposals, though, this would be my preferred proposal.

[cespare]

It would be helpful if this could be accompanied (at some stage of accepted-ness) by a tool to transform Go code to use try in some subset of error-returning functions where such a transformation can be easily performed without changing semantics. Three benefits occur to me:

• When evaluating this proposal, it would allow people to quickly get a sense for how try could be used in their codebase.
• If try lands in a future version of Go, people will likely want to change their code to make use of it. Having a tool to automate the easy cases will help a lot.
• Having a way to quickly transform a large codebase to use try will make it easy to examine the effects of the implementation at scale. (Correctness, performance, and code size, say.) The implementation may be simple enough to make this a negligible consideration, though.

[lestrrat]

I just would like to express that I think a bare try(foo()) actually bailing out of the calling function takes away from us the visual cue that function flow may change depending on the result.

I feel I can work with try given enough getting used, but I also do feel we will need extra IDE support (or some such) to highlight try to efficiently recognize the implicit flow in code reviews/debugging sessions

[Goodwine]

The thing I'm most concerned about is the need to have named return values just so that the defer statement is happy.

I think the overall error handling issue that the community complains about is a combination of the boilerplate of if err != nil AND adding context to errors. The FAQ clearly states that the latter is left out intentionally as a separate problem, but I feel like then this becomes an incomplete solution, but I'll be willing to give it a chance after thinking on these 2 things:

1. Declare err at the beginning of the function.
   Does this work? I recall issues with defer & unnamed results. If it doesn't the proposal needs to consider this.

func sample() (string, error) {
  var err error
  defer fmt.HandleErrorf(&err, "whatever")
  s := try(f())
  return s, nil
}

2. Assign values like we did in the past, but use a helper wrapf function that has the if err != nil boilerplate.

func sample() (string, error) {
  s, err := f()
  try(wrapf(err, "whatever"))
  return s, nil
}
func wrapf(err error, format string, ...v interface{}) error {
  if err != nil {
    // err = wrapped error
  }
  return err
}

If either work, I can deal with it.

[randall77]

func sample() (string, error) {
  var err error
  defer fmt.HandleErrorf(&err, "whatever")
  s := try(f())
  return s, nil
}

This will not work. The defer will update the local err variable, which is unrelated to the return value.

func sample() (string, error) {
  s, err := f()
  try(wrapf(err, "whatever"))
  return s, nil
}
func wrapf(err error, format string, ...v interface{}) error {
  if err != nil {
    // err = wrapped error
  }
  return err
}

That should work. It will call wrapf even on a nil error, though.
This will also (continue to) work, and is IMO a lot clearer:

func sample() (string, error) {
  s, err := f()
  if err != nil {
      return "", wrap(err)
  }
  return s, nil
}

No one is going to make you use try.

[singhpradeep]

Not sure why anyone ever would write a function like this but what would be the envisioned output for

try(foobar())

If foobar returned (error, error)

Why would you return more than one error from a function? If you are returning more than one error from function, perhaps function should be split into two separate ones in the first place, each returning just one error.

Could you elaborate with an example?

[griesemer]

@cespare: It should be possible for somebody to write a go fix that rewrites existing code suitable for try such that it uses try. It may be useful to get a feel for how existing code could be simplified. We don't expect any significant changes in code size or performance, since try is just syntactic sugar, replacing a common pattern by a shorter piece of source code that produces essentially the same output code. Note also that code that uses try will be bound to use a Go version that's at least the version at which try was introduced.

@lestrrat: Agreed that one will have to learn that try can change control flow. We suspect that IDE's could highlight that easily enough.

@Goodwine: As @randall77 already pointed out, your first suggestion won't work. One option we have thought about (but not discussed in the doc) is the possibility of having some predeclared variable that denotes the error result (if one is present in the first place). That would eliminate the need for naming that result just so it can be used in a defer. But that would be even more magic; it doesn't seem justified. The problem with naming the return result is essentially cosmetic, and where that matters most is in the auto-generated APIs served by go doc and friends. It would be easy to address this in those tools (see also the detailed design doc's FAQ on this subject).

[griesemer]

@nictuku: Regarding your suggestion for clarification (s/before/and/): I think the code immediately before the paragraph you're referring to makes it clear what happens exactly, but I see your point, s/before/and/ may make the prose clearer. I'll make the change.

See CL 180637.

[deanveloper]

I actually really like this proposal. However, I do have one criticism. The exit point of functions in Go have always been marked by a return. Panics are also exit points, however those are catastrophic errors that are typically not meant to ever be encountered.

Making an exit point of a function that isn't a return, and is meant to be commonplace, may lead to much less readable code. I had heard about this in a talk and it is hard to unsee the beauty of how this code is structured:

func CopyFile(src, dst string) error {
	r, err := os.Open(src)
	if err != nil {
		return fmt.Errorf("copy %s %s: %v", src, dst, err)
	}
	defer r.Close()

	w, err := os.Create(dst)
	if err != nil {
		return fmt.Errorf("copy %s %s: %v", src, dst, err)
	}

	if _, err := io.Copy(w, r); err != nil {
		w.Close()
		os.Remove(dst)
		return fmt.Errorf("copy %s %s: %v", src, dst, err)
	}

	if err := w.Close(); err != nil {
		os.Remove(dst)
		return fmt.Errorf("copy %s %s: %v", src, dst, err)
	}
}

This code may look like a big mess, and was meant to by the error handling draft, but let's compare it to the same thing with try.

func CopyFile(src, dst string) error {
	defer func() {
		err = fmt.Errorf("copy %s %s: %v", src, dst, err)
	}()
	r, err := try(os.Open(src))
	defer r.Close()

	w, err := try(os.Create(dst))

	defer w.Close()
	defer os.Remove(dst)
	try(io.Copy(w, r))
	try(w.Close())

	return nil
}

You may look at this at first glance and think it looks better, because there is a lot less repeated code. However, it was very easy to spot all of the spots that the function returned in the first example. They were all indented and started with return, followed by a space. This is because of the fact that all conditional returns must be inside of conditional blocks, thereby being indented by gofmt standards. return is also, as previously stated, the only way to leave a function without saying that a catastrophic error occurred. In the second example, there is only a single return, so it looks like the only thing that the function ever should return is nil. The last two try calls are easy to see, but the first two are a bit harder, and would be even harder if they were nested somewhere, ie something like proc := try(os.FindProcess(try(strconv.Atoi(os.Args[1])))).

Returning from a function has seemed to have been a "sacred" thing to do, which is why I personally think that all exit points of a function should be marked by return.

[s4n-gt]

Someone has already implemented this 5 years ago. If you are interested, you can
try this feature

https://news.ycombinator.com/item?id=20101417

I implemented try() in Go five years ago with an AST preprocessor and used it in real projects, it was pretty nice: https://github.com/lunixbochs/og

Here are some examples of me using it in error-check-heavy functions: https://github.com/lunixbochs/poxd/blob/master/tls.go#L13

[jasonmoo]

I appreciate the effort that went into this. I think it's the most go-ey solution I've seen so far. But I think it introduces a bunch of work when debugging. Unwrapping try and adding an if block every time I debug and rewrapping it when I'm done is tedious. And I also have some cringe about the magical err variable that I need to consider. I've never been bothered by the explicit error checking so perhaps I'm the wrong person to ask. It always struck me as "ready to debug".

[Goodwine]

@griesemer
My problem with your proposed use of defer as a way to handle the error wrapping is that the behavior from the snippet I showed (repeated below) is not very common AFAICT, and because it's very rare then I can imagine people writing this thinking it works when it doesn't.

Like.. a beginner wouldn't know this, if they have a bug because of this they won't go "of course, I need a named return", they would get stressed out because it should work and it doesn't.

var err error
defer fmt.HandleErrorf(err);

try is already too magic so you may as well go all the way and add that implicit error value. Think on the beginners, not on those who know all the nuances of Go. If it's not clear enough, I don't think it's the right solution.

Or... Don't suggest using defer like this, try another way that's safer but still readable.

[griesemer]

@deanveloper It is true that this proposal (and for that matter, any proposal trying to attempt the same thing) will remove explicitly visible return statements from the source code - that is the whole point of the proposal after all, isn't it? To remove the boilerplate of if statements and returns that are all the same. If you want to keep the return's, don't use try.

We are used to immediately recognize return statements (and panic's) because that's how this kind of control flow is expressed in Go (and many other languages). It seems not far fetched that we will also recognize try as changing control flow after some getting used to it, just like we do for return. I have no doubt that good IDE support will help with this as well.

[buchanae]

I have two concerns:

• named returns have been very confusing, and this encourages them with a new and important use case
• this will discourage adding context to errors

In my experience, adding context to errors immediately after each call site is critical to having code that can be easily debugged. And named returns have caused confusion for nearly every Go developer I know at some point.

A more minor, stylistic concern is that it's unfortunate how many lines of code will now be wrapped in try(actualThing()). I can imagine seeing most lines in a codebase wrapped in try(). That feels unfortunate.

I think these concerns would be addressed with a tweak:

a, b, err := myFunc()
check(err, "calling myFunc on %v and %v", a, b)

check() would behave much like try(), but would drop the behavior of passing through function return values generically, and instead would provide the ability to add context. It would still trigger a return.

This would retain many of the advantages of try():

• it's a built-in
• it follows the existing control flow WRT to defer
• it aligns with existing practice of adding context to errors well
• it aligns with current proposals and libraries for error wrapping, such as errors.Wrap(err, "context message")
• it results in a clean call site: there's no boilerplate on the a, b, err := myFunc() line
• describing errors with defer fmt.HandleError(&err, "msg") is still possible, but doesn't need to be encouraged.
• the signature of check is slightly simpler, because it doesn't need to return an arbitrary number of arguments from the function it is wrapping.

[griesemer]

@s4n-gt Thanks for this link. I was not aware of it.

[griesemer]

@Goodwine Point taken. The reason for not providing more direct error handling support is discussed in the design doc in detail. It is also a fact that over the course of a year or so (since the draft designs published at last year's Gophercon) no satisfying solution for explicit error handling has come up. Which is why this proposal leaves this out on purpose (and instead suggests to use a defer). This proposal still leaves the door open for future improvements in that regard.

[josharian]

The proposal mentions changing package testing to allow tests and benchmarks to return an error. Though it wouldn’t be “a modest library change”, we could consider accepting func main() error as well. It’d make writing little scripts much nicer. The semantics would be equivalent to:

func main() {
  if err := newmain(); err != nil {
    println(err.Error())
    os.Exit(1)
  }
}

[jtlapp]

#52175 proposes a variation of the try syntax that does not hide error behavior.

[guoliang1994]

我只是一个普通的 go 开发者，并且我也没有做很多底层的开发，主要用 go 来做web开发。从使用感觉上来讲，go 的错误处理机制真的很难用。真的很难理解为什么不用try catch？实现一套异常错误处理机制，就像大部分的语言一样。

这样用不是更好吗？（我自己使用 recover 机制实现了一个try catch）

1. 所有的函数将不会有错误返回值
2. 可以根据不同的异常做不同的处理
3. 代码大量减少

	response := NewJsonResponse(w, r)
	Try(func() {
         ...
           方法可能抛出各种异常
	 ...
	}).Catch(exception.NotLoggedIn{}, func(err error) {
		response.NeedLogin()
	}).Catch(exception.LoginExpired{}, func(err error) {
		response.LoginExpire()
	}).Catch(exception.NoPermission{}, func(err error) {
		response.PermissionDenied()
	}).Catch(exception.RedisBroken{}, func(err error) {
		response.RedisBroken()
	}).Catch(&httpErrs.HTTPError{}, func(err error) {
		lego.Error("系统可能正在受到 CC 攻击, 请求来源 ", r.RemoteAddr)
		response.RequestSpeedTooFast()
	}).CatchAll(func(err error) {
		response.SysError()
	}).Finally(func() {})

英文很差，见谅~

一点猜想

try catch 大概率不会被官方支持了，golang 的错误处理机制，积重难返。如果使用 try catch 这样的机制，golang的官方包，三方包统统都要改造~

[mfatihercik]

Although the discussion on this issue is concluded, I'd like to introduce my library, which offers a solution to the problem we've been addressing through try proposal.

One notable question from the try design document is:

If Go had “generics”, couldn’t we implement try as a generic function?
The implementation of try demands the ability to return from the function that calls try. Without a "super return" capability, try cannot be realized in Go, even with generic functions. Moreover, try necessitates a variadic parameter list that accepts different types. The anticipation for variadic generic functions of such nature is low.

My library leverages generics to replicate try's functionality, circumventing these limitations. It introduces distinct functions for handling functions with different return counts. For instance, Check1 is designed for functions with a single return value, while Check2 caters to those with two. This approach simplify function calls, exemplified by:

x := e.Check1(strconv.Atoi(a))

I know this issue is closed but I think it wort to share my library here that addressing the issue that we trying to solve with try or catch

One of the question in the try design document is as follow:

If Go had “generics”, couldn’t we implement try as a generic function?
Implementing try requires the ability to return from the function enclosing the try call. Absent such a “super return” statement, try cannot be implemented in Go even if there were generic functions. try also requires a variadic parameter list with parameters of different types. We do not anticipate support for such variadic generic functions.

I implemented a library that using generics to achieve the same functionality of the try. The difference in here for different return type I created a separate function.e.g Check1,work with function have 1 return, Check2 work with function have two return. In this way we can simplify calling functions
Example call will be:

x := e.Check1(strconv.Atoi(a))

To handle errors, the Check function triggers a panic. Errors can be caught using a defer statement alongside named returns in HandleReturn function to capture and assign the error.

Check function "panic" the error. Now we need to catch the error via defer statement.
We can use named return and HandleReturn function to catch error and set the error.

A complete usage example is as follows:

func printSum(a, b string) (err error) {
    defer e.HandleReturn(func (e error){
        err = e
    }
    x := e.Check1(strconv.Atoi(a))
    y := e.Check1(strconv.Atoi(a))
    fmt.Println("result:", x + y)
    return nil
}

Additionally, custom error handlers can be used with the Check function to tailor error handling or add context to errors.

Custom error handler can be used as follow:

func printSum(a, b string) (err error) {
    defer e.HandleReturn(func (e error){
        err = e
    }
    
    convertionError:= func (err error) error {
            return fmt.Errorf("failed to convert to int: %w", err)
     } 
    
    x := e.Check1W(strconv.Atoi(a)).With(convertionError)
    y := e.Check1W(strconv.Atoi(a)).With(convertionError)
    fmt.Println("result:", x + y)
    return nil
}

For more detailed examples and to explore the library further, please visit: https://github.com/mfatihercik/errless.

[zeroidentidad]

If it is a closed issue, why can it still be commented? There are a lot of comments over the years

[ianlancetaylor]

People can still discuss the idea even though we aren't going to adopt it.

[flysand7]