err2

The package extends Go's error handling with fully automatic error checking and propagation like other modern programming languages: Zig, Rust, Swift, etc. err2 isn't an exception handling library, but an entirely orthogonal package with Go's existing error handling mechanism.

func CopyFile(src, dst string) (err error) {
	defer err2.Handle(&err)

	r := try.To1(os.Open(src))
	defer r.Close()

	w := try.To1(os.Create(dst))
	defer err2.Handle(&err, err2.Err(func(error) {
		try.Out(os.Remove(dst)).Logf("cleaning error")
	}))
	defer w.Close()

	try.To1(io.Copy(w, r))
	return nil
}

go get github.com/lainio/err2

• Structure
• Performance
• Automatic Error Propagation
• Error handling
  • Error Stack Tracing
• Error Checks
  • Filters for non-errors like io.EOF
• Backwards Compatibility Promise for the API
• Assertion
  • Asserters
  • Assertion Package for Runtime Use
  • Assertion Package for Unit Testing
• Automatic Flags
  • Support for Cobra Flags
• Code Snippets
• Background
• Learnings by so far
• Support And Contributions
• Roadmap

Structure

err2 has the following package structure:

• The err2 (main) package includes declarative error handling functions.
• The try package offers error checking functions.
• The assert package implements assertion helpers for both unit-testing and design-by-contract.

Performance

All of the listed above without any performance penalty! You are welcome to run benchmarks in the project repo and see yourself.

Please note that many benchmarks run 'too fast' according to the common Go benchmarking rules, i.e., compiler optimizations (inlining) are working so well that there are no meaningful results. But for this type of package, where we compete with if-statements, that's precisely what we hope to achieve. The whole package is written toward that goal. Especially with parametric polymorphism, it's been quite the effort.

Automatic Error Propagation

The current version of Go tends to produce too much error checking and too little error handling. But most importantly, it doesn't help developers with automatic error propagation, which would have the same benefits as, e.g., automated garbage collection or automatic testing:

Automation is not just about efficiency but primarily about repeatability and resilience. -- Gregor Hohpe

Automatic error propagation is crucial because it makes your code change tolerant. And, of course, it helps to make your code error-safe:

The err2 package is your automation buddy:

1. It helps to declare error handlers with defer. If you're familiar with Zig language, you can think defer err2.Handle(&err,...) line exactly similar as Zig's errdefer.
2. It helps to check and transport errors to the nearest (the defer-stack) error handler.
3. It helps us use design-by-contract type preconditions.
4. It offers automatic stack tracing for every error, runtime error, or panic. If you are familiar with Zig, the err2 error traces are same as Zig's.

You can use all of them or just the other. However, if you use try for error checks, you must remember to use Go's recover() by yourself, or your error isn't transformed to an error return value at any point.

Error handling

The err2 relies on Go's declarative programming structure defer. The err2 helps to set deferred functions (error handlers) which are only called if err != nil.

Every function which uses err2 for error-checking should have at least one error handler. The current function panics if there are no error handlers and an error occurs. However, if any function above in the call stack has an err2 error handler, it will catch the error.

This is the simplest form of err2 automatic error handler:

func doSomething() (err error) {
    // below: if err != nil { return ftm.Errorf("%s: %w", CUR_FUNC_NAME, err) }
    defer err2.Handle(&err)

See more information from err2.Handle's documentation. It supports several error-handling scenarios. And remember that you can have as many error handlers per function as you need, as well as you can chain error handling functions per err2.Handle that allows you to build new error handling middleware for your own purposes.

Error Stack Tracing

The err2 offers optional stack tracing. It's automatic and optimized. Optimized means that the call stack is processed before output. That means that stack trace starts from where the actual error/panic is occurred, not where the error is caught. You don't need to search for the line where the pointer was nil or received an error. That line is in the first one you are seeing:

---
runtime error: index out of range [0] with length 0
---
goroutine 1 [running]:
main.test2({0x0, 0x0, 0x40XXXXXf00?}, 0x2?)
	/home/.../go/src/github.com/lainio/ic/main.go:43 +0x14c
main.main()
	/home/.../go/src/github.com/lainio/ic/main.go:77 +0x248

Just set the err2.SetErrorTracer or err2.SetPanicTracer to the stream you want traces to be written:

err2.SetErrorTracer(os.Stderr) // write error stack trace to stderr
// or, for example:
err2.SetPanicTracer(log.Writer()) // stack panic trace to std logger

If no Tracer is set no stack tracing is done. This is the default because in the most cases proper error messages are enough and panics are handled immediately by a programmer.

Note. Since v0.9.5 you can set these tracers through Go's standard flag package just by adding flag.Parse() to your program. See more information from Automatic Flags.

Read the package documentation for more information.

Error Checks

The try package provides convenient helpers to check the errors. Since the Go 1.18 we have been using generics to have fast and convenient error checking.

For example, instead of

b, err := io.ReadAll(r)
if err != nil {
        return err
}
...

we can call

b := try.To1(io.ReadAll(r))
...

but not without an error handler (err2.Handle). However, you can put your error handlers where ever you want in your call stack. That can be handy in the internal packages and certain types of algorithms.

In cases where you want to handle the error immediately after the function call return you can use Go's default if statement. However, we encourage you to use the errdefer concept, defer err2.Handle(&err) for all of your error handling.

Nevertheless, there might be cases where you might want to:

1. Suppress the error and use some default value.
2. Just write a logline and continue without a break.
3. Annotate the specific error value even when you have a general error handler.
4. You want to handle the specific error value, let's say, at the same line or statement.

The err2/try package offers other helpers based on the DSL concept where the DSL's domain is error-handling. It's based on functions try.Out, try.Out1, and try.Out2, which return instances of types Result, Result1, and Result2. The try.Result is similar to other programming languages, i.e., discriminated union. Please see more from its documentation.

Now we could have the following:

b := try.Out1(strconv.Atoi(s)).Logf("%s => 100", s).Catch(100)

The previous statement tries to convert incoming string value s, but if it doesn't succeed, it writes a warning to logs and uses the default value (100). The logging result includes the original error message as well.

It's easy to see that panicking about the errors at the start of the development is far better than not checking errors at all. But most importantly, err2/try keeps the code readable.

Filters for non-errors like io.EOF

When error values are used to transport some other information instead of actual errors we have functions like try.Is and even try.IsEOF for convenience.

With these you can write code where error is translated to boolean value:

notExist := try.Is(r2.err, plugin.ErrNotExist)

// real errors are cought and the returned boolean tells if value
// dosen't exist returned as `plugin.ErrNotExist`

Note. Any other error than plugin.ErrNotExist is treated as an real error:

1. try.Is function first checks if err == nil, and if yes, it returns false.
2. Then it checks if errors.Is(err, plugin.ErrNotExist) and if yes, it returns true.
3. Finally, it calls try.To for the non nil error, and we already know what then happens: nearest err2.Handle gets it first.

These try.Is functions help cleanup mesh idiomatic Go, i.e. mixing happy and error path, leads to.

For more information see the examples in the documentation of both functions.

Backwards Compatibility Promise for the API

The err2 package's API will be backward compatible. Before version 1.0.0 is released, the API changes occasionally, but we promise to offer automatic conversion scripts for your repos to update them for the latest API. We also mark functions deprecated before they become obsolete. Usually, one released version before. We have tested this with a large code base in our systems, and it works wonderfully.

More information can be found in the scripts/ directory readme file.

Assertion

The assert package is meant to be used for design-by-contract- type of development where you set pre- and post-conditions for your functions. It's not meant to replace the normal error checking but speed up the incremental hacking cycle. The default mode is to return an error value that includes a formatted and detailed assertion violation message. A developer gets immediate and proper feedback, allowing cleanup of the code and APIs before the release.

Asserters

The assert package offers a few pre-build asserters, which are used to configure how the assert package deals with assert violations. The line below exemplifies how the default asserter is set in the package.

assert.SetDefault(assert.Production)

If you want to suppress the caller info (source file name, line number, etc.) and get just the plain panics from the asserts, you should set the default asserter with the following line:

assert.SetDefault(assert.Debug)

For certain type of programs this is the best way. It allows us to keep all the error messages as simple as possible. And by offering option to turn additional information on, which allows super users and developers get more technical information when needed.

Note. Since v0.9.5 you can set these asserters through Go's standard flag package just by adding flag.Parse() to your program. See more information from Automatic Flags.

Assertion Package for Runtime Use

Following is example of use of the assert package:

func marshalAttestedCredentialData(json []byte, data *protocol.AuthenticatorData) []byte {
	assert.SLen(data.AttData.AAGUID, 16, "wrong AAGUID length")
	assert.NotEmpty(data.AttData.CredentialID, "empty credential id")
	assert.SNotEmpty(data.AttData.CredentialPublicKey, "empty credential public key")
	...

We have now described design-by-contract for development and runtime use. What makes err2's assertion packages unique, and extremely powerful, is its use for automatic testing as well.

Assertion Package for Unit Testing

The same asserts can be used and shared during the unit tests:

func TestWebOfTrustInfo(t *testing.T) {
	defer assert.PushTester(t)()

	common := dave.CommonChains(eve.Node)
	assert.SLen(common, 2)

	wot := dave.WebOfTrustInfo(eve.Node) //<- this includes asserts as well!!
	// And if there's violations during the test run they are reported as
	// test failures for this TestWebOfTrustInfo -test.

	assert.Equal(0, wot.CommonInvider)
	assert.Equal(1, wot.Hops)

	wot = NewWebOfTrust(bob.Node, carol.Node)
	assert.Equal(-1, wot.CommonInvider)
	assert.Equal(-1, wot.Hops)
	...

A compelling feature is that even if some assertion violation happens during the execution of called functions like the above NewWebOfTrust() function instead of the actual Test function, it's reported as a standard test failure. That means we don't need to open our internal pre- and post-conditions just for testing.

We can share the same assertions between runtime and test execution.

The err2 assert package integration to the Go testing package is completed at the cross-module level. Suppose package A uses package B. If package B includes runtime asserts in any function that A calls during testing and some of B's asserts fail, A's current test also fails. There is no loss of information, and even the stack trace is parsed to test logs for easy traversal. Packages A and B can be the same or different modules.

This means that where ever assertion violation happens during the test execution, we will find it and can even move thru every step in the call stack.

Automatic Flags

When you are using err2 or assert packages, i.e., just importing them, you have an option to automatically support for err2 configuration flags through Go's standard flag package. See more information about err2 settings from Error Stack Tracing and Asserters.

Now you can always deploy your applications and services with the simple end-user friendly error messages and no stack traces, but you can switch them on when ever you need.

Let's say you have build CLI (your-app) tool with the support for Go's flag package, and the app returns an error. Let's assume you're a developer. You can run it again with:

your-app -err2-trace stderr

Now you get full error trace addition to the error message. Naturally, this also works with assertions. You can configure their output with the flag asserter:

your-app -asserter Debug

That adds more information to the assertion statement, which in default is in production (Prod) mode, i.e., outputs a single-line assertion message.

All you need to do is to add flag.Parse to your main function.

Support for Cobra Flags

If you are using cobra you can still easily support packages like err2 and glog and their flags.

1. Add std flag package to imports in cmd/root.go:

   import (
       goflag "flag"
       ...
   )

2. Add the following to (usually) cmd/root.go's init function's end:

   func init() {
       ...
       // NOTE! Very important. Adds support for std flag pkg users: glog, err2
       pflag.CommandLine.AddGoFlagSet(goflag.CommandLine)
   }

3. And finally modify your PersistentPreRunE in cmd/root.go to something like:

   PersistentPreRunE: func(cmd *cobra.Command, args []string) (err error) {
       defer err2.Handle(&err)
   
       // NOTE! Very important. Adds support for std flag pkg users: glog, err2
       goflag.Parse()
   
       try.To(goflag.Set("logtostderr", "true"))
       handleViperFlags(cmd) // local helper with envs
       glog.CopyStandardLogTo("ERROR") // for err2
       return nil
   },

As a result you can have bunch of usable flags added to your CLI:

Flags:
      --asserter asserter                 asserter: Plain, Prod, Dev, Debug (default Prod)
      --err2-log stream                   stream for logging: nil -> log pkg (default nil)
      --err2-panic-trace stream           stream for panic tracing (default stderr)
      --err2-trace stream                 stream for error tracing: stderr, stdout (default nil)
      ...

Code Snippets

Most of the repetitive code blocks are offered as code snippets. They are in ./snippets in VC code format, which is well supported e.g. neovim, etc.

The snippets must be installed manually to your preferred IDE/editor. During the installation you can modify the according your style or add new ones. We would prefer if you could contribute some of the back to the err2 package.

Background

err2 implements similar error handling mechanism as drafted in the original check/handle proposal. The package does it by using internally panic/recovery, which some might think isn't perfect.

We have run many benchmarks try to minimise the performance penalty this kind of mechanism might bring. We have focused on the happy path analyses. If the performance of the error path is essential, don't use this mechanism presented here. But be aware that something is wrong if your code uses the error path as part of the algorithm itself.

For happy paths by using try.To* or assert.That error check functions there are no performance penalty at all. However, the mandatory use of the defer might prevent some code optimisations like function inlining. And still, we have cases where using the err2 and try package simplify the algorithm so that it's faster than the return value if err != nil version. (See the benchmarks for io.Copy in the repo.)

If you have a performance-critical use case, we always recommend you to write performance tests to measure the effect. As a general guideline for maximum performance we recommend to put error handlers as high in the call stack as possible, and use only error checking (try.To() calls) in the inner loops. And yes, that leads to non-local control structures, but it's the most performant solution of all. (The repo has benchmarks for that as well.)

The original goal was to make it possible to write similar code that the proposed Go2 error handling would allow and do it right now (summer 2019). The goal was well aligned with the Go2 proposal, where it would bring a try macro and let the error handling be implemented in defer blocks. The try-proposal was canceled at its latest form. Nevertheless, we have learned that using panics for early-stage error transport isn't bad but the opposite. It seems to help:

• to draft algorithms much faster,
• huge improvements for the readability,
• helps to bring a new blood (developers with different programming language background) to projects,
• and most importantly, it keeps your code more refactorable because you don't have to repeat yourself.

Learnings by so far

We have used the err2 and assert packages in several projects. The results have been so far very encouraging:

• If you forget to use handler, but you use checks from the package, you will get panics (and optionally stack traces) if an error occurs. That is much better than getting unrelated panic somewhere else in the code later. There have also been cases when code reports error correctly because the 'upper' handler catches it.

• Because the use of err2.Handle is so easy, error messages are much better and informative. When using err2.Handle's automatic annotation your error messages are always up-to-date. Even when you refactor your function name error message is also updated.

• When error handling is based on the actual error handlers, code changes have been much easier. There is an excellent blog post about the issues you are facing with Go's error handling without the help of the err2 package.

Support And Contributions

The package has been in experimental mode quite long time. Since the Go generics we are transiting towards more official mode. Currently we offer support by GitHub Discussions. Naturally, any issues and contributions are welcome as well!

Roadmap

Please see the full version history from CHANGELOG.

Latest Release

1.0.0

• Finally! We are very happy, and thanks to all who have helped!
• Lots of documentation updates and cleanups for version 1.0.0
• Catch/Handle take unlimited amount error handler functions
  • allows building e.g. error handling middlewares
  • this is major feature because it allows building helpers/add-ons
• automatic outputs aren't overwritten by given args, only with assert.Plain
• Minor API fixes to still simplify it:
  • remove exported vars, obsolete types and funcs from assert pkg
  • Result2.Def2() sets only Val2
• technical refactorings: variadic function calls only in API level