Go 1.18 with some fuzzing and a focus on generics

Go 1.181 is the biggest release since the Go version 1.0 came out in 2012. What made this release so important are two major features that got introduced to Go, those are:

  • generics
  • and fuzz testing support.

Before we go into more detail about the last two features, let's look at the general innovations first.

Regardless of Go 1.18 being a larger than normal release, it will be no exception of the rule, meaning the compatibility promise still holds and there will be no breaking changes. This is what makes Go a viable choice for production code.


Binaries compiled with the new version will automatically include version control information. This data can be accessed at runtime through runtime/debug.ReadBuildInfo or by running go version -m path/to/binary. A common workaround that is often used, and will be obsolete now, is to set version information on compile time using a go build flag like -ldflags "-X main.Version=$VERSION".

$ go1.18rc1 version -m hello
hello: go1.18rc1
        path    hello:
        mod     hello   (devel)
        build   -compiler=gc
        build   CGO_ENABLED=1
        build   CGO_CFLAGS=
        build   CGO_CPPFLAGS=
        build   CGO_CXXFLAGS=
        build   CGO_LDFLAGS=
        build   GOARCH=arm64
        build   GOOS=darwin
        build   vcs=git
        build   vcs.revision=9b40c4e23a9f695c5f2adb7886d7d45f3e395eb7
        build   vcs.time=2022-02-19T11:59:45Z
        build   vcs.modified=true


Tailscales' custom IP address struct landed in the standard library. In contrast to net.IPAddr this type is immutable, takes less memory and is comparable making it usable as a map key. Brad Fitzpatrick did a talk about Go at Tailscale in FOSDEM 2021 where he goes into the details why they needed a new IP type.

Miscellaneous changes

With strings.Cut() you can cut a string around the first instance of a separator, e.g. strings.Cut("a,b,c", ",") will result in "a", "b,c", true, where the last argument indicates if the separator was found or not. The strings.Title() function for making a string title-case is now deprecated because it doesn't handle unicode properly, use x/test/cases.Title instead.

Iteration in range pipelines of html/template or text/template templates can now be ended with {{break}} or the next iteration can be started immediately with {{continue}}.

Mutexes from sync now have non blocking TryLock/TryRLock methods that try to acquire the lock if it's not currently held.

Note that while correct uses of TryLock do exist, they are rare, and use of TryLock is often a sign of a deeper problem in a particular use of mutexes.

A small MaxBytesHandler middleware was added to the http package. This middleware limits the amount of data read from a request body or written in a response. Most Go HTTP routers and web frameworks already provide such a middleware, often to prevent denial-of-service attacks.

The go vet linting tool was updated to work with generic functions and types. Did you know that go vet is automatically run when you go test your application?

Our beloved formatting tool gofmt now reads and formats source code files concurrently making it significantly faster. To be honest, formatting speed was never an issue for any Go code base I worked with.

TLS 1.0 and TLS 1.1 are disabled by default now for client connections, the server side still allows TLS 1.0.


Go now supports four microarchitecture levels for the amd64 target. A specific level can be selected using the GOAMD64=<level> environment variable. To compile a binary for a recent x86-64 CPU that supports AVX instructions run GOAMD64=v3 GOARCH=amd64 go build my/application. A higher/newer microarchitecture version does not necessarily result in better performance, which means you need to benchmark. The actual performance benefit should be negligible for most web servers.

In the previous release the calling convention for functions on x86-64 targets switched from stack to register based, resulting in a minor speedup and slightly smaller binaries. Support for register based calling convention is now expanded to arm642 and amd64 on all operating systems.

Workspace Mode

The module workspace proposal was accepted and implemented. Workspace mode is activated if the go tool finds a go.work file in the current directory or any of its parents. A go.work file might look like this:

go 1.18

use ../foo/bar
use ./baz

replace example.com/foo v1.2.3 => example.com/bar v1.4.5

Note that, as for any other go.* file, there is a subcommand that should be used to create or edit its contents. In this case the command is go work. In workspace mode you can easily use development versions of a library for many go modules, without having to write a replace directives in each of every go.mod files. In the example above, that was copied from go help work, ./baz could be a fork of github.com/go-redis/redis or any other dependency you need to modify locally.


Regular unit tests use a fixed set of inputs to test a function, this often includes simple success cases and inputs that represent corner cases, e.g. the empty string, nil so on and so forth.

Fuzz testing, or fuzzing, on the other hand generates random inputs to test functions with the goal to trigger bugs. You can imagine that the search space, from which random inputs are taken, gets huge very quickly. Hence, fuzz testing implementations often use techniques to reduce the search space, e.g. instrumenting your code or some form of heuristics like starting with extreme values of an input type, those could be NaN, -/+inf for a float type or invalid unicode strings.

Anyways, we don't have to care about the implementation details since the Go team did all the hard work of implementing a fuzzing framework for us and we can just use it. A starting point to learn about fuzz testing in Go is the fuzzing landing page that also links to the official tutorial.

// copied from https://pkg.go.dev/testing@master#hdr-Fuzzing
func FuzzHex(f *testing.F) {
  for _, seed := range [][]byte{{}, {0}, {9}, {0xa}, {0xf}, {1, 2, 3, 4}} {
  f.Fuzz(func(t *testing.T, in []byte) {
    enc := hex.EncodeToString(in)
    out, err := hex.DecodeString(enc)
    if err != nil {
      t.Fatalf("%v: decode: %v", in, err)
    if !bytes.Equal(in, out) {
      t.Fatalf("%v: not equal after round trip: %v", in, out)

Fuzz tests look very similar to regular unit tests but their name must start with Fuzz. Inputs to the seed corpus, a set of default test inputs, are registered using f.Add(). A fuzz test must have only one fuzz target, that is the function passed to f.Fuzz() which takes an instance of *testing.T as their first argument. All remaining arguments must be in same order and type as those added to the seed corpus with f.Add(). To enable fuzzing pass the -fuzz=FuzzMyTest flag to go test. If the flag was not specified fuzz tests are still run, but only with their fixed seed corpus as input. Only a limited set of types is allowed as fuzz inputs, primitives, string and []byte among others.

Since there is no control over the inputs of a fuzz tested function only properties of it can be tested. Properties could be that the functions result is still a valid unicode string or that a number is in a certain range.

Be aware that fuzzing can create several gigabytes of data in the fuzz cache and might cause high load on the test machine. I would therefore advise against enabling fuzz tests in a CI pipeline.

Should fuzzing detect a bug in your code, consider adding it to the trophy case.


Generics might be the most controversial feature ever added to Go language. Not having Generics was one of Go's "features" since they make a language certainly more complex. It took multiple language change proposal until we landed with the type parameters proposal that was eventually implemented with Go 1.18. There was quite some buzz around that proposal and everyone and their mother wrote blog posts about using generics with Go, and so did I.

Let's start with a definition:

Generics allow you to write one, generic, function for a set of types without relying on reflection. The compiler will then implement one version of your generic function for every concrete type it was instantiated with.

This might sound intimidating to some, so here's an example:

func min[T int | uint | int64](x, y T) T {
	if x <= y {
		return x
	return y

What's different from a normal Go function is that there is a type parameter list in square brackets [T int | uint | int64] before the function arguments. Here we can define the set of types the function is allowed to be instantiated with. Thanks to Go's type inference we don't have to explicitly write out the type when calling the function, instead we can just do min(1, -5) and the compiler will be smart enough to infer the type for us.

One last minute change to the release was that the constraints package was moved out of the standard library. This package predefined a set of common interfaces, e.g. one for signed or unsigned integer numbers. This brings us to the next point, that is we don't need to define the set of allowed types inline, instead we can define them in an interface like:

type Signed interface {
	~int | ~int8 | ~int16 | ~int32 | ~int64

and then define our min function as

func min[T Signed](x, y T) T {
	if x <= y {
		return x
	return y

A new operator is shown in the example above, the tilde operator ~T specifies the underlying type of T. Sounds confusing but is actually pretty simple, in Go one often defines type alias, e.g. type MyEnum string and if a function only accepts [T string] then it could not be instantiated with an argument of type MyEnum even though the argument is a string. But [T ~string] on the other hand will happily accept any type whose underlying type is string, making the generic function much more useful.

For some functions there is no need to restrict the set of type at all and in such cases you can use [T interface{}], or even better, use any instead of interface{}. An example of such a function with great "general" use is the following one that returns a pointer to any type3:

func PointerOf[T any](t T) *T {
	return &t

Parameter lists are by no means limited to a single type. I hope the following function name leaves no doubt that this is a contrived example without practical use:

func weirdAddYankovic[T1 int | uint, T2 float32 | float64](x T1, y T2) string {
	sum := float64(x) + float64(y)
	return strconv.FormatFloat(sum, 'f', -1, 32)

Besides functions, type parameters can also be applied to structs and even interface's can take them:

type Container[T any] struct {
    elements []T

// c := Container[int]{elements: []int{2,3,4}}

For some reason Go was not able to infer T in the example above and so it must be instantiated explicitly.

Be aware that using generics comes with downsides, one of them is about 15% slower compile times at the moment, increased binary size since the compiler will generate a variant of the generic function or struct for any type it was instantiated with. Most importantly, in my opinion, it reduces the readability of your code, so think twice before adding type parameters.

Do not use type parameters when a regular interface would suffice:

// Wrong: func Write[T io.Writer](w T) error {}
func Write(w io.Writer) error {}

Also, do not use generics if the implementation of a common methods is different for each type.

Generics are no panacea and there are limitations with the current implementation of this feature. Most notably type parameters cannot be used in methods. This limitation might be lifted with Go 1.19 and there is also a workaround the facilitator pattern.

No type parameters will be used in the standard library with Go 1.18. I think it's a sensible decision of the Go team to first gather some experience with how the feature is used and then in a later release introduce generics to the standard library. However, there are already some generic utility functions for slices and maps. What I anticipate is a library that provides Rust iterator like functionality for slices, maps and sets.

I hope I could give you some viable insights into the features and changes that Go 1.18 will bring. Now go and write some, potentially generic, code!

  1. For this article Go 1.18 release candidate 1 was used. This release can be installed by running go install golang.org/dl/go1.18rc1@latest && go1.18rc1 download. Any other details were mostly found in the preliminary release notes↩︎

  2. Also for PowerPC, but I don't know anyone targeting those CPUs. ↩︎

  3. Someone took the opportunity to implement a new package↩︎