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SGo: a safer Go dialect Build Status Gitter GoDoc Go Report Card codecov

Disclaimer: alpha stage!

SGo is a dialect of the Go programming language that avoids nil-related panics at compile time. It is based on idiomatic Go patterns, so SGo code feels familiar, and straightforwarldy compiles to and works together with plain Go.

It looks like this:

// var p *Something = nil // Wrong! Doesn't compile.
var p *Something = &Something{} // OK!
_ = *p // OK! And it won't ever crash, as p can't be nil.

var op ?*Something = nil // OK!
// _ = *op // Wrong! Doesn't compile.
if op != nil {
	_ = *op // OK! You checked that op is not nil.
func giveMeSomethingMaybe() (*Something \ error) { ... }

s \ err := giveMeSomethingMaybe()

// _ = *s // Wrong! Doesn't compile.

if err != nil {
_ = *s // OK! You checked that err is nil, and thus s is usable.

See it running, plus its Go translation, here!

Table of contents


Provided you've correctly set up Go, this should work:

go get

A quick comparison with plain Go

So, what does SGo buy me? Let's see a comparison with potentially crashing Go code:

// Plain Go code

type Response struct {
	Body io.ReadCloser
response := &Response{Body: nil}
response.Body.Close() // This line crashes! response.Body is nil.

In SGo, this situation is avoided altogether. This line:

response := &Response{Body: nil} // Doesn't compile in SGo! An interface, like io.ReadCloser, can't be nil.

wouldn't compile: an io.ReadCloser isn't allowed be nil. You are forced to actually give something that can be readed and closed there.

In case you need something to accept nil as a value, you have to put a ? before its type:

type Response struct {
	Body ?io.ReadCloser

Only that in this case, this line wouldn't compile:

response.Body.Close() // Doesn't compile in SGo! You can't call methods on a ?-prefixed interface. 

Instead, SGo forces you to check that response.Body is not nil before using it, like this:

if body := response.Body; body != nil {

Note that this is something that you should be doing anyway in plain Go, if response.Body being nil is a real possibility. SGo just takes that and makes it mandatory.

This applies also to channels, maps, pointers, and functions:

var ch chan string = nil // Doesn't compile in SGo; channels can't be nil without ?.
ch <- "Hello!" // Doesn't compile in SGo; would cause panic.

var m map[string]int = nil // Also doesn't compile in SGo, because...
_ = m["needle"]            // ... would cause a panic if used before initialized.

var f func() = nil // Same deal...
f()                // ... because this would panic.

The billion dollar mistake

"It was the invention of the null reference in 1965. [...] This has led to innumerable errors, vulnerabilities, and system crashes, which have probably caused a billion dollars of pain and damage in the last forty years."

C. A. R. Hoare, inventor of nil

nil, in Go, is used to represent the idea of "you expected something to be here, but there is nothing" for certain types (pointers, slices, interfaces, maps, channels, and functions). This is a very useful concept; not only it can be used by the logic of your programs to express lack of something (e. g. a http.Request can have no body, in which case its Body field is nil), but also provides a meaningful way of initializing variables and fields of those types.

nil has a problem, though. Sometimes the code expect something to be something, but it is nil instead. In those situations, the program usually crashes; those are the infamous invalid memory address or nil pointer dereference panics. There is no way of preventing those situations except carefully reasoning about what do you write, and hoping that testing catches all possible misses. of This is often not the case. Even when it is, the process of achieving it is costly, if not because of crashes in production, because of developer effort spent on it.

In SGo, this nothingness concept is represented in a way that the compiler can track, so those situations never cause a crash while running the program, but simply prevent the program from even compiling.

Optional types

In SGo, pointers, interfaces, maps, channels and functions by themselves can't be nil. That is, if you get a pointer, you know that it is pointing somewhere; if you get an interface, you know there's some object implementing it that you can call methods on; if you get a map, you know you can immediately get and put things in it; if you get a channel, you know you can send and receive things through it without necessarily blocking forever and, if you get a function, you know you can call it. There's no "hey, I know you asked for this thing but I give you nothing instead". (Slices still can be nil; you can't do anything with it that would cause a nil pointer dereference, so there's no point forbidding that.)

If you do want to provide the option of not having something, you use optional types.

An optional wraps another type. It is denoted by prefixing some other type with a ?, and it reads like "an optional...", optionally a..." or "maybe a...". Some examples:

func (m *Map) Find(key string) ?*Value  { ... }

func (t *Template) Execute(w io.Writer, data ?interface{}) ?error  { ... }

type TreeNode struct {
	Value interface{}
	Left ?*TreeNode
	Right ?*TreeNode

There are two kind of values you can assign to an optional:

  • A value of its wrapped type.
  • nil
var maybeP ?*P // It is initially nil. Nil is an optional's zero value.
maybeP = &P{}  // You can assign a *P to it...
maybeP = nil   // ... and nil back. (Remember, here nil is _not_ a *P.)

The key thing about optionals is that you can't do anything with them. You can't call its wrapped type's methods on them, you can't call them if they are wrapping functions, you can put or get stuff in them if they are wrapping a map. Basically, you can't perform any operation that would cause the program to crash if the optional were nil. This is the way SGo makes sure this kind of situation doesn't happen.

err := template.Execute(w, 123)
// err has type ?error. The next line wouldn't compile, even if it
// would compile if err had type error.
// fmt.Println(err.Error())

The only thing that you can do with an optional is compare them with nil. By comparing an optional with nil, you can prove that in certain parts of the code an optional does have something instead of nothing, ie. it is not nil. Only in those parts you can then use the optional as its wrapped type.

err := template.Execute(w, 123)
if err != nil {
	// Here it is proved that err is not nil. It must be safe to use
	// it as an error instead of as an ?error, e. g. calling its Error()
	// method. SGo's compiler notices that, and it indeed converts err
	// to type error.
// Back outside of the if, err is again an ?error and you can't do
// anything with it.

By returning early, you can also prove that an optional is not nil pass that return.

func (m *Map) Find(key string) ?*Value  { ... }

maybeNeedle := haystack.Find("needle")
if maybeNeedle == nil {
// Because it's a 100% sure thing that we wouldn't reach this point if
// maybeNeedle were nil, we know that there is a needle something in
// maybeNeedle. The compiler thus lets us use maybeNeedle as a *Value
// until the end of the function.

To be precise about what "proving" means, and which are those parts of the code in which this is proven:

  • In a branch of an if, if-else or switch statement, if the branch condition requires that a variable of type ?T is not equal to nil, then the variable has type T instead inside that branch.
  • Given an if, if-else or switch statement in a block, if some branch condition requires that a variable in the block's scope of type ?T is equal to nil, and that branch makes the block return, break or continue, then the variable has type T instead in every statement below in that same block.

In short, a variable of type ?T has type T instead in a statement if the statement is only reachable when the variable is not nil.

Entangled optionals

It is a very common Go idiom to use multiple returns, such that one of them makes sense only if the other one is nil, true, or a similarly special value. We see this mainly when returning something may fail:

// Go code; not SGo.
response, err := http.Get("")
// You could use response here, but if err != nil it wouldn't be pretty.
if err != nil {
	// You shouldn't use response here. It is probably nil.
	return err
// It is not enforced by the compiler, but you are told by the documentation
// to trust response not to be nil here, given that err is nil.

SGo leverages optionals to also capture this pattern in a safer yet equally convenient manner.

func Get(url string) (*Response \ error) { ... }

response, err := http.Get("")
// You can't use response yet; it wouldn't compile.
if err != nil {
	// You can't use response yet; it wouldn't compile.
	return err
// Now it _is_ enforced by the compiler that response is _not_
// nil here.

A function signature may have at the end of its return list a backslash \ followed by a type (or a named return with a type), this type being a pointer, map, interface, channel, or function. (It will typically be the error interface.)

When calling this function, the last returned value will be an optional. Only once proved, as defined above, that this optional is nil you will be able to use the rest of the returned values. (Hence the name "entangled", inspired by quantum entanglement, in which collapsing the wavefunction of a particle also causes a collapse in a separate particle that is entangled with it.)

Let's see how to define a function that returns an entangled optional:

func NewRequest(method string, url string, body ?io.Reader) (*Request \ error) { ... }

func Get(url string) (*Response \ error) {
	req \ err := NewRequest("GET", url, nil)
	if err != nil {
		return \ err

	response, err = http.DefaultClient.Do(req)
	if err != nil {
		return \ err

	return response \
	// Just 'return response, err' would work here too.

You can entangle any number of variables with an optional, not just one.

Try in browser!

func Divide(dividend, divisor int64) (quotient int64, remainder int64 \ err error) {
	if divisor == 0 {
		err = errors.New("div by zero")
	quotient = dividend / divisor
	remainder = dividend % divisor

Entangled bools

The same idiom works for booleans, too. It's typical to use an "ok" boolean last return value to indicate whether the other return values are valid or not.

Try in browser!

func IndexOf(needle int, haystack []int) (index int \ ok bool) {
	for i, v := range haystack {
		if v == needle {
			return i \
	return \ false

Of course, you can still use the old (T, bool) multiple return. However, this way SGo forces you to get the logic right. For example, you aren't allowed to return \ true; and you aren't allowed to use the entangled return values until the associated boolean return value is proven to be true.

Comma-OK assignments

In Go, when performing some operations (receiving from a channel, type-asserting, reading from a map), you can additionally get a second boolean return value that tells you whether the operation succeeded.

// Go code; not SGo.
m = map[int]string{456: "foo"}
v, ok := m[123] // v is "", ok is false
v, ok = m[456]  // v is "foo", ok is true

m = m = map[int]interface{}{456: nil, 789: "qux"}
v, ok = m[123]  // v is nil, ok is false
v, ok = m[456]  // v is nil, ok is true
v, ok = m[789]  // v is "qux", ok is true

In SGo, such operations return an entangled bool instead.

Try in browser!

m = map[int]string{456: "foo"}
v \ ok := m[123] // v is uninitialized, ok is false
// fmt.Println(v) doesn't compile; ok might be false.
if ok {

In fact, when the first return type from such operations (receiving from a channel, reading from a map) is a pointer, map, interface, channel, or function, SGo will forbid you to perform it without expecting a second "OK" value. This is because those types don't have a zero value in SGo, so you need to make sure the operation succeeds.

Representation in Go code

Optionals and entanglement introduce absolutely no runtime costs. You can translate from SGo to Go in your head just by removing the ?s and the \s. When in SGo you assign nil to an optional variable, in Go you assign nil to a variable of the wrapped type. The only difference is that the resulting Go code is proven to be safe to execute (as in "won't crash due to nil") by the SGo compiler.

This is why only pointers, maps, interfaces, channels and functions can be wrapped in optionals. Those are the types which in Go can be nil. (Slices are excluded from this protection, since a nil slice is exactly as safe as a slice with zero elements. You can and should still use nil slices.) SGo keeps Go's feature that memory representation is totally obvious at all points, and doesn't introduce new, unfamiliar memory layouts such as tagged unions. Although it can be handy to have ?string, or ?int, that would defeat this purpose. You can either continue to use "" and 0 or -1 as nothingness for those types, or use an entangled bool, as you usually do in Go, or wrap them in a pointer in the middle (?*string, ?*int).

Zero values of pointers, maps, functions, channels, and interfaces

In Go, declaring a pointer, map, function, channel, or interface without initializing it results in implicitly initializing it to nil.

// Go code; not SGo.
var x interface{}
fmt.Println(x) // Prints '<nil>'.

Those types don't have a zero value in SGo. This is a new situation that never happens in Go, but it is a unavoidable price to pay.

What happens instead is that an uninitialized variable remains uninitialized, and you can't use it until it is proven that you have initialized it. In structs or arrays, you can't leave a field or element of one or those types unitialized.

Type assertions

SGo compiles to Go, and all information about optional types gets lost in translation.

Because of this, type-assertions to pointers, functions, maps, channels, or interfaces need extra runtime checks. Those are automatically added by SGo for you, to ensure type safety.

Try in browser!

var m = map[string]int{"foo": 123}
var x interface{} = m

// Although x was assigned a map[string]int, when you want to take it back, you can
// wrap it in an optional.
v := x.(?map[string]int)
if v != nil {
	// v is again map[string]int here.
	v["bar"] = 456

// Or not, in which case an implicit `if v == nil` check will be added by SGo.
v = x.(map[string]int) // OK!
x = (?map[string]int)(nil)
// v = x.(map[string]int)       // Panics!
v \ ok := x.(map[string]int) // ok is false!

This is unfortunate, but necessary due to the way SGo optionals get translated to Go. At runtime, SGo programs don't know whether pointers, functions, maps, channels, or interfaces were wrapped in an optional or not when they were defined in the code. To a running SGo program, which is just a running Go program, a value of an optional type, ?T, has the wrapped type instead, T; plus, it can be nil. Type assertions use this runtime information, and thus an optional value could be type-asserted to a its wrapped type if SGo didn't forbid the type assertion altogether.

Type-switches follow the same rules. Additionally, you can't have both T and ?T as clauses in a type-switch.


Because, at runtime, SGo programs are just Go, and thus know nothing of optionals, reflection will ignore them altogether, and just use their underlying Go representation.

Try in browser!

var x ?*int
fmt.Printf("%T\n", x) // Prints '*int', not '?*int'.

Unfortunately, this means that you can use reflection to bypass SGo's guarantees.

Try in browser!

type Point struct{ X, Y int }
var p *Point = &Point{2, 3}
var pp **Point = &p
v := reflect.ValueOf(pp)
// *pp = nil, which is the same as p = nil.
// It wouldn't be possible to say p = nil in normal SGo.
fmt.Println(p.X) // Causes a nil panic, because p is nil.

Importing from, and exporting to, Go

SGo is designed to be pleasant to use together with both other SGo code and plain old Go code.

Because SGo compiles to plain Go, you use SGo code in a separate plain Go file by just using its compiled Go counterpart, either importing its package or putting it in the same package. There's nothing special going on in this case.

This get more interesting when importing Go code into SGo.

When importing a package, SGo performs an automatic translation of the Go types it finds into SGo types. Basically, it puts a ? before everything that in Go can be nil but in SGo cannot.

This alone would be too cumbersome to use, though. Consider, for example, "net/http".HandleFunc:

func HandleFunc(pattern string, handler func(ResponseWriter, *Request))

Following this naive approach, SGo would import that as this:

func HandleFunc(pattern string, handler ?func(?ResponseWriter, ?*Request))

But we know that "net/http" won't give us a nil http.ResponseWriter nor a nil *http.Request, and that it requires us to give it a non-nil handler function. We need to tell SGo this somehow. There are three ways this can happen.

"For SGo:" doc comments

If something has a comment beginning with // For SGo: before, SGo takes the rest of the line and uses it as if it were the type of the thing being commented.

// For SGo: func(string, func(w http.ResponseWriter, req *http.Request)
func HandleFunc(pattern string, handler func(ResponseWriter, *Request))

SGo will see the func(string, func(w http.ResponseWriter, req *http.Request), and just believe that HandleFunc has that type, instead of the automatically inferred func(string, ?func(w ?http.ResponseWriter, req ?*http.Request).

Let's see a more interesting example, "net/http".Post:

func Post(url string, bodyType string, body io.Reader) (resp *Response, err error)

We know that the library expects body to possibly be nil, meaning that the POST request has no body. We also know that we aren't supposed to use resp if err is not nil. So we can annotate it like this:

// For SGo: func(url string, bodyType string, body ?io.Reader) (resp *Response \ err error)
func Post(url string, bodyType string, body io.Reader) (resp *Response, err error)

We can replace a multiple return value with an entangled return, making things much more usable from SGo.

SGo automatically inserts such "For SGo:" comments when compiling SGo code to Go, so if a library is written in SGo originally, another SGo package can import it and expect it to work. Having those annotations in the doc comment has the additional advantage of telling Go users of our originally SGo code what should and shouldn't be ever nil.

Of course, there's an important downside to "For SGo:" comments: we can't just add them to third-party code. When it's not our own code that we are annotating, SGo gives us another way: sgovendor.


You can annotate third-party code for your own SGo project by putting only the annotations in a special folder, called sgovendor, alongside your code.

A sgovendor folder should have a folder structure matching the path of the Go packages you want to annotate. In the last level, you should put one or more files with a .sgoann extension.

Those .sgoann files must have the following syntax:

File -> List
List -> Item*
Item -> Name Def /[\n;]*/
Name -> Ident | Receiver
Receiver -> "(" "*" Ident ")"
Ident -> (Go identifier)
Def -> Type | "{" List "}"
Type -> /[^{][^\n;]*/

For example, let's say that our project uses "".(*Upgrader).Upgrade. SGo would naively translate it into this:

func (u ?*Upgrader) Upgrade(w ?http.ResponseWriter, r ?*http.Request, responseHeader ?http.Header) (?*Conn, ?error)

But we can do better. So we make a "sgovendor" folder alongside our code, then a "" folder inside, then a "gorilla" folder inside that one, and a "websocket" folder inside that, and then create a "websocket.sgoann" file inside with this:

(*Upgrader) {
	Upgrade func(w http.ResponseWriter, r *http.Request, responseHeader ?http.Header) (*Conn \ error)

(In fact, that's exactly what sgoplayground does.)

Built-in annotations

For the standard library, SGo comes with predefined SGo annotations. You can check those here.

Ideally, that file would have annotations for the whole standard library; please contribute!


There are forks of both gofmt:

go get

And goimports:

go get

There's not much editor support beyond that. For Sublime Text 3, I hacked together a fork of GoSublime that might come handy (it does for me!).


A dialect of Go with optional types and without nil references.




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