Go (programming language)

The lack of support for generic programming in initial versions of Go drew considerable criticism.[34] The designers expressed an openness to generic programming and noted that built-in functions were in fact type-generic, but are treated as special cases; Pike called this a weakness that might at some point be changed.[35] The Google team built at least one compiler for an experimental Go dialect with generics, but did not release it.[36]

In August 2018, the Go principal contributors published draft designs for generic programming and error handling and asked users to submit feedback.[37][38] However, the error handling proposal was eventually abandoned.[39]

In June 2020, a new draft design document[40] was published that would add the necessary syntax to Go for declaring generic functions and types. A code translation tool, go2go, was provided to allow users to try the new syntax, along with a generics-enabled version of the online Go Playground.[41]

Generics were finally added to Go in version 1.18.[42]

Go 1 guarantees compatibility[43] for the language specification and major parts of the standard library. All versions up to the current Go 1.19 release[44] have maintained this promise.

Each major Go release is supported until there are two newer major releases.[45]

Go's syntax includes changes from C aimed at keeping code concise and readable. A combined declaration/initialization operator was introduced that allows the programmer to write i := 3 or s := "Hello, world!", without specifying the types of variables used. This contrasts with C's int i = 3; and const char *s = "Hello, world!";. Semicolons still terminate statements;[lower-alpha 2] but are implicit when the end of a line occurs.[lower-alpha 3] Methods may return multiple values, and returning a result, err pair is the conventional way a method indicates an error to its caller in Go.[lower-alpha 4] Go adds literal syntaxes for initializing struct parameters by name and for initializing maps and slices. As an alternative to C's three-statement for loop, Go's range expressions allow concise iteration over arrays, slices, strings, maps, and channels.[54]

Go has a number of built-in types, including numeric ones (byte, int64, float32, etc.), booleans, and character strings (string). Strings are immutable; built-in operators and keywords (rather than functions) provide concatenation, comparison, and UTF-8 encoding/decoding.[55] Record types can be defined with the struct keyword.[56]

For each type T and each non-negative integer constant n, there is an array type denoted [n]T; arrays of differing lengths are thus of different types. Dynamic arrays are available as "slices", denoted []T for some type T. These have a length and a capacity specifying when new memory needs to be allocated to expand the array. Several slices may share their underlying memory.[35][57][58]

Pointers are available for all types, and the pointer-to-T type is denoted *T. Address-taking and indirection use the & and * operators, as in C, or happen implicitly through the method call or attribute access syntax.[59][60] There is no pointer arithmetic,[lower-alpha 5] except via the special unsafe.Pointer type in the standard library.[61]

For a pair of types K, V, the type map[K]V is the type of hash tables mapping type-K keys to type-V values. Hash tables are built into the language, with special syntax and built-in functions. chan T is a channel that allows sending values of type T between concurrent Go processes.[62]

Aside from its support for interfaces, Go's type system is nominal: the type keyword can be used to define a new named type, which is distinct from other named types that have the same layout (in the case of a struct, the same members in the same order). Some conversions between types (e.g., between the various integer types) are pre-defined and adding a new type may define additional conversions, but conversions between named types must always be invoked explicitly.[63] For example, the type keyword can be used to define a type for IPv4 addresses, based on 32-bit unsigned integers:

type ipv4addr uint32

With this type definition, ipv4addr(x) interprets the uint32 value x as an IP address. Simply assigning x to a variable of type ipv4addr is a type error.[64]

Constant expressions may be either typed or "untyped"; they are given a type when assigned to a typed variable if the value they represent passes a compile-time check.[65]

Function types are indicated by the func keyword; they take zero or more parameters and return zero or more values, all of which are typed. The parameter and return values determine a function type; thus, func(string, int32) (int, error) is the type of functions that take a string and a 32-bit signed integer, and return a signed integer (of default width) and a value of the built-in interface type error.[66]

Any named type has a method set associated with it. The IP address example above can be extended with a method for checking whether its value is a known standard:

// ZeroBroadcast reports whether addr is 255.255.255.255.
func (addr ipv4addr) ZeroBroadcast() bool {
    return addr == 0xFFFFFFFF
}

Due to nominal typing, this method definition adds a method to ipv4addr, but not on uint32. While methods have special definition and call syntax, there is no distinct method type.[67]

Go provides two features that replace class inheritance.

The first is embedding, which can be viewed as an automated form of composition.[68]

The second are its interfaces, which provides runtime polymorphism.[69]^: 266 Interfaces are a class of types and provide a limited form of structural typing in the otherwise nominal type system of Go. An object which is of an interface type is also of another type, much like C++ objects being simultaneously of a base and derived class. Go interfaces were designed after protocols from the Smalltalk programming language.[70] Multiple sources use the term duck typing when describing Go interfaces.[71][72] Although the term duck typing is not precisely defined and therefore not wrong, it usually implies that type conformance is not statically checked. Since conformance to a Go interface is checked statically by the Go compiler (except when performing a type assertion), the Go authors prefer the term structural typing.[73]

The definition of an interface type lists required methods by name and type. Any object of type T for which functions exist matching all the required methods of interface type I is an object of type I as well. The definition of type T need not (and cannot) identify type I. For example, if Shape, Square and Circle are defined as

import "math"

type Shape interface {
    Area() float64
}

type Square struct { // Note: no "implements" declaration
    side float64
}

func (sq Square) Area() float64 { return sq.side * sq.side }

type Circle struct { // No "implements" declaration here either
    radius float64
}

func (c Circle) Area() float64 { return math.Pi * math.Pow(c.radius, 2) }

then both a Square and a Circle are implicitly a Shape and can be assigned to a Shape-typed variable.[69]^{: 263–268} In formal language, Go's interface system provides structural rather than nominal typing. Interfaces can embed other interfaces with the effect of creating a combined interface that is satisfied by exactly the types that implement the embedded interface and any methods that the newly defined interface adds.[69]^: 270

The Go standard library uses interfaces to provide genericity in several places, including the input/output system that is based on the concepts of Reader and Writer.[69]^{: 282–283}

Besides calling methods via interfaces, Go allows converting interface values to other types with a run-time type check. The language constructs to do so are the type assertion,[74] which checks against a single potential type:

var shp Shape = Square{5}
square, ok := shp.(Square) // Asserts Square type on shp, should work
if ok {
	fmt.Printf("%#v\n", square)
} else {
	fmt.Println("Can't print shape as Square")
}

and the type switch,[75] which checks against multiple types:

func (sq Square) Diagonal() float64 { return sq.side * math.Sqrt2 }
func (c Circle) Diameter() float64 { return 2 * c.radius }

func LongestContainedLine(shp Shape) float64 {
	switch v := shp.(type) {
	case Square:
		return v.Diagonal() // Or, with type assertion, shp.(Square).Diagonal()
	case Circle:
		return v.Diameter() // Or, with type assertion, shp.(Circle).Diameter()
	default:
		return 0 // In practice, this should be handled with errors
	}
}

The empty interface interface{} is an important base case because it can refer to an item of any concrete type. It is similar to the Object class in Java or C# and is satisfied by any type, including built-in types like int.[69]^: 284 Code using the empty interface cannot simply call methods (or built-in operators) on the referred-to object, but it can store the interface{} value, try to convert it to a more useful type via a type assertion or type switch, or inspect it with Go's reflect package.[76] Because interface{} can refer to any value, it is a limited way to escape the restrictions of static typing, like void* in C but with additional run-time type checks.

The interface{} type can be used to model structured data of any arbitrary schema in Go, such as JSON or YAML data, by representing it as a map[string]interface{} (map of string to empty interface). This recursively describes data in the form of a dictionary with string keys and values of any type.[77]

Interface values are implemented using pointer to data and a second pointer to run-time type information.[78] Like some other types implemented using pointers in Go, interface values are nil if uninitialized.[79]

Since version 1.18, Go supports generic code using parameterized types.[80]

In Go's package system, each package has a path (e.g., "compress/bzip2" or "golang.org/x/net/html") and a name (e.g., bzip2 or html). References to other packages' definitions must always be prefixed with the other package's name, and only the capitalized names from other packages are accessible: io.Reader is public but bzip2.reader is not.[81] The go get command can retrieve packages stored in a remote repository[82] and developers are encouraged to develop packages inside a base path corresponding to a source repository (such as example.com/user_name/package_name) to reduce the likelihood of name collision with future additions to the standard library or other external libraries.[83]

The Go language has built-in facilities, as well as library support, for writing concurrent programs. Concurrency refers not only to CPU parallelism, but also to asynchrony: letting slow operations like a database or network read run while the program does other work, as is common in event-based servers.[84]

The primary concurrency construct is the goroutine, a type of light-weight process. A function call prefixed with the go keyword starts a function in a new goroutine. The language specification does not specify how goroutines should be implemented, but current implementations multiplex a Go process's goroutines onto a smaller set of operating-system threads, similar to the scheduling performed in Erlang.[85]^: 10

While a standard library package featuring most of the classical concurrency control structures (mutex locks, etc.) is available,[85]^{: 151–152} idiomatic concurrent programs instead prefer channels, which send messages between goroutines.[86] Optional buffers store messages in FIFO order[87]^: 43 and allow sending goroutines to proceed before their messages are received.

Channels are typed, so that a channel of type chan T can only be used to transfer messages of type T. Special syntax is used to operate on them; <-ch is an expression that causes the executing goroutine to block until a value comes in over the channel ch, while ch <- x sends the value x (possibly blocking until another goroutine receives the value). The built-in switch-like select statement can be used to implement non-blocking communication on multiple channels; see below for an example. Go has a memory model describing how goroutines must use channels or other operations to safely share data.[88]

The existence of channels sets Go apart from actor model-style concurrent languages like Erlang, where messages are addressed directly to actors (corresponding to goroutines). The actor style can be simulated in Go by maintaining a one-to-one correspondence between goroutines and channels, but the language allows multiple goroutines to share a channel or a single goroutine to send and receive on multiple channels.[85]^: 147

From these tools one can build concurrent constructs like worker pools, pipelines (in which, say, a file is decompressed and parsed as it downloads), background calls with timeout, "fan-out" parallel calls to a set of services, and others.[89] Channels have also found uses further from the usual notion of interprocess communication, like serving as a concurrency-safe list of recycled buffers,[90] implementing coroutines (which helped inspire the name goroutine),[91] and implementing iterators.[92]

Concurrency-related structural conventions of Go (channels and alternative channel inputs) are derived from Tony Hoare's communicating sequential processes model. Unlike previous concurrent programming languages such as Occam or Limbo (a language on which Go co-designer Rob Pike worked),[93] Go does not provide any built-in notion of safe or verifiable concurrency.[94] While the communicating-processes model is favored in Go, it is not the only one: all goroutines in a program share a single address space. This means that mutable objects and pointers can be shared between goroutines; see § Lack of race condition safety, below.

Although Go's concurrency features are not aimed primarily at parallel processing,[84] they can be used to program shared-memory multi-processor machines. Various studies have been done into the effectiveness of this approach.[95] One of these studies compared the size (in lines of code) and speed of programs written by a seasoned programmer not familiar with the language and corrections to these programs by a Go expert (from Google's development team), doing the same for Chapel, Cilk and Intel TBB. The study found that the non-expert tended to write divide-and-conquer algorithms with one go statement per recursion, while the expert wrote distribute-work-synchronize programs using one goroutine per processor. The expert's programs were usually faster, but also longer.[96]

Go's approach to concurrency can be summarized as "don't communicate by sharing memory; share memory by communicating".[97] There are no restrictions on how go-routines access shared data, making race conditions possible. Specifically, unless a program explicitly synchronizes via channels or other means, writes from one go-routine might be partly, entirely, or not at all visible to another, often with no guarantees about ordering of writes.[94] Furthermore, Go's internal data structures like interface values, slice headers, hash tables, and string headers are not immune to race conditions, so type and memory safety can be violated in multithreaded programs that modify shared instances of those types without synchronization.[98][99] Instead of language support, safe concurrent programming thus relies on conventions; for example, Chisnall recommends an idiom called "aliases xor mutable", meaning that passing a mutable value (or pointer) over a channel signals a transfer of ownership over the value to its receiver.[85]^: 155

The linker in the gc toolchain creates statically linked binaries by default; therefore all Go binaries include the Go runtime.[100][101]

Go deliberately omits certain features common in other languages, including (implementation) inheritance, assertions,[lower-alpha 6] pointer arithmetic,[lower-alpha 5] implicit type conversions, untagged unions,[lower-alpha 7] and tagged unions.[lower-alpha 8] The designers added only those facilities that all three agreed on.[104]

Of the omitted language features, the designers explicitly argue against assertions and pointer arithmetic, while defending the choice to omit type inheritance as giving a more useful language, encouraging instead the use of interfaces to achieve dynamic dispatch[lower-alpha 9] and composition to reuse code. Composition and delegation are in fact largely automated by struct embedding; according to researchers Schmager et al., this feature "has many of the drawbacks of inheritance: it affects the public interface of objects, it is not fine-grained (i.e, no method-level control over embedding), methods of embedded objects cannot be hidden, and it is static", making it "not obvious" whether programmers will overuse it to the extent that programmers in other languages are reputed to overuse inheritance.[68]

Exception handling was initially omitted in Go due to lack of a "design that gives value proportionate to the complexity".[105] An exception-like panic/recover mechanism that avoids the usual try-catch control structure was proposed[106] and released in the March 30, 2010 snapshot.[107] The Go authors advise using it for unrecoverable errors such as those that should halt an entire program or server request, or as a shortcut to propagate errors up the stack within a package.[108][109] Across package boundaries, Go includes a canonical error type, and multi-value returns using this type are the standard idiom.[12]

package main

import "fmt"

func main() {
    fmt.Println("hello world")
}

where "fmt" is the package for formatted I/O, similar to C's C file input/output.[118]

The following simple program demonstrates Go's concurrency features to implement an asynchronous program. It launches two lightweight threads ("goroutines"): one waits for the user to type some text, while the other implements a timeout. The select statement waits for either of these goroutines to send a message to the main routine, and acts on the first message to arrive (example adapted from David Chisnall's book).[85]^: 152

package main

import (
    "fmt"
    "time"
)

func readword(ch chan string) {
    fmt.Println("Type a word, then hit Enter.")
    var word string
    fmt.Scanf("%s", &word)
    ch <- word
}

func timeout(t chan bool) {
    time.Sleep(5 * time.Second)
    t <- false
}

func main() {
    t := make(chan bool)
    go timeout(t)

    ch := make(chan string)
    go readword(ch)

    select {
    case word := <-ch:
        fmt.Println("Received", word)
    case <-t:
        fmt.Println("Timeout.")
    }
}

The testing package provides support for automated testing of go packages.[119] Target function example:

func ExtractUsername(email string) string {
	at := strings.Index(email, "@")
	return email[:at]
}

Test code (note that assert keyword is missing in Go; tests live in <filename>_test.go at the same package):

import (
    "testing"
)

func TestExtractUsername(t *testing.T) {
	t.Run("withoutDot", func(t *testing.T) {
		username := ExtractUsername("r@google.com")
		if username != "r" {
			t.Fatalf("Got: %v\n", username)
		}
	})

	t.Run("withDot", func(t *testing.T) {
		username := ExtractUsername("jonh.smith@example.com")
		if username != "jonh.smith" {
			t.Fatalf("Got: %v\n", username)
		}
	})
}

It is possible to run tests in parallel.

The net/http package provides support for creating web applications.

This example would show "Hello world!" when localhost:8080 is visited.

package main

import (
    "fmt"
    "log"
    "net/http"
)

func helloFunc(w http.ResponseWriter, r *http.Request) {
    fmt.Fprintf(w, "Hello world!")
}

func main() {
    http.HandleFunc("/", helloFunc)
    log.Fatal(http.ListenAndServe(":8080", nil))
}

On November 10, 2009, the day of the general release of the language, Francis McCabe, developer of the Go! programming language (note the exclamation point), requested a name change of Google's language to prevent confusion with his language, which he had spent 10 years developing.[158] McCabe raised concerns that "the 'big guy' will end up steam-rollering over" him, and this concern resonated with the more than 120 developers who commented on Google's official issues thread saying they should change the name, with some[159] even saying the issue contradicts Google's motto of: Don't be evil.[160]

On October 12, 2010, the issue was closed by Google developer Russ Cox (@rsc) with the custom status "Unfortunate" accompanied by the following comment:

"There are many computing products and services named Go. In the 11 months since our release, there has been minimal confusion of the two languages."[160]

• Go's nil combined with the lack of algebraic types leads to difficulty handling failures and base cases.[161][162]
• Go does not allow an opening brace to appear on its own line, which forces all Go programmers to use the same brace style.[163]
• File semantics in Go standard library are heavily based on POSIX semantics, and they do not map well to the Windows platform.[164][165] Note that this problem is not particular to Go, but other programming languages have solved it through well defined standard libraries.
• A study showed that it is as easy to make concurrency bugs with message passing as with shared memory, sometimes even more.[166]