Go (programming language)

Go

Paradigm	compiled, concurrent, imperative, structured
Designed by	Robert Griesemer Rob Pike Ken Thompson
Developer	Google Inc.
First appeared	2009
Stable release	version 1.1.2 / 13 August 2013 (2013-08-13)
Typing discipline	strong, static
OS	Linux, Mac OS X, FreeBSD, NetBSD, OpenBSD, MS Windows, Plan 9
License	BSD-style + Patent grant
Filename extensions	.go
Website	golang.org
Major implementations
gc (8g, 6g, 5g), gccgo
Influenced by
C, Limbo, Modula, Newsqueak, Oberon, Pascal, Python

Go, also called golang, is a programming language initially developed at Google in 2007 by Robert Griesemer, Rob Pike, and Ken Thompson. It is a statically-typed language with syntax loosely derived from that of C, adding automatic memory management, type safety, some dynamic-typing capabilities, additional built-in types such as variable-length arrays and key-value maps, and a large standard library. Beyond that broad category, Go is defined by:

• A syntax and environment adopting patterns more common in dynamic languages:
  • Concise variable declaration and initialization through type inference (x := 0 not int x = 0;)
  • Fast compilation times
  • Remote package management (go get) and online package documentation
• Distinctive approaches to particular problems
  • Built-in concurrency primitives: light-weight processes (goroutines), channels, and the select statement
  • An interface system in place of virtual inheritance
  • Compilers that, by default, produce statically linked native binaries without external dependencies
• A desire to keep the language specification simple enough to hold in a programmer's head, in part by omitting features common to similar languages:
  • No type inheritance
  • No method or operator overloading
  • No circular dependencies among packages
  • No pointer arithmetic
  • No assertions
  • No generic programming

The language was announced in November 2009 and is now used in some of Google's production systems. Go's "gc" compiler targets the Linux, Mac OS X, FreeBSD, OpenBSD, Plan 9, and Microsoft Windows operating systems and the i386, amd64, and ARM processor architectures. A second compiler, gccgo, is a GCC frontend.

Syntax and package system

Go's syntax, while recognizably in the C family, makes many changes aimed at keeping code concise. The Go compiler infers the types of expressions, allowing just i := 3 or w := "some words" to replace C's int i = 3; or char* s = "some words";. When a variable or struct field isn't explicitly initialized (e.g., var x, y, z int), the compiler gives it a type-appropriate zero value like 0, nil, or "". Semicolons at the end of lines aren't required. Functions may return multiple, named values, and returning a result, err pair is the standard way to handle errors in Go. (The language does have an exception-like mechanism, panic/recover, but it's intentionally cumbersome and only meant for rare circumstances.)

Go also adds syntax to support facilities that aren't in C. The interface system adds type assertions (str := iface.(string)) and type switches (switch iface.(type) {...}). For concurrency, the go statement starts a lightweight process, the <- operator tries to communicate over a channel, and the select statement tries to communicate on multiple channels. Go adds syntax for array slices, so that, for example, a[:3] refers to the first three items of array a. As in many C-family languages, function definitions can appear in expressions and close over any variables in the surrounding scope.

Go has a standard style covering indentation, spacing, and many other details, usually applied to user code by the go fmt tool in the Go distribution. Go requires that programs not contain unused variables or imports or discard the results of evaluating certain built-in functions. Banning unnecessary imports is particularly important because Go does not support circular dependencies.

There is much more to Go's syntax; for further reading, try Go's language specification, the official language tour, and the Effective Go document, as well as the community-written gobyexample.com.

In Go's package system, each package has a path (e.g., "compress/bzip2" or "code.google.com/p/go.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 modules are accessible: io.Reader is public but bzip2.reader is not. The go get command can retrieve packages stored in a remote repository such as Github or Google Code.

Go deliberately omits certain features common in other languages, including generic programming, assertions, pointer arithmetic, and inheritance. The Go authors express an openness to generic programming, explicitly argue against assertions and pointer arithmetic, while defending the choice to omit type inheritance as giving a more useful language, encouraging heavy use of interfaces instead.

Concurrency

Go provides facilities for writing concurrent programs that share state by communicating. Concurrency refers not only to multithreading and CPU parallelism, which Go supports, 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.

Goroutines, channels, and select

Go's concurrency-related syntax and types include:

• The go statement, go func(), starts a function in a new light-weight process, or goroutine
• Channel types, chan type, provide a type-safe, synchronized, optionally buffered channels between goroutines, and are useful mostly with two other facilities:
  • The send statement, ch <- x sends x over ch
  • The receive operator, <- ch receives a value from ch
  • Both operations block until the other goroutine is ready to communicate
• The select statement uses a switch-like syntax to wait for communication on any of a set of channels

These tools can be used, sometimes in nonobvious ways, to build constructs like worker pools, pipelines in which multiple stages run at once (for example, a file is decompressed as it downloads), asynchronous calls with timeout, "fan-out" parallel calls to a set of services, and others. Channels have also found uses further from the usual notion of interprocess communication, like serving as a concurrency-safe list of recycled buffers, implementing coroutines (which helped inspire the name goroutine), and implementing iterators.

While communicating-processes model is the favored one in Go, it isn't the only one: memory can be shared across goroutines (see below), and the standard sync module provides locks and other primitives.

Safety

There are no restrictions on how goroutines access shared data, making race conditions possible. Specifically, unless a program explicitly synchronizes via channels or mutexes, writes from one goroutine might be partly, entirely, or not at all visible to another, often with no guarantees about ordering of writes. Furthermore, Go's internal data structures like interface values, slice headers, 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.

Idiomatic Go minimizes sharing of data (and thus potential race conditions) by communicating over channels, and a race-condition tester is included in the standard distribution to help catch unsafe behavior. Still, it is important to realize that while Go provides building blocks that can be used to write correct, comprehensible concurrent code, arbitrary code isn't guaranteed to be safe.

Some 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), Go does not provide any built-in notion of safe or verifiable concurrency.

Interface system

In place of virtual inheritance, Go uses interfaces. An interface declaration is a list of required methods: for example, the interface io.Reader just requires the method Read(p []byte) (n int, err error). Code that uses an io.Reader could potentially read from an HTTP connection, a file, an in-memory buffer, or any other source.

Go's standard library defines interfaces for a number of concepts: input sources and output sinks, sortable collections, objects printable as strings, cryptographic hashes, and so on.

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, which checks against a single potential type, and the type switch, which checks against multiple types.

Go types don't declare which interfaces they implement: having the required methods is implementing the interface.

interface{}, the empty interface, is an important corner case because it can refer to an item of any concrete type. Code using the empty interface can't 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. Because interface{} can refer to any value, it's a limited way to escape the restrictions of static typing, like void* in C but with additional run-time type checks.

Go does not have interface inheritance, but one interface type can embed another; then the embedding interface requires all of the methods required by the embedded interface.

The example below uses the io.Reader and io.Writer interfaces to test Go's implementation of SHA-256 on a standard test input, 1,000,000 repeats of the character "a". RepeatByte implements an io.Reader yielding an infinite stream of repeats of a byte, similar to Unix /dev/zero. The main() function uses RepeatByte to stream a million repeats of "a" into the hash function, then prints the result, which matches the expected value published online. Even though both reader and writer interfaces are needed to make this work, the code needn't mention either; the compiler infers what types implement what interfaces:

package main
 
import (
        "fmt"
        "io"
	"crypto/sha256"
)

type RepeatByte byte

func (r RepeatByte) Read(p []byte) (n int, err error) {
        for i := range p {
                p[i] = byte(r)
        }
        return len(p), nil
}
 
func main() {
        testStream := RepeatByte('a')
	hasher := sha256.New()
        io.CopyN(hasher, testStream, 1000000)
        fmt.Printf("%x", hasher.Sum(nil))
}

(You can run or edit this example online.)

Interface values are stored in memory as a pointer to data and a second pointer to run-time type information. Like other pointers in Go, interface values are nil if uninitialized. Unlike in environments like Java's virtual machine, there is no object header; the run-time type information is only attached to interface values. So, the system imposes no per-object memory overhead for objects not accessed via interface, similar to C structs or C# ValueTypes.

Examples

Hello world

The following is a Hello world program in Go:

package main

import "fmt"

func main() {
    fmt.Println("Hello, World")
}

(Run or edit this example online.)

Go's automatic semicolon insertion feature requires that opening braces not be placed on their own lines, and this is thus the preferred brace style. The examples shown comply with this style.

Echo

Example illustrating how to write a program like the Unix echo command in Go:

package main

import (
    "flag"
    "fmt"
    "strings"
)

func main() {
    var omitNewline bool
    flag.BoolVar(&omitNewline, "n", false, "don't print final newline")
    flag.Parse() // Scans the arg list and sets up flags.

    str := strings.Join(flag.Args(), " ")
    if omitNewline {
        fmt.Print(str)
    } else {
        fmt.Println(str)
    }
}

Reception

Go's initial release led to much discussion.

Michele Simionato wrote in an article for artima.com:

Here I just wanted to point out the design choices about interfaces and inheritance. Such ideas are not new and it is a shame that no popular language has followed such particular route in the design space. I hope Go will become popular; if not, I hope such ideas will finally enter in a popular language, we are already 10 or 20 years too late :-(

Dave Astels at Engine Yard wrote:

Go is extremely easy to dive into. There are a minimal number of fundamental language concepts and the syntax is clean and designed to be clear and unambiguous. Go is still experimental and still a little rough around the edges.

Ars Technica interviewed Rob Pike, one of the authors of Go, and asked why a new language was needed. He replied that:

It wasn't enough to just add features to existing programming languages, because sometimes you can get more in the long run by taking things away. They wanted to start from scratch and rethink everything. ... [But they did not want] to deviate too much from what developers already knew because they wanted to avoid alienating Go's target audience.

Go was named Programming Language of the Year by the TIOBE Programming Community Index in its first year, 2009, for having a larger 12-month increase in popularity (in only 2 months, after its introduction in November) than any other language that year, and reached 13th place by January 2010, surpassing established languages like Pascal. As of March 2013, it ranked 24th in the index. Go is already in commercial use by several large organizations.

Bruce Eckel stated:

The complexity of C++ (even more complexity has been added in the new C++), and the resulting impact on productivity, is no longer justified. All the hoops that the C++ programmer had to jump through in order to use a C-compatible language make no sense anymore -- they're just a waste of time and effort. Now, Go makes much more sense for the class of problems that C++ was originally intended to solve.

Mascot

Go's mascot is a gopher designed by Renée French, who also designed Glenda, the Plan 9 Bunny. The logo and mascot are licensed under Creative Commons Attribution 3.0 license.

Naming dispute

On 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. The issue was closed by a Google developer on 12 October 2010 with the custom status "Unfortunate" and with 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."

See also

• Comparison of programming languages

References

This article incorporates material from the official Go tutorial, which is licensed under the Creative Commons Attribution 3.0 license.

External links

• Official website
• A Tour of Go (official)
• Go Programming Language Resources (unofficial)
• Pike, Rob (28 April 2010). "Another Go at Language Design". Stanford EE Computer Systems Colloquium. Stanford University. (video) — A university lecture