Go (programming language)

Not to be confused with Go! (programming language), an agent-based language released in 2003.

Go

Paradigm(s)	compiled, concurrent, imperative, structured
Designed by	Robert Griesemer Rob Pike Ken Thompson
Developer	Google Inc.
Appeared in	2009 (2009)
Stable release	version 1.3.3 / 1 October 2014 (2014-10-01)
Typing discipline	strong, static, inferred, nominal
Major implementations	gc (8g, 6g, 5g), gccgo
Influenced by	C, occam, Limbo, Modula, Newsqueak, Oberon, Pascal, Python
Implementation language	C, Go, Asm
OS	Linux, Mac OS X, FreeBSD, NetBSD, OpenBSD, MS Windows, Plan 9
License	BSD-style + Patent grant
Filename extension(s)	.go
Website	golang.org

Go, also commonly referred to as 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 garbage collection, type safety, some dynamic-typing capabilities, additional built-in types such as variable-length arrays and key-value maps, and a large standard library.

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, NetBSD, OpenBSD, Plan 9, and Microsoft Windows operating systems and the i386, amd64, and ARM processor architectures. A second compiler, gccgo, is a GCC frontend.

History

Ken Thompson states that, initially, Go was purely an experimental project. Referring to himself along with the other original authors of Go, he states:

When the three of us [Thompson, Rob Pike, and Robert Griesemer] got started, it was pure research. The three of us got together and decided that we hated C++. [laughter] ... [Returning to Go,] we started off with the idea that all three of us had to be talked into every feature in the language, so there was no extraneous garbage put into the language for any reason.

The history of the language before its first release, back to 2007, is covered in the language's FAQ.

Language design

Go is recognizably in the tradition of C, but makes many changes aimed at conciseness, simplicity, and safety. The following is a brief overview of the features which define Go (for more information see the language specification):

• 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, and type embedding instead of non-virtual inheritance.
  • A toolchain that, by default, produces 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

Syntax

Go's syntax includes changes from C aimed at keeping code concise and readable. The programmer needn't specify the types of expressions, allowing just i := 3 or s := "some words" to replace C's int i = 3; or char* s = "some words";. Semicolons at the end of lines aren't required. Functions may return multiple, named values, and returning a result, err pair is the conventional way a function indicates an error to its caller in Go. 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, and maps.

Types

Go adds some basic types not present in C for safety and convenience:

• Slices (written []type) point into an array of objects in memory, storing a pointer to the start of the slice, a length, and a capacity specifying when new memory needs to be allocated to expand the array. Slice contents are passed by reference, and their contents are always mutable.
• Go's immutable string type typically holds UTF-8 text (though it can hold arbitrary bytes as well).
• map[keytype]valtype provides a hashtable.
• Go also adds channel types, which support concurrency and are discussed below, and interfaces, which replace virtual inheritance and are discussed in Interface system section.

Structurally, Go's type system has a few differences from C and most C derivatives. Unlike C typedefs, Go's named types are not aliases for each other, and rules limit when different types can be assigned to each other without explicit conversion. Unlike in C, conversions between number types are explicit; to ensure that doesn't create verbose conversion-heavy code, numeric constants in Go represent abstract, untyped numbers. Finally, in place of non-virtual inheritance, Go has a feature called type embedding in which one object can contain others and pick up their methods.

Package system

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, and package paths often look like partial URLs for compatibility.

Concurrency: goroutines, channels, and select

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.

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

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. 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 the communicating-processes model is favored 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.

Race condition safety

There are no restrictions on how goroutines access shared data, making race conditions possible. Specifically, unless a program explicitly synchronizes via channels or other means, 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 nothing but a list of required methods: for example, implementing io.Reader requires a Read method that takes a []byte and returns a count of bytes read and any error. Code calling Read needn't know whether it's reading 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.

Go types don't declare which interfaces they implement: having the required methods is implementing the interface. In formal language, Go's interface system provides structural rather than nominal typing.

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))
}

(Run or edit this example online.)

Also note type RepeatByte is defined as a byte, not a struct. Named types in Go needn't be structs, and any named type can have methods defined, satisfy interfaces, and act, for practical purposes, as objects; the standard library, for example, stores IP addresses in byte slices.

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.

interface{}, the empty interface, is an important corner case because it can refer to an item of any concrete type, including primitive types like string. 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.

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.

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.

Omissions

Go deliberately omits certain features common in other languages, including generic programming, assertions, pointer arithmetic, and inheritance. After initially omitting exceptions, the language added the panic/recover mechanism, but it is only meant for rare circumstances.

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.

Conventions and code style

The Go authors and community put substantial effort into molding the style and design of Go programs:

• Indentation, spacing, and other surface-level details of code are automatically standardized by the go fmt tool. go vet and golint do additional checking automatically.
• Tools and libraries distributed with Go suggest standard approaches to things like API documentation (godoc), testing (go test), building (go build), package management (go get), and so on.
• Syntax rules require things that are optional in other languages, for example by banning cyclic dependencies, unused variables or imports, and implicit type conversions.
• The omission of certain features (for example, functional-programming shortcuts like map and C++-style try/finally blocks) tends to encourage a particular explicit, concrete, and imperative programming style.
• Core developers write extensively about Go idioms, style, and philosophy, in the Effective Go document and code review comments reference, presentations, blog posts, and public mailing list messages.

When adapting to the Go ecosystem after working in other languages, differences in style and approach can be as important as low-level language and library differences.

Language tools

Go includes the same sort of debugging, testing, and code-vetting tools as many language distributions. The Go distribution includes go vet, which analyzes code searching for common stylistic problems and mistakes. A profiler, unit testing tool, gdb debugging support, and a race condition tester are also in the distribution. The Go distribution includes its own build system, which requires only information in the Go files themselves, no separate build files.

There is an ecosystem of third-party tools that add to the standard distribution, such as gocode, which enables code autocompletion in many text editors, goimports (by a Go team member), which automatically adds/removes package imports as needed, errcheck, which detects code that might unintentionally ignore errors, and more. Plugins exist to add language support in widely used text editors, and at least one IDE, LiteIDE, targets Go in particular.

Examples

Hello world

Here is a Hello world program in Go:

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

(Run or edit this example online.)

Echo

This imitates 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)
    }
}

File Read

// Reading and writing files are basic tasks needed for
// many Go programs. First we'll look at some examples of
// reading files.
 
package main
 
import (
    "bufio"
    "fmt"
    "io"
    "io/ioutil"
    "os"
)
 
// Reading files requires checking most calls for errors.
// This helper will streamline our error checks below.
func check(e error) {
    if e != nil {
        panic(e)
    }
}
 
func main() {
 
    // Perhaps the most basic file reading task is
    // slurping a file's entire contents into memory.
    dat, err := ioutil.ReadFile("/tmp/dat")
    check(err)
    fmt.Print(string(dat))
 
    // You'll often want more control over how and what
    // parts of a file are read. For these tasks, start
    // by `Open`ing a file to obtain an `os.File` value.
    f, err := os.Open("/tmp/dat")
    check(err)
 
    // Read some bytes from the beginning of the file.
    // Allow up to 5 to be read but also note how many
    // actually were read.
    b1 := make([]byte, 5)
    n1, err := f.Read(b1)
    check(err)
    fmt.Printf("%d bytes: %s\n", n1, string(b1))
 
    // You can also `Seek` to a known location in the file
    // and `Read` from there.
    o2, err := f.Seek(6, 0)
    check(err)
    b2 := make([]byte, 2)
    n2, err := f.Read(b2)
    check(err)
    fmt.Printf("%d bytes @ %d: %s\n", n2, o2, string(b2))
 
    // The `io` package provides some functions that may
    // be helpful for file reading. For example, reads
    // like the ones above can be more robustly
    // implemented with `ReadAtLeast`.
    o3, err := f.Seek(6, 0)
    check(err)
    b3 := make([]byte, 2)
    n3, err := io.ReadAtLeast(f, b3, 2)
    check(err)
    fmt.Printf("%d bytes @ %d: %s\n", n3, o3, string(b3))
 
    // There is no built-in rewind, but `Seek(0, 0)`
    // accomplishes this.
    _, err = f.Seek(0, 0)
    check(err)
 
    // The `bufio` package implements a buffered
    // reader that may be useful both for its efficiency
    // with many small reads and because of the additional
    // reading methods it provides.
    r4 := bufio.NewReader(f)
    b4, err := r4.Peek(5)
    check(err)
    fmt.Printf("5 bytes: %s\n", string(b4))
 
    // Close the file when you're done (usually this would
    // be scheduled immediately after `Open`ing with
    // `defer`).
    f.Close()
 
}

Notable users

Some notable open-source applications in Go include:

• Docker, a set of tools for deploying Linux containers
• Flynn, a PaaS powered by Docker
• Juju, a service orchestration tool by Canonical, packagers of Ubuntu Linux
• nsq, a message queue by bit.ly
• Doozer, a lock service by managed hosting provider Heroku

Other companies and sites using Go (generally together with other languages, not exclusively) include:

• Google, for many projects, notably including download server dl.google.com
• Dropbox, migrated some of their critical components from Python to Go
• CloudFlare, for their delta-coding proxy Railgun, their distributed DNS service, as well as tools for cryptography, logging, stream processing, and accessing SPDY sites.
• SoundCloud, for "dozens of systems"
• The BBC, in some games and internal projects
• Novartis, for an internal inventory system
• Cloud Foundry, a platform as a service
• CoreOS, an operating system based on Chrome OS
• MongoDB, tools for administrating MongoDB instances

Libraries

Go's open-source libraries include:

• Go's standard library, which covers a lot of fundamental functionality:
  • Algorithms: compression, cryptography, sorting, math, indexing, and text and string manipulation.
  • External interfaces: I/O, network clients and servers, parsing and writing common formats, running system calls, and interacting with C code.
  • Development tools: reflection, runtime control, debugging, profiling, unit testing, synchronization, and parsing Go.
• Third-party libraries with more specialized tools:
  • Web toolkits, including the Gorilla Web Toolkit, Revel, and goweb
  • Database, stream, and caching tools, including groupcache and kv and ql
  • Parsers for common formats, such as HTML, JSON, and Google Protocol Buffers
  • Protocol implementations, such as ssh, SPDY, and websocket
  • Database drivers, such as sqlite3, mysql, and redis
  • Bindings to C libraries, such as cgzip, qml, and GTK
  • Specialized tools like biogo for bioinformatics, meeus for astronomy, and gogeos for GIS

Some sites help index the libraries outside the Go distribution:

• godoc.org
• GitHub's most starred repositories in Go
• The Go Wiki's project page

Community and conferences

• Gopher Academy, Gopher Academy is a group of developers working to educate and promote the golang community.
• Golangprojects.com, lists programming jobs and projects where companies are looking for people that know Go
• GopherCon The first Go conference. Denver, Colorado, USA April 24-26 2014
• dotGo European conference. Paris, France October 10 2014
• GopherConIndia The first Go conference in India. Bangalore Feb. 19-21 2015

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 August 2014, its ranking had dropped to 38th in the index, placing it lower than COBOL and Fortran. Go is already in commercial use by several large organizations.

Regarding Go, Bruce Eckel has 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
• Dart, another Google programming language

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