No major systems language has emerged in over a decade, but over that time the computing landscape has changed tremendously. There are several trends:
We believe it's worth trying again with a new language, a concurrent, garbage-collected language with fast compilation. Regarding the points above:
“Ogle” would be a good name for a Go debugger.
The mascot and logo were designed by Renée French, who also designed Glenda, the Plan 9 bunny. The gopher is derived from one she used for an WFMU T-shirt design some years ago. The logo and mascot are covered by the Creative Commons Attribution 3.0 license.
The 6g (and 8g and 5g) compiler is named in the
tradition of the Plan 9 C compilers, described in
http://plan9.bell-labs.com/sys/doc/compiler.html
(see the table in section 2).
6 is the architecture letter for amd64 (or x86-64, if you prefer), while
g stands for Go.
Robert Griesemer, Rob Pike and Ken Thompson started sketching the goals for a new language on the white board on September 21, 2007. Within a few days the goals had settled into a plan to do something and a fair idea of what it would be. Design continued part-time in parallel with unrelated work. By January 2008, Ken had started work on a compiler with which to explore ideas; it generated C code as its output. By mid-year the language had become a full-time project and had settled enough to attempt a production compiler. In May 2008, Ian Taylor independently started on a GCC front end for Go using the draft specification. Russ Cox joined in late 2008 and helped move the language and libraries from prototype to reality.
Many others have contributed ideas, discussions, and code.
Go was born out of frustration with existing languages and environments for systems programming. Programming had become too difficult and the choice of languages was partly to blame. One had to choose either efficient compilation, efficient execution, or ease of programming; all three were not available in the same mainstream language. Programmers who could were choosing ease over safety and efficiency by moving to dynamically typed languages such as Python and JavaScript rather than C++ or, to a lesser extent, Java.
Go is an attempt to combine the ease of programming of an interpreted, dynamically typed language with the efficiency and safety of a statically typed, compiled language. It also aims to be modern, with support for networked and multicore computing. Finally, it is intended to be fast: it should take at most a few seconds to build a large executable on a single computer. To meet these goals required addressing a number of linguistic issues: an expressive but lightweight type system; concurrency and garbage collection; rigid dependency specification; and so on. These cannot be addressed well by libraries or tools; a new language was called for.
Go is mostly in the C family (basic syntax), with significant input from the Pascal/Modula/Oberon family (declarations, packages), plus some ideas from languages inspired by Tony Hoare's CSP, such as Newsqueak and Limbo (concurrency). However, it is a new language across the board. In every respect the language was designed by thinking about what programmers do and how to make programming, at least the kind of programming we do, more effective, which means more fun.
Programming today involves too much bookkeeping, repetition, and clerical work. As Dick Gabriel says, “Old programs read like quiet conversations between a well-spoken research worker and a well-studied mechanical colleague, not as a debate with a compiler. Who'd have guessed sophistication bought such noise?” The sophistication is worthwhile—no one wants to go back to the old languages—but can it be more quietly achieved?
Go attempts to reduce the amount of typing in both senses of the word.
Throughout its design, we have tried to reduce clutter and
complexity. There are no forward declarations and no header files;
everything is declared exactly once. Initialization is expressive,
automatic, and easy to use. Syntax is clean and light on keywords.
Stuttering (foo.Foo* myFoo = new(foo.Foo)) is reduced by
simple type derivation using the :=
declare-and-initialize construct. And perhaps most radically, there
is no type hierarchy: types just are, they don't have to
announce their relationships. These simplifications allow Go to be
expressive yet comprehensible without sacrificing, well, sophistication.
Another important principle is to keep the concepts orthogonal. Methods can be implemented for any type; structures represent data while interfaces represent abstraction; and so on. Orthogonality makes it easier to understand what happens when things combine.
Go is an experiment. We hope adventurous users will give it a try and see if they enjoy it. Not every programmer will, but we hope enough will find satisfaction in the approach it offers to justify further development.
Yes. There are now several Go programs deployed in
production inside Google. For instance, the server behind
http://golang.org is a Go program;
in fact it's just the godoc
document server running in a production configuration.
There are two Go compiler implementations, 6g and friends,
generically called gc, and gccgo.
Gc uses a different calling convention and linker and can
therefore only be linked with C programs using the same convention.
There is such a C compiler but no C++ compiler.
Gccgo is a GCC front-end that can, with care, be linked with
GCC-compiled C or C++ programs.
The cgo program provides the mechanism for a “foreign function interface” to allow safe calling of C libraries from Go code. SWIG extends this capability to C++ libraries.
A separate open source project provides the necessary compiler plugin and library. It is available at http://code.google.com/p/goprotobuf/
Absolutely. We encourage developers to make Go Language sites in their own languages. However, if you choose to add the Google logo or branding to your site (it does not appear on golang.org), you will need to abide by the guidelines at http://www.google.com/permissions/guidelines.html
It was important to us to extend the space of identifiers from the confines of ASCII. Go's rule—identifier characters must be letters or digits as defined by Unicode—is simple to understand and to implement but has restrictions. Combining characters are excluded by design, for instance. Until there is an agreed external definition of what an identifier might be, plus a definition of canonicalization of identifiers that guarantees no ambiguity, it seemed better to keep combining characters out of the mix. Thus we have a simple rule that can be expanded later without breaking programs, one that avoids bugs that would surely arise from a rule that admits ambiguous identifiers.
On a related note, since an exported identifier must begin with an
upper-case letter, identifiers created from “letters”
in some languages can, by definition, not be exported. For now the
only solution is to use something like X日本語, which
is clearly unsatisfactory; we are considering other options. The
case-for-visibility rule is unlikely to change however; it's one
of our favorite features of Go.
Every language contains novel features and omits someone's favorite feature. Go was designed with an eye on felicity of programming, speed of compilation, orthogonality of concepts, and the need to support features such as concurrency and garbage collection. Your favorite feature may be missing because it doesn't fit, because it affects compilation speed or clarity of design, or because it would make the fundamental system model too difficult.
If it bothers you that Go is missing feature X, please forgive us and investigate the features that Go does have. You might find that they compensate in interesting ways for the lack of X.
Generics may well be added at some point. We don't feel an urgency for them, although we understand some programmers do.
Generics are convenient but they come at a cost in complexity in the type system and run-time. We haven't yet found a design that gives value proportionate to the complexity, although we continue to think about it. Meanwhile, Go's built-in maps and slices, plus the ability to use the empty interface to construct containers (with explicit unboxing) mean in many cases it is possible to write code that does what generics would enable, if less smoothly.
This remains an open issue.
We believe that coupling exceptions to a control
structure, as in the try-catch-finally idiom, results in
convoluted code. It also tends to encourage programmers to label
too many ordinary errors, such as failing to open a file, as
exceptional.
Go takes a different approach. Instead of exceptions, it has a couple of built-in functions to signal and recover from truly exceptional conditions. The recovery mechanism is executed only as part of a function's state being torn down after an error, which is sufficient to handle catastrophe but requires no extra control structures and, when used well, can result in clean error-handling code.
See the Defer, Panic, and Recover article for details.
Go doesn't provide assertions. They are undeniably convenient, but our experience has been that programmers use them as a crutch to avoid thinking about proper error handling and reporting. Proper error handling means that servers continue operation after non-fatal errors instead of crashing. Proper error reporting means that errors are direct and to the point, saving the programmer from interpreting a large crash trace. Precise errors are particularly important when the programmer seeing the errors is not familiar with the code.
The same arguments apply to the use of assert() in test programs. Proper
error handling means letting other tests run after one has failed, so
that the person debugging the failure gets a complete picture of what is
wrong. It is more useful for a test to report that
isPrime gives the wrong answer for 2, 3, 5, and 7 (or for
2, 4, 8, and 16) than to report that isPrime gives the wrong
answer for 2 and therefore no more tests were run. The programmer who
triggers the test failure may not be familiar with the code that fails.
Time invested writing a good error message now pays off later when the
test breaks.
In testing, if the amount of extra code required to write good errors seems repetitive and overwhelming, it might work better as a table-driven test instead. Go has excellent support for data structure literals.
We understand that this is a point of contention. There are many things in the Go language and libraries that differ from modern practices, simply because we feel it's sometimes worth trying a different approach.
Concurrency and multi-threaded programming have a reputation for difficulty. We believe the problem is due partly to complex designs such as pthreads and partly to overemphasis on low-level details such as mutexes, condition variables, and even memory barriers. Higher-level interfaces enable much simpler code, even if there are still mutexes and such under the covers.
One of the most successful models for providing high-level linguistic support for concurrency comes from Hoare's Communicating Sequential Processes, or CSP. Occam and Erlang are two well known languages that stem from CSP. Go's concurrency primitives derive from a different part of the family tree whose main contribution is the powerful notion of channels as first class objects.
Goroutines are part of making concurrency easy to use. The idea, which has been around for a while, is to multiplex independently executing functions—coroutines, really—onto a set of threads. When a coroutine blocks, such as by calling a blocking system call, the run-time automatically moves other coroutines on the same operating system thread to a different, runnable thread so they won't be blocked. The programmer sees none of this, which is the point. The result, which we call goroutines, can be very cheap: unless they spend a lot of time in long-running system calls, they cost little more than the memory for the stack.
To make the stacks small, Go's run-time uses segmented stacks. A newly minted goroutine is given a few kilobytes, which is almost always enough. When it isn't, the run-time allocates (and frees) extension segments automatically. The overhead averages about three cheap instructions per function call. It is practical to create hundreds of thousands of goroutines in the same address space. If goroutines were just threads, system resources would run out at a much smaller number.
After long discussion it was decided that the typical use of maps did not require safe access from multiple threads, and in those cases where it did, the map was probably part of some larger data structure or computation that was already synchronized. Therefore requiring that all map operations grab a mutex would slow down most programs and add safety to few. This was not an easy decision, however, since it means uncontrolled map access can crash the program.
The language does not preclude atomic map updates. When required, such as when hosting an untrusted program, the implementation could interlock map access.
Yes and no. Although Go has types and methods and allows an object-oriented style of programming, there is no type hierarchy. The concept of “interface” in Go provides a different approach that we believe is easy to use and in some ways more general. There are also ways to embed types in other types to provide something analogous—but not identical—to subclassing. Moreover, methods in Go are more general than in C++ or Java: they can be defined for any sort of data, not just structs.
Also, the lack of type hierarchy makes “objects” in Go feel much more lightweight than in languages such as C++ or Java.
The only way to have dynamically dispatched methods is through an interface. Methods on structs or other types are always resolved statically.
Object-oriented programming, at least in the best-known languages, involves too much discussion of the relationships between types, relationships that often could be derived automatically. Go takes a different approach.
Rather than requiring the programmer to declare ahead of time that two types are related, in Go a type automatically satisfies any interface that specifies a subset of its methods. Besides reducing the bookkeeping, this approach has real advantages. Types can satisfy many interfaces at once, without the complexities of traditional multiple inheritance. Interfaces can be very lightweight—having one or even zero methods in an interface can express useful concepts. Interfaces can be added after the fact if a new idea comes along or for testing—without annotating the original types. Because there are no explicit relationships between types and interfaces, there is no type hierarchy to manage or discuss.
It's possible to use these ideas to construct something analogous to
type-safe Unix pipes. For instance, see how fmt.Fprintf
enables formatted printing to any output, not just a file, or how the
bufio package can be completely separate from file I/O,
or how the crypto packages stitch together block and
stream ciphers. All these ideas stem from a single interface
(io.Writer) representing a single method
(Write). And that's only scratching the surface.
It takes some getting used to but this implicit style of type dependency is one of the most exciting things about Go.
len a function and not a method?
We debated this issue but decided
implementing len and friends as functions was fine in practice and
didn't complicate questions about the interface (in the Go type sense)
of basic types.
Method dispatch is simplified if it doesn't need to do type matching as well. Experience with other languages told us that having a variety of methods with the same name but different signatures was occasionally useful but that it could also be confusing and fragile in practice. Matching only by name and requiring consistency in the types was a major simplifying decision in Go's type system.
Regarding operator overloading, it seems more a convenience than an absolute requirement. Again, things are simpler without it.
The convenience of automatic conversion between numeric types in C is outweighed by the confusion it causes. When is an expression unsigned? How big is the value? Does it overflow? Is the result portable, independent of the machine on which it executes? It also complicates the compiler; “the usual arithmetic conversions” are not easy to implement and inconsistent across architectures. For reasons of portability, we decided to make things clear and straightforward at the cost of some explicit conversions in the code. The definition of constants in Go—arbitrary precision values free of signedness and size annotations—ameliorates matters considerably, though.
A related detail is that, unlike in C, int and int64
are distinct types even if int is a 64-bit type. The int
type is generic; if you care about how many bits an integer holds, Go
encourages you to be explicit.
The same reason strings are: they are such a powerful and important data structure that providing one excellent implementation with syntactic support makes programming more pleasant. We believe that Go's implementation of maps is strong enough that it will serve for the vast majority of uses. If a specific application can benefit from a custom implementation, it's possible to write one but it will not be as convenient syntactically; this seems a reasonable tradeoff.
Map lookup requires an equality operator, which structs and arrays do not implement. They don't implement equality because equality is not well defined on such types; there are multiple considerations involving shallow vs. deep comparison, pointer vs. value comparison, how to deal with recursive structures, and so on. We may revisit this issue—and implementing equality for structs and arrays will not invalidate any existing programs—but without a clear idea of what equality of structs and arrays should mean, it was simpler to leave it out for now.
There's a lot of history on that topic. Early on, maps and channels were syntactically pointers and it was impossible to declare or use a non-pointer instance. Also, we struggled with how arrays should work. Eventually we decided that the strict separation of pointers and values made the language harder to use. Introducing reference types, including slices to handle the reference form of arrays, resolved these issues. Reference types add some regrettable complexity to the language but they have a large effect on usability: Go became a more productive, comfortable language when they were introduced.
There is a program, godoc, written in Go, that extracts
package documentation from the source code. It can be used on the
command line or on the web. An instance is running at
http://golang.org/pkg/.
In fact, godoc implements the full site at
http://golang.org/.
Eventually, there may be a small number of rules to guide things
like naming, layout, and file organization.
The document Effective Go
contains some style advice.
More directly, the program gofmt is a pretty-printer
whose purpose is to enforce layout rules; it replaces the usual
compendium of do's and don'ts that allows interpretation.
All the Go code in the repository has been run through gofmt.
The library sources are in go/src/pkg.
If you want to make a significant change, please discuss on the mailing list before embarking.
See the document Contributing to the Go project for more information about how to proceed.
Everything in Go is passed by value. A function always gets a copy of the thing being passed, as if there were an assignment statement assigning the value to the parameter. For instance, copying a pointer value makes a copy of the pointer, not the data it points to.
Map and slice values behave like pointers; they are descriptors that contain pointers to the underlying map or slice data. Copying a map or slice value doesn't copy the data it points to. Copying an interface value makes a copy of the thing stored in the interface value. If the interface value holds a struct, copying the interface value makes a copy of the struct. If the interface value holds a pointer, copying the interface value makes a copy of the pointer, but again not the data it points to.
func (s *MyStruct) someMethod() { } // method on pointer
func (s MyStruct) someMethod() { } // method on value
When defining a method on a type, the receiver (s in the above
example) behaves exactly is if it were an argument to the method. Define the
method on a pointer type if you need the method to modify the data the receiver
points to. Otherwise, it is often cleaner to define the method on a value type.
In short: new allocates memory, make initializes
the slice, map, and channel types.
See the relevant section of Effective Go for more details.
int 32 bits on 64 bit machines?
The size of int and float is implementation-specific.
The 64 bit Go compilers (both 6g and gccgo) use a 32 bit representation for
both int and float. Code that relies on a particular
size of value should use an explicitly sized type, like int64 or
float64.
We haven't fully defined it all yet, but some details about atomicity are available in the Go Memory Model specification.
Regarding mutexes, the sync package implements them, but we hope Go programming style will encourage people to try higher-level techniques. In particular, consider structuring your program so that only one goroutine at a time is ever responsible for a particular piece of data.
Do not communicate by sharing memory. Instead, share memory by communicating.
See the Share Memory By Communicating code walk and its associated article for a detailed discussion of this concept.
Under the gc compilers you must set GOMAXPROCS to allow the
runtime to utilise more than one OS thread. Under gccgo an OS
thread will be created for each goroutine, and GOMAXPROCS is
effectively equal to the number of running goroutines.
Programs that perform concurrent computation should benefit from an increase in
GOMAXPROCS. (See the runtime package
documentation.)
GOMAXPROCS > 1 sometimes make my program
slower?(This is specific to the gc compilers. See above.)
It depends on the nature of your program. Programs that contain several goroutines that spend a lot of time communicating on channels will experience performance degradation when using multiple OS threads. This is because of the significant context-switching penalty involved in sending data between threads.
The Go runtime's scheduler is not as good as it needs to be. In future, it
should recognise such cases and optimize its use of OS threads. For now,
GOMAXPROCS should be set on a per-application basis.
From the Go Spec:
The method set of any other named typeTconsists of all methods with receiver typeT. The method set of the corresponding pointer type*Tis the set of all methods with receiver*TorT(that is, it also contains the method set ofT).
If an interface value contains a pointer *T,
a method call can obtain a value by dereferencing the pointer,
but if an interface value contains a value T,
there is no useful way for a method call to obtain a pointer.
If not for this restriction, this code:
var buf bytes.Buffer io.Copy(buf, os.Stdin)
would copy standard input into a copy of buf,
not into buf itself.
This is almost never the desired behavior.
Some confusion may arise when using closures with concurrency. Consider the following program:
func main() {
done := make(chan bool)
values = []string{ "a", "b", "c" }
for _, v := range values {
go func() {
fmt.Println(v)
done <- true
}()
}
// wait for all goroutines to complete before exiting
for i := range values {
<-done
}
}
One might mistakenly expect to see a, b, c as the output.
What you'll probably see instead is c, c, c. This is because
each closure shares the same variable v. Each closure prints the
value of v at the time fmt.Println is executed,
rather than the value of v when the goroutine was launched.
To bind the value of v to each closure as they are launched, one
could modify the inner loop to read:
for _, v := range values {
go func(u) {
fmt.Println(u)
done <- true
}(v)
}
In this example, the value of v is passed as an argument to the
anonymous function. That value is then accessible inside the function as
the variable u.
?: operator?There is no ternary form in Go. You may use the following to achieve the same result:
if expr {
n = trueVal
} else {
n = falseVal
}
Put all the source files for the package in a directory by themselves. Source files can refer to items from different files at will; there is no need for forward declarations or a header file.
Other than being split into multiple files, the package will compile and test just like a single-file package.
Create a new file ending in _test.go in the same directory
as your package sources. Inside that file, import "testing"
and write functions of the form
func TestFoo(t *testing.T) {
...
}
Run gotest in that directory.
That script finds the Test functions,
builds a test binary, and runs it.
See the How to Write Go Code document for more details.
Gccgo has a C++ front-end with a recursive descent parser coupled to the
standard GCC back end. Gc is written in C using
yacc/bison for the parser.
Although it's a new program, it fits in the Plan 9 C compiler suite
(http://plan9.bell-labs.com/sys/doc/compiler.html)
and uses a variant of the Plan 9 loader to generate ELF binaries.
We considered writing 6g, the original Go compiler, in Go itself but
elected not to do so because of the difficulties of bootstrapping and
especially of open source distribution—you'd need a Go compiler to
set up a Go environment. Gccgo, which came later, makes it possible to
consider writing a compiler in Go, which might well happen. (Go would be a
fine language in which to implement a compiler; a native lexer and
parser are already available in /pkg/go.)
We also considered using LLVM for 6g but we felt it was too large and
slow to meet our performance goals.
Again due to bootstrapping issues, the runtime is mostly in C (with a
tiny bit of assembler) although Go is capable of implementing most of
it now. Gccgo's runtime uses glibc.
Gc uses a custom library, to keep the footprint under
control; it is
compiled with a version of the Plan 9 C compiler that supports
segmented stacks for goroutines.
Work is underway to provide the same stack management in
gccgo.
One of Go's design goals is to approach the performance of C for comparable programs, yet on some benchmarks it does quite poorly, including several in test/bench. The slowest depend on libraries for which versions of comparable performance are not available in Go. For instance, pidigits depends on a multi-precision math package, and the C versions, unlike Go's, use GMP (which is written in optimized assembler). Benchmarks that depend on regular expressions (regex-dna, for instance) are essentially comparing Go's stopgap regexp package to mature, highly optimized regular expression libraries like PCRE.
Benchmark games are won by extensive tuning and the Go versions of most of the benchmarks need attention. If you measure comparable C and Go programs (reverse-complement is one example), you'll see the two languages are much closer in raw performance than this suite would indicate.
Still, there is room for improvement. The compilers are good but could be better, many libraries need major performance work, and the garbage collector isn't fast enough yet (even if it were, taking care not to generate unnecessary garbage can have a huge effect).
Other than declaration syntax, the differences are not major and stem from two desires. First, the syntax should feel light, without too many mandatory keywords, repetition, or arcana. Second, the language has been designed to be easy to analyze and can be parsed without a symbol table. This makes it much easier to build tools such as debuggers, dependency analyzers, automated documentation extractors, IDE plug-ins, and so on. C and its descendants are notoriously difficult in this regard.
They're only backwards if you're used to C. In C, the notion is that a
variable is declared like an expression denoting its type, which is a
nice idea, but the type and expression grammars don't mix very well and
the results can be confusing; consider function pointers. Go mostly
separates expression and type syntax and that simplifies things (using
prefix * for pointers is an exception that proves the rule). In C,
the declaration
int* a, b;
declares a to be a pointer but not b; in Go
var a, b *int;
declares both to be pointers. This is clearer and more regular.
Also, the := short declaration form argues that a full variable
declaration should present the same order as := so
var a uint64 = 1;has the same effect as
a := uint64(1);
Parsing is also simplified by having a distinct grammar for types that
is not just the expression grammar; keywords such as func
and chan keep things clear.
See the Go's Declaration Syntax article for more details.
Safety. Without pointer arithmetic it's possible to create a language that can never derive an illegal address that succeeds incorrectly. Compiler and hardware technology have advanced to the point where a loop using array indices can be as efficient as a loop using pointer arithmetic. Also, the lack of pointer arithmetic can simplify the implementation of the garbage collector.
++ and -- statements and not expressions? And why postfix, not prefix?
Without pointer arithmetic, the convenience value of pre- and postfix
increment operators drops. By removing them from the expression
hierarchy altogether, expression syntax is simplified and the messy
issues around order of evaluation of ++ and --
(consider f(i++) and p[i] = q[++i])
are eliminated as well. The simplification is
significant. As for postfix vs. prefix, either would work fine but
the postfix version is more traditional; insistence on prefix arose
with the STL, a library for a language whose name contains, ironically, a
postfix increment.
Go uses brace brackets for statement grouping, a syntax familiar to programmers who have worked with any language in the C family. Semicolons, however, are for parsers, not for people, and we wanted to eliminate them as much as possible. To achieve this goal, Go borrows a trick from BCPL: the semicolons that separate statements are in the formal grammar but are injected automatically, without lookahead, by the lexer at the end of any line that could be the end of a statement. This works very well in practice but has the effect that it forces a brace style. For instance, the opening brace of a function cannot appear on a line by itself.
Some have argued that the lexer should do lookahead to permit the
brace to live on the next line. We disagree. Since Go code is meant
to be formatted automatically by
gofmt,
some style must be chosen. That style may differ from what
you've used in C or Java, but Go is a new language and
gofmt's style is as good as any other. More
important—much more important—the advantages of a single,
programmatically mandated format for all Go programs greatly outweigh
any perceived disadvantages of the particular style.
Note too that Go's style means that an interactive implementation of
Go can use the standard syntax one line at a time without special rules.
One of the biggest sources of bookkeeping in systems programs is memory management. We feel it's critical to eliminate that programmer overhead, and advances in garbage collection technology in the last few years give us confidence that we can implement it with low enough overhead and no significant latency. (The current implementation is a plain mark-and-sweep collector but a replacement is in the works.)
Another point is that a large part of the difficulty of concurrent and multi-threaded programming is memory management; as objects get passed among threads it becomes cumbersome to guarantee they become freed safely. Automatic garbage collection makes concurrent code far easier to write. Of course, implementing garbage collection in a concurrent environment is itself a challenge, but meeting it once rather than in every program helps everyone.
Finally, concurrency aside, garbage collection makes interfaces simpler because they don't need to specify how memory is managed across them.