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diff --git a/doc/go_faq.html b/doc/go_faq.html new file mode 100644 index 000000000..aeed53795 --- /dev/null +++ b/doc/go_faq.html @@ -0,0 +1,1496 @@ +<!-- FAQ --> + +<h2 id="Origins">Origins</h2> + +<h3 id="What_is_the_purpose_of_the_project"> +What is the purpose of the project?</h3> + +<p> +No major systems language has emerged in over a decade, but over that time +the computing landscape has changed tremendously. There are several trends: +</p> + +<ul> +<li> +Computers are enormously quicker but software development is not faster. +<li> +Dependency management is a big part of software development today but the +“header files” of languages in the C tradition are antithetical to clean +dependency analysis—and fast compilation. +<li> +There is a growing rebellion against cumbersome type systems like those of +Java and C++, pushing people towards dynamically typed languages such as +Python and JavaScript. +<li> +Some fundamental concepts such as garbage collection and parallel computation +are not well supported by popular systems languages. +<li> +The emergence of multicore computers has generated worry and confusion. +</ul> + +<p> +We believe it's worth trying again with a new language, a concurrent, +garbage-collected language with fast compilation. Regarding the points above: +</p> + +<ul> +<li> +It is possible to compile a large Go program in a few seconds on a single computer. +<li> +Go provides a model for software construction that makes dependency +analysis easy and avoids much of the overhead of C-style include files and +libraries. +<li> +Go's type system has no hierarchy, so no time is spent defining the +relationships between types. Also, although Go has static types the language +attempts to make types feel lighter weight than in typical OO languages. +<li> +Go is fully garbage-collected and provides fundamental support for +concurrent execution and communication. +<li> +By its design, Go proposes an approach for the construction of system +software on multicore machines. +</ul> + +<h3 id="What_is_the_origin_of_the_name"> +What is the origin of the name?</h3> + +<p> +“Ogle” would be a good name for a Go debugger. +</p> + +<h3 id="Whats_the_origin_of_the_mascot"> +What's the origin of the mascot?</h3> + +<p> +The mascot and logo were designed by +<a href="http://reneefrench.blogspot.com">Renée French</a>, who also designed +<a href="http://plan9.bell-labs.com/plan9/glenda.html">Glenda</a>, +the Plan 9 bunny. +The gopher is derived from one she used for an <a href="http://wfmu.org/">WFMU</a> +T-shirt design some years ago. +The logo and mascot are covered by the +<a href="http://creativecommons.org/licenses/by/3.0/">Creative Commons Attribution 3.0</a> +license. +</p> + +<h3 id="What_kind_of_a_name_is_6g"> +What kind of a name is 6g?</h3> + +<p> +The <code>6g</code> (and <code>8g</code> and <code>5g</code>) compiler is named in the +tradition of the Plan 9 C compilers, described in +<a href="http://plan9.bell-labs.com/sys/doc/compiler.html"> +http://plan9.bell-labs.com/sys/doc/compiler.html</a> +(see the table in section 2). + +<code>6</code> is the architecture letter for amd64 (or x86-64, if you prefer), while +<code>g</code> stands for Go. +</p> + +<h3 id="history"> +What is the history of the project?</h3> +<p> +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. +</p> + +<p> +Go became a public open source project on November 10, 2009. +Many people from the community have contributed ideas, discussions, and code. +</p> + +<h3 id="creating_a_new_language"> +Why are you creating a new language?</h3> +<p> +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. +</p> + +<p> +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 <i>fast</i>: 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. +</p> + +<h3 id="ancestors"> +What are Go's ancestors?</h3> +<p> +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. +</p> + +<h3 id="principles"> +What are the guiding principles in the design?</h3> +<p> +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? +</p> +<p> +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 (<code>foo.Foo* myFoo = new(foo.Foo)</code>) is reduced by +simple type derivation using the <code>:=</code> +declare-and-initialize construct. And perhaps most radically, there +is no type hierarchy: types just <i>are</i>, they don't have to +announce their relationships. These simplifications allow Go to be +expressive yet comprehensible without sacrificing, well, sophistication. +</p> +<p> +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. +</p> + +<h2 id="Usage">Usage</h2> + +<h3 id="Is_Google_using_go_internally"> Is Google using Go internally?</h3> + +<p> +Yes. There are now several Go programs deployed in +production inside Google. For instance, the server behind +<a href="http://golang.org">http://golang.org</a> is a Go program; +in fact it's just the <a href="/cmd/godoc"><code>godoc</code></a> +document server running in a production configuration. +</p> + +<h3 id="Do_Go_programs_link_with_Cpp_programs"> +Do Go programs link with C/C++ programs?</h3> + +<p> +There are two Go compiler implementations, <code>6g</code> and friends, +generically called <code>gc</code>, and <code>gccgo</code>. +<code>Gc</code> 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. +<code>Gccgo</code> is a GCC front-end that can, with care, be linked with +GCC-compiled C or C++ programs. +</p> + +<p> +The <a href="/cmd/cgo/">cgo</a> 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. +</p> + + +<h3 id="Does_Go_support_Google_protocol_buffers"> +Does Go support Google's protocol buffers?</h3> + +<p> +A separate open source project provides the necessary compiler plugin and library. +It is available at +<a href="http://code.google.com/p/goprotobuf/">http://code.google.com/p/goprotobuf/</a> +</p> + + +<h3 id="Can_I_translate_the_Go_home_page"> +Can I translate the Go home page into another language?</h3> + +<p> +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 <a href="http://golang.org/">golang.org</a>), +you will need to abide by the guidelines at +<a href="http://www.google.com/permissions/guidelines.html">http://www.google.com/permissions/guidelines.html</a> +</p> + +<h2 id="Design">Design</h2> + +<h3 id="unicode_identifiers"> +What's up with Unicode identifiers?</h3> + +<p> +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. +</p> + +<p> +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 <code>X日本語</code>, 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. +</p> + +<h3 id="Why_doesnt_Go_have_feature_X">Why does Go not have feature X?</h3> + +<p> +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. +</p> + +<p> +If it bothers you that Go is missing feature <var>X</var>, +please forgive us and investigate the features that Go does have. You might find that +they compensate in interesting ways for the lack of <var>X</var>. +</p> + +<h3 id="generics"> +Why does Go not have generic types?</h3> +<p> +Generics may well be added at some point. We don't feel an urgency for +them, although we understand some programmers do. +</p> + +<p> +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. +</p> + +<p> +This remains an open issue. +</p> + +<h3 id="exceptions"> +Why does Go not have exceptions?</h3> +<p> +We believe that coupling exceptions to a control +structure, as in the <code>try-catch-finally</code> 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. +</p> + +<p> +Go takes a different approach. For plain error handling, Go's multi-value +returns make it easy to report an error without overloading the return value. +<a href="http://blog.golang.org/2011/07/error-handling-and-go.html">A +canonical error type, coupled +with Go's other features</a>, makes error +handling pleasant but quite different from that in other languages. +</p> + +<p> +Go also 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. +</p> + +<p> +See the <a href="http://blog.golang.org/2010/08/defer-panic-and-recover.html">Defer, Panic, and Recover</a> article for details. +</p> + +<h3 id="assertions"> +Why does Go not have assertions?</h3> + +<p> +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. +</p> + +<p> +The same arguments apply to the use of <code>assert()</code> 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 +<code>isPrime</code> gives the wrong answer for 2, 3, 5, and 7 (or for +2, 4, 8, and 16) than to report that <code>isPrime</code> 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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<h3 id="csp"> +Why build concurrency on the ideas of CSP?</h3> +<p> +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 memory barriers. +Higher-level interfaces enable much simpler code, even if there are still +mutexes and such under the covers. +</p> + +<p> +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. +</p> + +<h3 id="goroutines"> +Why goroutines instead of threads?</h3> +<p> +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—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, which is just a few kilobytes. +</p> + +<p> +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. +</p> + +<h3 id="atomic_maps"> +Why are map operations not defined to be atomic?</h3> + +<p> +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. +</p> + +<p> +The language does not preclude atomic map updates. When required, such +as when hosting an untrusted program, the implementation could interlock +map access. +</p> + +<h2 id="types">Types</h2> + +<h3 id="Is_Go_an_object-oriented_language"> +Is Go an object-oriented language?</h3> + +<p> +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. +</p> + +<p> +Also, the lack of type hierarchy makes “objects” in Go feel much more +lightweight than in languages such as C++ or Java. +</p> + +<h3 id="How_do_I_get_dynamic_dispatch_of_methods"> +How do I get dynamic dispatch of methods?</h3> + +<p> +The only way to have dynamically dispatched methods is through an +interface. Methods on structs or other types are always resolved statically. +</p> + +<h3 id="inheritance"> +Why is there no type inheritance?</h3> +<p> +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. +</p> + +<p> +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—an interface with +one or even zero methods can express a useful concept. +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. +</p> + +<p> +It's possible to use these ideas to construct something analogous to +type-safe Unix pipes. For instance, see how <code>fmt.Fprintf</code> +enables formatted printing to any output, not just a file, or how the +<code>bufio</code> package can be completely separate from file I/O, +or how the <code>crypto</code> packages stitch together block and +stream ciphers. All these ideas stem from a single interface +(<code>io.Writer</code>) representing a single method +(<code>Write</code>). And that's only scratching the surface. +</p> + +<p> +It takes some getting used to but this implicit style of type +dependency is one of the most productive things about Go. +</p> + +<h3 id="methods_on_basics"> +Why is <code>len</code> a function and not a method?</h3> +<p> +We debated this issue but decided +implementing <code>len</code> and friends as functions was fine in practice and +didn't complicate questions about the interface (in the Go type sense) +of basic types. +</p> + +<h3 id="overloading"> +Why does Go not support overloading of methods and operators?</h3> +<p> +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. +</p> + +<p> +Regarding operator overloading, it seems more a convenience than an absolute +requirement. Again, things are simpler without it. +</p> + +<h3 id="implements_interface"> +Why doesn't Go have "implements" declarations?</h3> + +<p> +A Go type satisfies an interface by implementing the methods of that interface, +nothing more. This property allows interfaces to be defined and used without +having to modify existing code. It enables a kind of "duck typing" that +promotes separation of concerns and improves code re-use, and makes it easier +to build on patterns that emerge as the code develops. +The semantics of interfaces is one of the main reasons for Go's nimble, +lightweight feel. +</p> + +<p> +See the <a href="#inheritance">question on type inheritance</a> for more detail. +</p> + +<h3 id="guarantee_satisfies_interface"> +How can I guarantee my type satisfies an interface?</h3> + +<p> +You can ask the compiler to check that the type <code>T</code> implements the +interface <code>I</code> by attempting an assignment: +</p> + +<pre> +type T struct{} +var _ I = T{} +</pre> + +<p> +If <code>T</code> doesn't implement <code>I</code>, the mistake will be caught +at compile time. +</p> + +<p> +If you wish the users of an interface to explicitly declare that they implement +it, you can add a method with a descriptive name to the interface's method set. +For example: +</p> + +<pre> +type Fooer interface { + Foo() + ImplementsFooer() +} +</pre> + +<p> +A type must then implement the <code>ImplementsFooer</code> method to be a +<code>Fooer</code>, clearly documenting the fact and announcing it in +<a href="/cmd/godoc/">godoc</a>'s output. +</p> + +<pre> +type Bar struct{} +func (b Bar) ImplementsFooer() {} +func (b Bar) Foo() {} +</pre> + +<p> +Most code doesn't make use of such constraints, since they limit the utility of +the interface idea. Sometimes, though, they're necessary to resolve ambiguities +among similar interfaces. +</p> + +<h3 id="t_and_equal_interface"> +Why doesn't type T satisfy the Equal interface?</h3> + +<p> +Consider this simple interface to represent an object that can compare +itself with another value: +</p> + +<pre> +type Equaler interface { + Equal(Equaler) bool +} +</pre> + +<p> +and this type, <code>T</code>: +</p> + +<pre> +type T int +func (t T) Equal(u T) bool { return t == u } // does not satisfy Equaler +</pre> + +<p> +Unlike the analogous situation in some polymorphic type systems, +<code>T</code> does not implement <code>Equaler</code>. +The argument type of <code>T.Equal</code> is <code>T</code>, +not literally the required type <code>Equaler</code>. +</p> + +<p> +In Go, the type system does not promote the argument of +<code>Equal</code>; that is the programmer's responsibility, as +illustrated by the type <code>T2</code>, which does implement +<code>Equaler</code>: +</p> + +<pre> +type T2 int +func (t T2) Equal(u Equaler) bool { return t == u.(T2) } // satisfies Equaler +</pre> + +<p> +Even this isn't like other type systems, though, because in Go <em>any</em> +type that satisfies <code>Equaler</code> could be passed as the +argument to <code>T2.Equal</code>, and at run time we must +check that the argument is of type <code>T2</code>. +Some languages arrange to make that guarantee at compile time. +</p> + +<p> +A related example goes the other way: +</p> + +<pre> +type Opener interface { + Open(name) Reader +} + +func (t T3) Open() *os.File +</pre> + +<p> +In Go, <code>T3</code> does not satisfy <code>Opener</code>, +although it might in another language. +</p> + +<p> +While it is true that Go's type system does less for the programmer +in such cases, the lack of subtyping makes the rules about +interface satisfaction very easy to state: are the function's names +and signatures exactly those of the interface? +Go's rule is also easy to implement efficiently. +We feel these benefits offset the lack of +automatic type promotion. Should Go one day adopt some form of generic +typing, we expect there would be a way to express the idea of these +examples and also have them be statically checked. +</p> + +<h3 id="convert_slice_of_interface"> +Can I convert a []T to an []interface{}?</h3> + +<p> +Not directly because they do not have the same representation in memory. +It is necessary to copy the elements individually to the destination +slice. This example converts a slice of <code>int</code> to a slice of +<code>interface{}</code>: +</p> + +<pre> +t := []int{1, 2, 3, 4} +s := make([]interface{}, len(t)) +for i, v := range t { + s[i] = v +} +</pre> + +<h3 id="unions"> +Why are there no untagged unions, as in C?</h3> + +<p> +Untagged unions would violate Go's memory safety +guarantees. +</p> + +<h3 id="variant_types"> +Why does Go not have variant types?</h3> + +<p> +Variant types, also known as algebraic types, provide a way to specify +that a value might take one of a set of other types, but only those +types. A common example in systems programming would specify that an +error is, say, a network error, a security error or an application +error and allow the caller to discriminate the source of the problem +by examining the type of the error. Another example is a syntax tree +in which each node can be a different type: declaration, statement, +assignment and so on. +</p> + +<p> +We considered adding variant types to Go, but after discussion +decided to leave them out because they overlap in confusing ways +with interfaces. What would happen if the elements of a variant type +were themselves interfaces? +</p> + +<p> +Also, some of what variant types address is already covered by the +language. The error example is easy to express using an interface +value to hold the error and a type switch to discriminate cases. The +syntax tree example is also doable, although not as elegantly. +</p> + +<h2 id="values">Values</h2> + +<h3 id="conversions"> +Why does Go not provide implicit numeric conversions?</h3> +<p> +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. +</p> + +<p> +A related detail is that, unlike in C, <code>int</code> and <code>int64</code> +are distinct types even if <code>int</code> is a 64-bit type. The <code>int</code> +type is generic; if you care about how many bits an integer holds, Go +encourages you to be explicit. +</p> + +<h3 id="builtin_maps"> +Why are maps built in?</h3> +<p> +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. +</p> + +<h3 id="map_keys"> +Why don't maps allow structs and arrays as keys?</h3> +<p> +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. +</p> + +<h3 id="references"> +Why are maps, slices, and channels references while arrays are values?</h3> +<p> +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. +</p> + +<h2 id="Writing_Code">Writing Code</h2> + +<h3 id="How_are_libraries_documented"> +How are libraries documented?</h3> + +<p> +There is a program, <code>godoc</code>, 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 +<a href="http://golang.org/pkg/">http://golang.org/pkg/</a>. +In fact, <code>godoc</code> implements the full site at +<a href="http://golang.org/">http://golang.org/</a>. +</p> + +<h3 id="Is_there_a_Go_programming_style_guide"> +Is there a Go programming style guide?</h3> + +<p> +Eventually, there may be a small number of rules to guide things +like naming, layout, and file organization. +The document <a href="effective_go.html">Effective Go</a> +contains some style advice. +More directly, the program <code>gofmt</code> 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 <code>gofmt</code>. +</p> + +<h3 id="How_do_I_submit_patches_to_the_Go_libraries"> +How do I submit patches to the Go libraries?</h3> + +<p> +The library sources are in <code>go/src/pkg</code>. +If you want to make a significant change, please discuss on the mailing list before embarking. +</p> + +<p> +See the document +<a href="contribute.html">Contributing to the Go project</a> +for more information about how to proceed. +</p> + +<h2 id="Pointers">Pointers and Allocation</h2> + +<h3 id="pass_by_value"> +When are function parameters passed by value?</h3> + +<p> +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. +</p> + +<p> +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. +</p> + +<h3 id="methods_on_values_or_pointers"> +Should I define methods on values or pointers?</h3> + +<pre> +func (s *MyStruct) pointerMethod() { } // method on pointer +func (s MyStruct) valueMethod() { } // method on value +</pre> + +<p> +For programmers unaccustomed to pointers, the distinction between these +two examples can be confusing, but the situation is actually very simple. +When defining a method on a type, the receiver (<code>s</code> in the above +example) behaves exactly as if it were an argument to the method. +Whether to define the receiver as a value or as a pointer is the same +question, then, as whether a function argument should be a value or +a pointer. +There are several considerations. +</p> + +<p> +First, and most important, does the method need to modify the +receiver? +If it does, the receiver <em>must</em> be a pointer. +(Slices and maps are reference types, so their story is a little +more subtle, but for instance to change the length of a slice +in a method the receiver must still be a pointer.) +In the examples above, if <code>pointerMethod</code> modifies +the fields of <code>s</code>, +the caller will see those changes, but <code>valueMethod</code> +is called with a copy of the caller's argument (that's the definition +of passing a value), so changes it makes will be invisible to the caller. +</p> + +<p> +By the way, pointer receivers are identical to the situation in Java, +although in Java the pointers are hidden under the covers; it's Go's +value receivers that are unusual. +</p> + +<p> +Second is the consideration of efficiency. If the receiver is large, +a big <code>struct</code> for instance, it will be much cheaper to +use a pointer receiver. +</p> + +<p> +Next is consistency. If some of the methods of the type must have +pointer receivers, the rest should too, so the method set is +consistent regardless of how the type is used. +See the section on <a href="#different_method_sets">method sets</a> +for details. +</p> + +<p> +For types such as basic types, slices, and small <code>structs</code>, +a value receiver is very cheap so unless the semantics of the method +requires a pointer, a value receiver is efficient and clear. +</p> + + +<h3 id="new_and_make"> +What's the difference between new and make?</h3> + +<p> +In short: <code>new</code> allocates memory, <code>make</code> initializes +the slice, map, and channel types. +</p> + +<p> +See the <a href="/doc/effective_go.html#allocation_new">relevant section +of Effective Go</a> for more details. +</p> + +<h3 id="q_int_sizes"> +Why is <code>int</code> 32 bits on 64 bit machines?</h3> + +<p> +The sizes of <code>int</code> and <code>uint</code> are implementation-specific +but the same as each other on a given platform. +The 64 bit Go compilers (both 6g and gccgo) use a 32 bit representation for +<code>int</code>. Code that relies on a particular +size of value should use an explicitly sized type, like <code>int64</code>. +On the other hand, floating-point scalars and complex +numbers are always sized: <code>float32</code>, <code>complex64</code>, +etc., because programmers should be aware of precision when using +floating-point numbers. +The default size of a floating-point constant is <code>float64</code>. +</p> + +<h3 id="stack_or_heap"> +How do I know whether a variable is allocated on the heap or the stack?</h3> + +<p> +From a correctness standpoint, you don't need to know. +Each variable in Go exists as long as there are references to it. +The storage location chosen by the implementation is irrelevant to the +semantics of the language. +</p> + +<p> +The storage location does have an effect on writing efficient programs. +When possible, the Go compilers will allocate variables that are +local to a function in that function's stack frame. However, if the +compiler cannot prove that the variable is not referenced after the +function returns, then the compiler must allocate the variable on the +garbage-collected heap to avoid dangling pointer errors. +</p> + +<p> +In the current compilers, the analysis is crude: if a variable has its address +taken, that variable is allocated on the heap. We are working to improve this +analysis so that more data is kept on the stack. +</p> + +<h2 id="Concurrency">Concurrency</h2> + +<h3 id="What_operations_are_atomic_What_about_mutexes"> +What operations are atomic? What about mutexes?</h3> + +<p> +We haven't fully defined it all yet, but some details about atomicity are +available in the <a href="go_mem.html">Go Memory Model specification</a>. +</p> + +<p> +Regarding mutexes, the <a href="/pkg/sync">sync</a> +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. +</p> + +<p> +Do not communicate by sharing memory. Instead, share memory by communicating. +</p> + +<p> +See the <a href="/doc/codewalk/sharemem/">Share Memory By Communicating</a> code walk and its <a href="http://blog.golang.org/2010/07/share-memory-by-communicating.html">associated article</a> for a detailed discussion of this concept. +</p> + +<h3 id="Why_no_multi_CPU"> +Why doesn't my multi-goroutine program use multiple CPUs?</h3> + +<p> +Under the gc compilers you must set <code>GOMAXPROCS</code> to allow the +run-time support to utilise more than one OS thread. Under <code>gccgo</code> an OS +thread will be created for each goroutine, and <code>GOMAXPROCS</code> is +effectively equal to the number of running goroutines. +</p> + +<p> +Programs that perform concurrent computation should benefit from an increase in +<code>GOMAXPROCS</code>. (See the <a +href="http://golang.org/pkg/runtime/#GOMAXPROCS"><code>runtime</code> package's +documentation</a>.) +</p> + +<h3 id="Why_GOMAXPROCS"> +Why does using <code>GOMAXPROCS</code> > 1 sometimes make my program +slower?</h3> + +<p> +(This is specific to the gc compilers. See above.) +</p> + +<p> +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. +</p> + +<p> +Go's goroutine scheduler is not as good as it needs to be. In future, it +should recognize such cases and optimize its use of OS threads. For now, +<code>GOMAXPROCS</code> should be set on a per-application basis. +</p> + +<h2 id="Functions_methods">Functions and Methods</h2> + +<h3 id="different_method_sets"> +Why do T and *T have different method sets?</h3> + +<p> +From the <a href="http://golang.org/doc/go_spec.html#Types">Go Spec</a>: +</p> + +<blockquote> +The method set of any other named type <code>T</code> consists of all methods +with receiver type <code>T</code>. The method set of the corresponding pointer +type <code>*T</code> is the set of all methods with receiver <code>*T</code> or +<code>T</code> (that is, it also contains the method set of <code>T</code>). +</blockquote> + +<p> +If an interface value contains a pointer <code>*T</code>, +a method call can obtain a value by dereferencing the pointer, +but if an interface value contains a value <code>T</code>, +there is no useful way for a method call to obtain a pointer. +</p> + +<p> +If not for this restriction, this code: +</p> + +<pre> +var buf bytes.Buffer +io.Copy(buf, os.Stdin) +</pre> + +<p> +would copy standard input into a <i>copy</i> of <code>buf</code>, +not into <code>buf</code> itself. +This is almost never the desired behavior. +</p> + +<h3 id="closures_and_goroutines"> +Why am I confused by the way my closures behave as goroutines?</h3> + +<p> +Some confusion may arise when using closures with concurrency. +Consider the following program: +</p> + +<pre> +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 _ = range values { + <-done + } +} +</pre> + +<p> +One might mistakenly expect to see <code>a, b, c</code> as the output. +What you'll probably see instead is <code>c, c, c</code>. This is because +each closure shares the same variable <code>v</code>. Each closure prints the +value of <code>v</code> at the time <code>fmt.Println</code> is executed, +rather than the value of <code>v</code> when the goroutine was launched. +</p> + +<p> +To bind the value of <code>v</code> to each closure as they are launched, one +could modify the inner loop to read: +</p> + +<pre> + for _, v := range values { + go func(<b>u</b> string) { + fmt.Println(<b>u</b>) + done <- true + }(<b>v</b>) + } +</pre> + +<p> +In this example, the value of <code>v</code> is passed as an argument to the +anonymous function. That value is then accessible inside the function as +the variable <code>u</code>. +</p> + +<h2 id="Control_flow">Control flow</h2> + +<h3 id="Does_Go_have_a_ternary_form"> +Does Go have the <code>?:</code> operator?</h3> + +<p> +There is no ternary form in Go. You may use the following to achieve the same +result: +</p> + +<pre> +if expr { + n = trueVal +} else { + n = falseVal +} +</pre> + +<h2 id="Packages_Testing">Packages and Testing</h2> + +<h3 id="How_do_I_create_a_multifile_package"> +How do I create a multifile package?</h3> + +<p> +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. +</p> + +<p> +Other than being split into multiple files, the package will compile and test +just like a single-file package. +</p> + +<h3 id="How_do_I_write_a_unit_test"> +How do I write a unit test?</h3> + +<p> +Create a new file ending in <code>_test.go</code> in the same directory +as your package sources. Inside that file, <code>import "testing"</code> +and write functions of the form +</p> + +<pre> +func TestFoo(t *testing.T) { + ... +} +</pre> + +<p> +Run <code>gotest</code> in that directory. +That script finds the <code>Test</code> functions, +builds a test binary, and runs it. +</p> + +<p>See the <a href="/doc/code.html">How to Write Go Code</a> document for more details.</p> + + +<h2 id="Implementation">Implementation</h2> + +<h3 id="What_compiler_technology_is_used_to_build_the_compilers"> +What compiler technology is used to build the compilers?</h3> + +<p> +<code>Gccgo</code> has a C++ front-end with a recursive descent parser coupled to the +standard GCC back end. <code>Gc</code> is written in C using +<code>yacc</code>/<code>bison</code> for the parser. +Although it's a new program, it fits in the Plan 9 C compiler suite +(<a href="http://plan9.bell-labs.com/sys/doc/compiler.html">http://plan9.bell-labs.com/sys/doc/compiler.html</a>) +and uses a variant of the Plan 9 loader to generate ELF binaries. +</p> + +<p> +We considered writing <code>6g</code>, 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. <code>Gccgo</code>, 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 <a href="/pkg/go/"><code>/pkg/go</code></a>.) +</p> + +<p> +We also considered using LLVM for <code>6g</code> but we felt it was too large and +slow to meet our performance goals. +</p> + +<h3 id="How_is_the_run_time_support_implemented"> +How is the run-time support implemented?</h3> + +<p> +Again due to bootstrapping issues, the run-time code is mostly in C (with a +tiny bit of assembler) although Go is capable of implementing most of +it now. <code>Gccgo</code>'s run-time support uses <code>glibc</code>. +<code>Gc</code> 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 +<code>gccgo</code>. +</p> + +<h3 id="Why_is_my_trivial_program_such_a_large_binary"> +Why is my trivial program such a large binary?</h3> + +<p> +The gc tool chain (<code>5l</code>, <code>6l</code>, and <code>8l</code>) only +generate statically linked binaries. All Go binaries therefore include the Go +run-time, along with the run-time type information necessary to support dynamic +type checks, reflection, and even panic-time stack traces. +</p> + +<p> +A trivial C "hello, world" program compiled and linked statically using gcc +on Linux is around 750 kB. An equivalent Go program is around 1.1 MB, but +that includes more powerful run-time support. We believe that with some effort +the size of Go binaries can be reduced. +</p> + +<h3 id="unused_variables_and_imports"> +Can I stop these complaints about my unused variable/import?</h3> + +<p> +The presence of an unused variable may indicate a bug, while +unused imports just slow down compilation. +Accumulate enough unused imports in your code tree and +things can get very slow. +For these reasons, Go allows neither. +</p> + +<p> +When developing code, it's common to create these situations +temporarily and it can be annoying to have to edit them out before the +program will compile. +</p> + +<p> +Some have asked for a compiler option to turn those checks off +or at least reduce them to warnings. +Such an option has not been added, though, +because compiler options should not affect the semantics of the +language and because the Go compiler does not report warnings, only +errors that prevent compilation. +</p> + +<p> +There are two reasons for having no warnings. First, if it's worth +complaining about, it's worth fixing in the code. (And if it's not +worth fixing, it's not worth mentioning.) Second, having the compiler +generate warnings encourages the implementation to warn about weak +cases that can make compilation noisy, masking real errors that +<em>should</em> be fixed. +</p> + +<p> +It's easy to address the situation, though. Use the blank identifier +to let unused things persist while you're developing. +</p> + +<pre> +import "unused" + +// This declaration marks the import as used by referencing an +// item from the package. +var _ = unused.Item // TODO: Delete before committing! + +func main() { + debugData := debug.Profile() + _ = debugData // Used only during debugging. + .... +} +</pre> + +<h2 id="Performance">Performance</h2> + +<h3 id="Why_does_Go_perform_badly_on_benchmark_x"> +Why does Go perform badly on benchmark X?</h3> + +<p> +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 <a href="/test/bench/">test/bench</a>. 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 <a href="http://gmplib.org/">GMP</a> (which is +written in optimized assembler). +Benchmarks that depend on regular expressions (regex-dna, for instance) are +essentially comparing Go's stopgap <a href="/pkg/regexp">regexp package</a> to +mature, highly optimized regular expression libraries like PCRE. +</p> + +<p> +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. +</p> + +<p> +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). +</p> + +<p> +In any case, Go can often be very competitive. See the blog post about +<a href="http://blog.golang.org/2011/06/profiling-go-programs.html">profiling +Go programs</a> for an informative example. + +<h2 id="change_from_c">Changes from C</h2> + +<h3 id="different_syntax"> +Why is the syntax so different from C?</h3> +<p> +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. +</p> + +<h3 id="declarations_backwards"> +Why are declarations backwards?</h3> +<p> +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 <code>*</code> for pointers is an exception that proves the rule). In C, +the declaration +</p> +<pre> + int* a, b; +</pre> +<p> +declares <code>a</code> to be a pointer but not <code>b</code>; in Go +</p> +<pre> + var a, b *int; +</pre> +<p> +declares both to be pointers. This is clearer and more regular. +Also, the <code>:=</code> short declaration form argues that a full variable +declaration should present the same order as <code>:=</code> so +</p> +<pre> + var a uint64 = 1; +</pre> +has the same effect as +<pre> + a := uint64(1); +</pre> +<p> +Parsing is also simplified by having a distinct grammar for types that +is not just the expression grammar; keywords such as <code>func</code> +and <code>chan</code> keep things clear. +</p> + +<p> +See the article about +<a href="http://blog.golang.org/2010/07/gos-declaration-syntax.html">Go's Declaration Syntax</a> +for more details. +</p> + +<h3 id="no_pointer_arithmetic"> +Why is there no pointer arithmetic?</h3> +<p> +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. +</p> + +<h3 id="inc_dec"> +Why are <code>++</code> and <code>--</code> statements and not expressions? And why postfix, not prefix?</h3> +<p> +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 <code>++</code> and <code>--</code> +(consider <code>f(i++)</code> and <code>p[i] = q[++i]</code>) +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. +</p> + +<h3 id="semicolons"> +Why are there braces but no semicolons? And why can't I put the opening +brace on the next line?</h3> +<p> +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. +</p> + +<p> +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 +<a href="http://golang.org/cmd/gofmt/"><code>gofmt</code></a>, +<i>some</i> style must be chosen. That style may differ from what +you've used in C or Java, but Go is a new language and +<code>gofmt</code>'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. +</p> + +<h3 id="garbage_collection"> +Why do garbage collection? Won't it be too expensive?</h3> +<p> +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.) +</p> + +<p> +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. +</p> + +<p> +Finally, concurrency aside, garbage collection makes interfaces +simpler because they don't need to specify how memory is managed across them. +</p> + +<p> +On the topic of performance, keep in mind that Go gives the programmer +considerable control over memory layout and allocation, much more than +is typical in garbage-collected languages. A careful programmer can reduce +the garbage collection overhead dramatically by using the language well; +see the article about +<a href="http://blog.golang.org/2011/06/profiling-go-programs.html">profiling +Go programs</a> for a worked example, including a demonstration of Go's +profiling tools. +</p> |