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diff --git a/doc/go_faq.html b/doc/go_faq.html deleted file mode 100644 index 560ab3617..000000000 --- a/doc/go_faq.html +++ /dev/null @@ -1,1254 +0,0 @@ -<!-- 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: - -<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> - -<p> -We believe it's worth trying again with a new language, a concurrent, -garbage-collected language with fast compilation. Regarding the points above: - -<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> -</p> - -<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> -Many others 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. 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. -</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 even 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, 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. -</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—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. -</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 exciting 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="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> - -<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) someMethod() { } // method on pointer -func (s MyStruct) someMethod() { } // method on value -</pre> - -<p> -When defining a method on a type, the receiver (<code>s</code> 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. -</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> - -<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> - -<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 <a href="http://blog.golang.org/2010/07/gos-declaration-syntax.html">Go's Declaration Syntax</a> article 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> |