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diff --git a/doc/go_faq.html b/doc/go_faq.html index 4f11baa80..1c7b85ef8 100644 --- a/doc/go_faq.html +++ b/doc/go_faq.html @@ -56,6 +56,21 @@ What is the origin of the name?</h3> <p> “Ogle” would be a good name for a Go debugger. +<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> @@ -69,42 +84,102 @@ http://plan9.bell-labs.com/sys/doc/compiler.html</a> <code>6</code> is the architecture letter for amd64 (or x86-64, if you prefer), while <code>g</code> stands for Go. -<h3 id="Why_not_just_write_some_libraries_for_Cpp_to_do_communication"> -Why not just write some libraries for C++ to do communication?</h3> +<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>We considered doing that, but too many of the problems—lack of -garbage collection, long dependency chains, nested include files, -lack of concurrency awareness—are rooted in the design of -the C and C++ languages themselves. -We felt a viable solution required a more complete approach. +<p> +Many others have contributed ideas, discussions, and code. +</p> -<h3 id="Why_doesnt_Go_run_on_Windows"> -Why doesn't Go run on Windows yet?</h3> +<h3 id="creating_a_new_language"> +Why are you creating a new language?</h3> <p> -We understand that a significant fraction of computers in the world -run Windows and it would be great if those computers could run Go -programs. A group of volunteers has made significant progress toward -porting Go to <a href="http://www.mingw.org/">MinGW</a>. -You can follow their progress at the Go Wiki's -<a href="http://code.google.com/p/go/wiki/WindowsPort">WindowsPort</a> page. +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="Whats_the_origin_of_the_mascot"> -What's the origin of the mascot?</h3> +<h3 id="ancestors"> +What are Go's ancestors?</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. +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="Who_should_use_the_language"> @@ -116,37 +191,31 @@ if they enjoy it. Not every programmer will, but we hope enough will find satisfaction in the approach it offers to justify further development. -<h3 id="Is_Google_using_go_internally"> Is Google using Go -internally?</h3> +<h3 id="Is_Google_using_go_internally"> Is Google using Go internally?</h3> -<p> The Go project was conceived to make it easier to write the kind -of servers and other software Google uses internally, but the -implementation isn't quite mature enough yet for large-scale -production use. While we continue development we are also doing -experiments with the language as a candidate server environment. It's -getting there. 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>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. <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>. +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. However, because Go is garbage-collected it will be -unwise to do so, at least naively. +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> -There is a “foreign function interface” to allow safe calling of C-written -libraries from Go code. We expect to use SWIG to extend this capability -to C++ libraries. There is no safe way to call Go code from C or C++ yet. +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. <h3 id="Does_Go_support_Google_protocol_buffers"> Does Go support Google's protocol buffers?</h3> @@ -170,7 +239,35 @@ you will need to abide by the guidelines at <h2 id="Design">Design</h2> -<h3 id="Why_doesnt_Go_have_feature_X">Why doesn't Go have feature X?</h3> +<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 @@ -186,15 +283,145 @@ 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>. -<h3 id="Why_is_the_syntax_so_different_from_Cpp"> -Why is the syntax so different from C++?</h3> +<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> -This and other language design questions are answered in -the separate <a href="go_lang_faq.html">language design FAQ</a>. +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> -<h2 id="Object_Oriented_Programming"> -Object-Oriented Programming</h2> + +<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> +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> +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> +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. + +<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> @@ -220,6 +447,130 @@ How do I get dynamic dispatch of methods?</h3> The only way to have dynamically dispatched methods is through an interface. Methods on structs or other types are always resolved statically. +<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> + + +<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> @@ -259,41 +610,284 @@ See the document <a href="contribute.html">Contributing to the Go project</a> for more information about how to proceed. -<h3 id="Where_is_assert"> -Where is assert?</h3> + +<h2 id="Pointers">Pointers and Allocation</h2> + +<h3 id="pass_by_value"> +When are function parameters passed by value?</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. +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> -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. +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> -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. +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> -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. +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 size of <code>int</code> and <code>float</code> is implementation-specific. +The 64 bit Go compilers (both 6g and gccgo) use a 32 bit representation for +both <code>int</code> and <code>float</code>. Code that relies on a particular +size of value should use an explicitly sized type, like <code>int64</code> or +<code>float64</code>. +</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 +runtime 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">runtime package +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> +The Go runtime's scheduler is not as good as it needs to be. In future, it +should recognise such cases and optimize its use of OS threads. For now, +<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 i := 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>) { + 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> @@ -368,3 +962,143 @@ 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> + |