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diff --git a/doc/go_lang_faq.html b/doc/go_lang_faq.html deleted file mode 100644 index b8deb1534..000000000 --- a/doc/go_lang_faq.html +++ /dev/null @@ -1,491 +0,0 @@ -<!-- Language Design FAQ --> - -<h2 id="origins">Origins</h2> - -<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="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> - -<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> - -<h2 id="unicode_identifiers">What's up with Unicode identifiers?</h2> - -<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> - -<h2 id="absent_features">Absent features</h2> - -<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> - -<h3 id="assertions"> -Why does Go not have assertions?</h3> -<p> -This is answered in the general <a href="go_faq.html#Where_is_assert">FAQ</a>. -</p> - -<h2 id="types">Types</h2> - -<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="concurrency">Concurrency</h2> - -<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> |