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<h1 id="The_Go_Programming_Language_Design_FAQ">The Go Programming Language Design FAQ</h1>
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<!-- The Table of Contents is automatically inserted in this <div>.
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Do not delete this <div>. -->
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<div id="nav"></div>
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<h2 id="origins">Origins</h2>
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<h3 id="history">
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What is the history of the project?</h3>
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<p>
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Robert Griesemer, Rob Pike and Ken Thompson started sketching the
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goals for a new language on the white board on September 21, 2007.
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Within a few days the goals had settled into a plan to do something
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and a fair idea of what it would be. Design continued part-time in
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parallel with unrelated activities. By January 2008, Ken started work
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on a compiler with which to explore ideas; it generated C code as its
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output. By mid-year the language had become a full-time project and
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had settled enough to attempt a production compiler. Meanwhile, Ian
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Taylor had read the draft specification and written an independent GCC
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front end.
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</p>
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<p>
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In the last few months of 2008, Russ Cox joined the team and Go had
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reached the point where it was usable as the main programming language
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for the team's own work.
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</p>
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<h3 id="creating_a_new_language">
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Why are you creating a new language?</h3>
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<p>
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Go was born out of frustration with existing languages and
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environments for systems programming. Programming had become too
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difficult and the choice of languages was partly to blame. One had to
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choose either efficient compilation, efficient execution, or ease of
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programming; all three were not available in the same commonly
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available language. Programmers who could were choosing ease over
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safety and efficiency by moving to dynamic languages such as
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Python and JavaScript rather than C++ or, to a lesser extent, Java.
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</p>
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<p>
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Go is an attempt to combine the ease of programming of a dynamic
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language with the efficiency and type safety of a compiled language.
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It also aims to be modern, with support for networked and multicore
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computing. Finally, it is intended to be <i>fast</i>: it should take
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at most a few seconds to build a large executable on a single computer.
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To meet these goals required addressing a number of
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linguistic issues: an expressive but lightweight type system;
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concurrency and garbage collection; rigid dependency specification;
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and so on. These cannot be addressed well by libraries or tools; a new
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language was called for.
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</p>
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<h3 id="ancestors">
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What are Go's ancestors?</h3>
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<p>
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Go is mostly in the C family (basic syntax),
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with significant input from the Pascal/Modula/Oberon
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family (declarations, packages),
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plus it borrows some ideas from languages
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inspired by Tony Hoare's CSP,
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such as Newsqueak and Limbo (concurrency).
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However, it is a new language across the board.
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In every respect the language was designed by thinking
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about what programmers do and how to make programming, at least the
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kind of programming we do, more effective, which means more fun.
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</p>
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<h3 id="protagonists">
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Who are the protagonists?</h3>
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<p>
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Robert Griesemer, Rob Pike and Ken Thompson laid out the goals and
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original specification of the language. Ian Taylor read the draft
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specification and decided to write <code>gccgo</code>. Russ
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Cox joined later and helped move the language and libraries from
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prototype to reality.
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</p>
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<h3 id="principles">
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What are the guiding principles in the design?</h3>
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<p>
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Programming today involves too much bookkeeping, repetition, and
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clerical work. As Dick Gabriel says, “Old programs read
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like quiet conversations between a well-spoken research worker and a
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well-studied mechanical colleague, not as a debate with a compiler.
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Who'd have guessed sophistication bought such noise?”
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The sophistication is worthwhile—no one wants to go back to
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the old languages—but can it be more quietly achieved?
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</p>
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<p>
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Go attempts to reduce the amount of typing in both senses of the word.
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Throughout its design, we have tried to reduce the clutter and
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complexity. There are no forward declarations and no header files;
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everything is declared exactly once. Initialization is expressive,
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automatic, and easy to use. Syntax is clean and light on keywords.
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Stuttering (<code>foo.Foo* myFoo = new(foo.Foo)</code>) is reduced by
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simple type derivation using the <code>:=</code>
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declare-and-initialize construct. And perhaps most radically, there
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is no type hierarchy: types just <i>are</i>, they don't have to
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announce their relationships. These simplifications allow Go to be
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expressive yet comprehensible without sacrificing, well, sophistication.
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</p>
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<h2 id="change_from_c">Changes from C</h2>
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<h3 id="different_syntax">
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Why is the syntax so different from C?</h3>
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<p>
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Other than declaration syntax, the differences are not major and stem
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from two desires. First, the syntax should feel light, without too
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many mandatory keywords, repetition, or arcana. Second, the language
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has been designed to be easy to parse. The grammar is conflict-free
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and can be parsed without a symbol table. This makes it much easier
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to build tools such as debuggers, dependency analyzers, automated
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documentation extractors, IDE plug-ins, and so on. C and its
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descendants are notoriously difficult in this regard but it's not hard
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to fix things up.
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</p>
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<h3 id="declarations_backwards">
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Why are declarations backwards?</h3>
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<p>
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They're only backwards if you're used to C. In C, the notion is that a
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variable is declared like an expression denoting its type, which is a
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nice idea, but the type and expression grammars don't mix very well and
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the results can be confusing; consider function pointers. Go mostly
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separates expression and type syntax and that simplifies things (using
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prefix <code>*</code> for pointers is an exception that proves the rule). In C,
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the declaration
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</p>
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<pre>
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int* a, b;
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</pre>
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<p>
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declares <code>a</code> to be a pointer but not <code>b</code>; in Go
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</p>
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<pre>
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var a, b *int;
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</pre>
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<p>
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declares both to be pointers. This is clearer and more regular.
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Also, the <code>:=</code> short declaration form argues that a full variable
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declaration should present the same order as <code>:=</code> so
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</p>
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<pre>
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var a uint64 = 1;
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</pre>
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has the same effect as
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<pre>
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a := uint64(1);
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</pre>
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<p>
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Parsing is also simplified by having a distinct grammar for types that
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is not just the expression grammar; keywords such as <code>func</code>
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and <code>chan</code> keep things clear.
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</p>
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<h3 id="no_pointer_arithmetic">
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Why is there no pointer arithmetic?</h3>
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<p>
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Safety. Without pointer arithmetic it's possible to create a
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language that can never derive an illegal address that succeeds
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incorrectly. Compiler and hardware technology have advanced to the
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point where a loop using array indices can be as efficient as a loop
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using pointer arithmetic. Also, the lack of pointer arithmetic can
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simplify the implementation of the garbage collector.
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</p>
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<h3 id="inc_dec">
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Why are <code>++</code> and <code>--</code> statements and not expressions? And why postfix, not prefix?</h3>
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<p>
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Without pointer arithmetic, the convenience value of pre- and postfix
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increment operators drops. By removing them from the expression
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hierarchy altogether, expression syntax is simplified and the messy
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issues around order of evaluation of <code>++</code> and <code>--</code>
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(consider <code>f(i++)</code> and <code>p[i] = q[++i]</code>)
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are eliminated as well. The simplification is
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significant. As for postfix vs. prefix, either would work fine but
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the postfix version is more traditional; insistence on prefix arose
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with the STL, part of a language whose name contains, ironically, a
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postfix increment.
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</p>
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<h2 id="absent_features">Absent features</h2>
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<h3 id="generics">
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Why does Go not have generic types?</h3>
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<p>
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Generics may well come at some point. We don't feel an urgency for
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them, although we understand some programmers do.
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</p>
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<p>
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Generics are convenient but they come at a cost in
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complexity in the type system and run-time. We haven't yet found a
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design that gives value proportionate to the complexity, although we
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continue to think about it. Meanwhile, Go's built-in maps and slices,
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plus the ability to use the empty interface to construct containers
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(with explicit unboxing) mean in many cases it is possible to write
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code that does what generics would enable, if less smoothly.
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</p>
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<p>
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This remains an open issue.
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</p>
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<h3 id="exceptions">
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Why does Go not have exceptions?</h3>
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<p>
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Exceptions are a similar story. A number of designs for exceptions
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have been proposed but each adds significant complexity to the
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language and run-time. By their very nature, exceptions span functions and
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perhaps even goroutines; they have wide-ranging implications. There
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is also concern about the effect they would have on the
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libraries. They are, by definition, exceptional yet experience with
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other languages that support them show they have profound effect on
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library and interface specification. It would be nice to find a design
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that allows them to be truly exceptional without encouraging common
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errors to turn into special control flow requiring every programmer to
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compensate.
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</p>
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<p>
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Like generics, exceptions remain an open issue.
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</p>
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<h3 id="assertions">
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Why does Go not have assertions?</h3>
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<p>
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This is answered in the general <a href="go_faq.html#Where_is_assert">FAQ</a>.
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</p>
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<h2 id="types">Types</h2>
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<h3 id="inheritance">
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Why is there no type inheritance?</h3>
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<p>
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Object-oriented programming, at least in the languages we've used,
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involves too much discussion of the relationships between types,
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relationships that often could be derived automatically. Go takes a
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different approach that we're still learning about but that feels
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useful and powerful.
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</p>
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<p>
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Rather than requiring the programmer to declare ahead of time that two
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types are related, in Go a type automatically satisfies any interface
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that specifies a subset of its methods. Besides reducing the
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bookkeeping, this approach has real advantages. Types can satisfy
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many interfaces at once, without the complexities of traditional
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multiple inheritance.
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Interfaces can be very lightweight—one or even zero methods
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in an interface can express useful concepts.
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Interfaces can be added after the fact if a new idea comes along
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or for testing—without annotating the original type.
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Because there are no explicit relationships between types
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and interfaces, there is no type hierarchy to manage.
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</p>
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<p>
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It's possible to use these ideas to construct something analogous to
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type-safe Unix pipes. For instance, see how <code>fmt.Fprintf</code>
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enables formatted printing to any output, not just a file, or how the
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<code>bufio</code> package can be completely separate from file I/O,
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or how the <code>crypto</code> packages stitch together block and
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stream ciphers. All these ideas stem from a single interface
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(<code>io.Writer</code>) representing a single method
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(<code>Write</code>). We've only scratched the surface.
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</p>
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<p>
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It takes some getting used to but this implicit style of type
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dependency is one of the most exciting things about Go.
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</p>
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<h3 id="methods_on_basics">
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Why is <code>len</code> a function and not a method?</h3>
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<p>
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To be blunt, Go isn't that kind of language. We debated this issue but decided
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implementing <code>len</code> and friends as functions was fine in practice and
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didn't complicate questions about the interface (in the Go type sense)
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of basic types. The issue didn't seem important enough to resolve that way.
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</p>
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<h3 id="overloading">
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Why does Go not support overloading of methods and operators?</h3>
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<p>
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Method dispatch is simplified if it doesn't need to do type matching as well.
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Experience with other languages told us that having a variety of
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methods with the same name but different signatures was occasionally useful
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but that it could also be confusing and fragile in practice. Matching only by name
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and requiring consistency in the types was a major simplifying decision
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in Go's type system.
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</p>
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<p>
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Regarding operator overloading, it seems more a convenience than an absolute
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requirement. Again, things are simpler without it.
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</p>
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<h3 id="builtin_maps">
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Why are maps built in?</h3>
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<p>
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The same reason strings are: they are such a powerful and important data
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structure that providing one excellent implementation with syntactic support
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makes programming more pleasant. We believe that Go's implementation of maps
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is strong enough that it will serve for the vast majority of uses.
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If a specific application can benefit from a custom implementation, it's possible
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to write one but it will not be as convenient to use; this seems a reasonable tradeoff.
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</p>
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<h3 id="map_keys">
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Why don't maps allow structs and arrays as keys?</h3>
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<p>
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Map lookup requires an equality operator, which structs and arrays do not implement.
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They don't implement equality because equality is not well defined on such types;
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there are multiple considerations involving shallow vs. deep comparison, pointer vs.
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value comparison, how to deal with recursive structures, and so on.
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We may revisit this issue—and implementing equality for structs and arrays
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will not invalidate any existing programs—but without a clear idea of what
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equality of structs and arrays should mean, it was simpler to leave it out for now.
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</p>
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<h2 id="concurrency">Concurrency</h2>
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<h3 id="csp">
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Why build concurrency on the ideas of CSP?</h3>
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<h3 id="goroutines">
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What's the idea behind goroutines?</h3>
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<h3 id="atomic_maps">
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Why are map operations not defined to be atomic?</h3>
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<p>
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After long discussion it was decided that the typical use of maps did not require
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safe access from multiple threads, and in those cases where it did, the map was
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probably part of some larger data structure or computation that was already
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synchronized. Therefore requiring that all map operations grab a mutex would slow
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down most programs and add safety to few. This was not an easy decision,
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however, since it means uncontrolled map access can crash the program.
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</p>
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<p>
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The language does not preclude atomic map updates. When required, such
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as when hosting an untrusted program, the implementation could interlock
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map access.
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</p>
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<h3 id="TODO">
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TODO</h3>
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<p>TODO:</p>
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<pre>
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explain:
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package design
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slices
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oo separate from storage (abstraction vs. implementation)
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why garbage collection?
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inheritance?
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embedding?
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dependency declarations in the language
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oo questions
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no data in interfaces
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dynamic dispatch
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clean separation of interface and implementation
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why no automatic numeric conversions?
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make vs new
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</pre>
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</div>
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<div id="footer">
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<p>Except as noted, this content is
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licensed under <a href="http://creativecommons.org/licenses/by/3.0/">
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Creative Commons Attribution 3.0</a>.
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</div>
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</body>
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