doc/go_faq.html

<!-- The Go Programming Language FAQ -->

<h2 id="Origins">Origins</h2>

<h3 id="What_is_the_purpose_of_the_project">
What is the purpose of the project?</h3>

<p>
No major systems language has emerged in over a decade, but over that time
the computing landscape has changed tremendously. There are several trends:

<ul>
<li>
Computers are enormously quicker but software development is not faster.
<li>
Dependency management is a big part of software development today but the
"header files" of languages in the C tradition are antithetical to clean
dependency analysis&mdash;and fast compilation.
<li>
There is a growing rebellion against cumbersome type systems like those of
Java and C++, pushing people towards dynamically typed languages such as
Python and Javascript.
<li>
Some fundamental concepts such as garbage collection and parallel computation
are not well supported by popular systems languages.
<li>
The emergence of multicore computers has generated worry and confusion.
</ul>

<p>
We believe it's worth trying again with a new language, a concurrent,
garbage-collected language with fast compilation. Regarding the points above:

<ul>
<li>
It should be possible to compile a major binary in a few seconds on a
single processor.
<li>
Go provides a model for software construction that makes dependency
analysis easy and avoids much of the overhead of C-style include files and
libraries.
<li>
The type system is not hierarchical, so less time should be spent
rearranging the hierarchy. Also, although Go has static types the language
attempts to make types feel lighter weight than in typical OO languages.
<li>
Go is fully garbage-collected and provides fundamental support for
concurrent execution and communication.
<li>
By its design, Go proposes an approach for the construction of system
software on multicore machines.
</ul>

<h3 id="What_is_the_origin_of_the_name">
What is the origin of the name?</h3>

<p>
"Ogle" would be a good name for a Go debugger.

<h3 id="Why_is_the_compiler_called_6g">Why is the compiler called 6g?</h3>

<p>
The <code>6g</code> (and <code>8g</code>) compiler is named in the
tradition of the Plan 9 C compilers, described in
<a href="http://plan9.bell-labs.com/sys/doc/compiler.html ">
http://plan9.bell-labs.com/sys/doc/compiler.html</a>
(see the table in section 2).

6 is the architecture letter for amd64 (or x86-64, if you prefer).
g 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>

<p>We considered doing that, but too many of the problems&mdash;lack of
garbage collection, long dependency chains, nested include files,
lack of concurrency awareness&mdash;are caused by the C++ language itself.
We felt a viable solution required a more complete approach.

<h2 id="Usage">Usage</h2>

<h3 id="Who_do_you_expect_to_use_the_language">
Who do you expect to use the language?</h3>

<p>
Go is an experiment. We hope adventurous users will give it a try and see
if it provides a viable alternative to the status quo. Not every programmer
will like it but we hope enough will find satisfaction in the approach it
offers to justify further development.

<h3 id="Do_Go_programs_link_with_Cpp_programs">
Do Go programs link with C/C++ programs?</h3>

<p>
There are two Go compilers, <code>6g</code> and <code>gccgo</code>.
<code>6g</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
<code>gcc</code> 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.

<p>
Work is underway to provide a 'foreign function interface' to allow safe
interoperation of languages in a running program.

<h3 id="Does_Go_support_protocol_buffers">
Does Go support protocol buffers?</h3>

<p>
Protocol buffers are supported.  The next release of the
protocol compiler project will include Go code generators
and a Go library for them.
It is still being tweaked but works well.

<h2 id="Design">Design</h2>

<h3 id="Why_doesn_t_Go_have_feature_X">Why doesn't Go have feature X?</h3>

<p>
Every language contains novel features and omits someone's favorite
feature. Go was designed with an eye on felicity of programming, speed of
compilation, orthogonality of concepts, and the need to support features
such as concurrency and garbage collection. Your favorite feature may be
missing because it doesn't fit, because it affects compilation speed or
clarity of design, or because it would make the fundamental system model
too difficult.

<p>
Before you get too worried about Go not having feature <var>X</var>,
please 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>

<p>
This and other language design questions are answered in
the separate <a href="go_lang_faq.html">language FAQ</a>.

<h2 id="Object_Oriented_Programming">
Object-Oriented Programming</h2>

<h3 id="Is_Go_an_object-oriented_language">
Is Go an object-oriented language?</h3>

<p>
Yes and no. Although Go has types and methods and allows an
object-oriented style of programming, there is no type hierarchy.
The concept of "interface" in Go provides a different approach that
we believe is easy to use and in some ways more general. There are
also ways to embed types in other types to provide something
analogous&mdash;but not identical&mdash;to subclassing.

<p>
The lack of type hierarchy makes "objects" in Go feel much more
lightweight than in languages such as C++ and Java.

<h3 id="How_do_I_get_dynamic_dispatch_of_methods">
How do I get dynamic dispatch of methods?</h3>

<p>
The only way to have dynamically dispatched methods is through an
interface. Methods on structs or other types are always resolved statically.

<h2 id="Concurrency">Concurency</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 are available in the
<a href="go_mem.html">Go Memory Model specification</a>.

<p>
One point that has come up: at least for now, maps do not guarantee atomic
update from multiple threads of execution. This is not the place for a
full discussion but in essence it was felt that shared maps are usually
part of some larger synchronized object and forcing synchronization in the
map implementation would be too costly considering how rarely it would
actually help.

<p>
The <a href="/pkg/sync">sync</a>
package implements mutexes, 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>
Do not communicate by sharing memory. Instead, share memory by communicating.

<h2 id="Writing_Code">Writing Code</h2>

<h3 id="How_are_libraries_documented">
How are libraries documented?</h3>

<p>
There is a program, <code>godoc</code>, written in Go, that extracts
package documentation from the source code. It can be used on the
command line or on the web. An instance is running at
<a href="http://go/go">http://go/go</a>.


<h3 id="Is_there_a_Go_programming_style_guide">
Is there a Go programming style guide?</h3>

<p>
Eventually, there will be a small number of rules that guide things
like naming, layout, and file organization. We are thinking of
enforcing layout rules using a pretty-printing program that
implements the rules, rather than writing a compendium of do's and
don'ts that allows interpretation.

<h3 id="How_do_I_submit_patches_to_the_Go_libraries">
How do I submit patches to the Go libraries?</h3>

<ol>
<li>If it's a significant change, discuss on the mailing list before embarking.

<li>Check out the Go source code files.  The library sources are in <code>go/src/pkg</code>.

<li>Make changes; add tests as appropriate. Try to follow existing style,
    including tabs for indentation, and no trailing whitespace. In
    documentation comments for public declarations, use full sentences
    and begin with the name of the thing being described, because godoc
    (or other tools) may someday display these comments out of context.

<li>Write the <code>Makefile</code> by following existing examples.

<li>Run <code>make</code> and <code>make test</code> in the affected
    directories.

<li>If you have added a new dependency, you may need to <code>cd go/src/lib;
    ./deps.bash</code> to update the Make.deps file included in the Makefile.
    For a new component, update the <code>Makefile</code> and then run
    <code>deps.bash</code>.
<li><code>cd go/src; ./all.bash</code>

<li>Once <code>all.bash</code> succeeds (output like
    "N known bugs; 0 unexpected bugs" is OK),
    <a href="/doc/contribute.html">submit a CL</a>.
</ol>

<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 header file or need for forward declarations.

<p>
Other than being split into multiple files, the package will compile and test
just like a single-file package.

<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

<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.

<h3 id="Where_is_assert">
Where is assert?</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 be someone editing the code months later
or even someone editing a different package on which the code depends.
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. Unlike Java (jUnit's home language),
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 worth trying to do better.

<h2 id="Implementation">Implementation</h2>

<h3 id="What_compiler_technology_is_used_to_build_the_compilers">
What compiler technology is used to build the compilers?</h3>

<p>
Gccgo has a C++ front-end with a recursive descent parser coupled to the
standard gcc back end. 6g is written in C using yacc/bison for the parser.
Although it's a new program, it fits in the Plan 9 C compiler suite
(<a href="http://plan9.bell-labs.com/sys/doc/compiler.html">http://plan9.bell-labs.com/sys/doc/compiler.html</a>)
and uses a variant of the Plan 9 loader to generate ELF binaries.

<p>
We considered writing 6g, the original Go compiler, in Go itself but
elected not to do so because of the difficulties of bootstrapping and
especially of open source distribution - you'd need a Go compiler to
set up a Go environment. Gccgo, which came later, makes it possible to
consider rewriting 6g in Go, which might well happen. (Go would be a
fine language in which to implement a compiler; a native lexer and
parser are already available in pkg/go.)

<p>
We also considered using LLVM for 6g but we felt it was too large and
slow to meet our performance goals.

<h3 id="How_is_the_runtime_implemented">
How is the runtime implemented?</h3>

<p>
Again due to bootstrapping issues, the runtime is mostly in C (with a
tiny bit of assembler) although Go is capable of implementing most of
it now. Gccgo's runtime uses glibc; 6g uses a custom library,
compiled with 6c (the Plan 9 C compiler) to keep the footprint under
control. The version of 6c used supports segmented stacks for
goroutines; work is underway to provide the same stack management in
gccgo.