<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:
+</p>
<ul>
<li>
<li>
The emergence of multicore computers has generated worry and confusion.
</ul>
-</p>
<p>
We believe it's worth trying again with a new language, a concurrent,
garbage-collected language with fast compilation. Regarding the points above:
+</p>
<ul>
<li>
By its design, Go proposes an approach for the construction of system
software on multicore machines.
</ul>
-</p>
<h3 id="What_is_the_origin_of_the_name">
What is the origin of the name?</h3>
</p>
<p>
-Many others have contributed ideas, discussions, and code.
+Go became a public open source project on November 10, 2009.
+Many people from the community have contributed ideas, discussions, and code.
</p>
<h3 id="creating_a_new_language">
</p>
<p>
-Go takes a different approach. Instead of exceptions, it has a couple
+Go takes a different approach. For plain error handling, Go's multi-value
+returns make it easy to report an error without overloading the return value.
+<a href="http://blog.golang.org/2011/07/error-handling-and-go.html">A
+canonical error type, coupled
+with Go's other features</a>, makes error
+handling pleasant but quite different from that in other languages.
+</p>
+
+<p>
+Go also 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
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.
+such as mutexes, condition variables, and memory barriers.
Higher-level interfaces enable much simpler code, even if there are still
mutexes and such under the covers.
</p>
<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.
+functions—coroutines—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.
+for the stack, which is just a few kilobytes.
</p>
<p>
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 very lightweight—an interface with
+one or even zero methods can express a useful concept.
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
<p>
It takes some getting used to but this implicit style of type
-dependency is one of the most exciting things about Go.
+dependency is one of the most productive things about Go.
</p>
<h3 id="methods_on_basics">
among similar interfaces.
</p>
+<h3 id="t_and_equal_interface">
+Why doesn't type T satisfy the Equal interface?</h3>
+
+<p>
+Consider this simple interface to represent an object that can compare
+itself with another value:
+</p>
+
+<pre>
+type Equaler interface {
+ Equal(Equaler) bool
+}
+</pre>
+
+<p>
+and this type, <code>T</code>:
+</p>
+
+<pre>
+type T int
+func (t T) Equal(u T) bool { return t == u } // does not satisfy Equaler
+</pre>
+
+<p>
+Unlike the analogous situation in some polymorphic type systems,
+<code>T</code> does not implement <code>Equaler</code>.
+The argument type of <code>T.Equal</code> is <code>T</code>,
+not literally the required type <code>Equaler</code>.
+</p>
+
+<p>
+In Go, the type system does not promote the argument of
+<code>Equal</code>; that is the programmer's responsibility, as
+illustrated by the type <code>T2</code>, which does implement
+<code>Equaler</code>:
+</p>
+
+<pre>
+type T2 int
+func (t T2) Equal(u Equaler) bool { return t == u.(T2) } // satisfies Equaler
+</pre>
+
+<p>
+Even this isn't like other type systems, though, because in Go <em>any</em>
+type that satisfies <code>Equaler</code> could be passed as the
+argument to <code>T2.Equal</code>, and at run time we must
+check that the argument is of type <code>T2</code>.
+Some languages arrange to make that guarantee at compile time.
+</p>
+
+<p>
+A related example goes the other way:
+</p>
+
+<pre>
+type Opener interface {
+ Open(name) Reader
+}
+
+func (t T3) Open() *os.File
+</pre>
+
+<p>
+In Go, <code>T3</code> does not satisfy <code>Opener</code>,
+although it might in another language.
+</p>
+
+<p>
+While it is true that Go's type system does less for the programmer
+in such cases, the lack of subtyping makes the rules about
+interface satisfaction very easy to state: are the function's names
+and signatures exactly those of the interface?
+Go's rule is also easy to implement efficiently.
+We feel these benefits offset the lack of
+automatic type promotion. Should Go one day adopt some form of generic
+typing, we expect there would be a way to express the idea of these
+examples and also have them be statically checked.
+</p>
+
<h3 id="convert_slice_of_interface">
Can I convert a []T to an []interface{}?</h3>
Should I define methods on values or pointers?</h3>
<pre>
-func (s *MyStruct) someMethod() { } // method on pointer
-func (s MyStruct) someMethod() { } // method on value
+func (s *MyStruct) pointerMethod() { } // method on pointer
+func (s MyStruct) valueMethod() { } // method on value
</pre>
<p>
+For programmers unaccustomed to pointers, the distinction between these
+two examples can be confusing, but the situation is actually very simple.
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.
+example) behaves exactly as if it were an argument to the method.
+Whether to define the receiver as a value or as a pointer is the same
+question, then, as whether a function argument should be a value or
+a pointer.
+There are several considerations.
+</p>
+
+<p>
+First, and most important, does the method need to modify the
+receiver?
+If it does, the receiver <em>must</em> be a pointer.
+(Slices and maps are reference types, so their story is a little
+more subtle, but for instance to change the length of a slice
+in a method the receiver must still be a pointer.)
+In the examples above, if <code>pointerMethod</code> modifies
+the fields of <code>s</code>,
+the caller will see those changes, but <code>valueMethod</code>
+is called with a copy of the caller's argument (that's the definition
+of passing a value), so changes it makes will be invisible to the caller.
+</p>
+
+<p>
+By the way, pointer receivers are identical to the situation in Java,
+although in Java the pointers are hidden under the covers; it's Go's
+value receivers that are unusual.
+</p>
+
+<p>
+Second is the consideration of efficiency. If the receiver is large,
+a big <code>struct</code> for instance, it will be much cheaper to
+use a pointer receiver.
</p>
+<p>
+Next is consistency. If some of the methods of the type must have
+pointer receivers, the rest should too, so the method set is
+consistent regardless of how the type is used.
+See the section on <a href="#different_method_sets">method sets</a>
+for details.
+</p>
+
+<p>
+For types such as basic types, slices, and small <code>structs</code>,
+a value receiver is very cheap so unless the semantics of the method
+requires a pointer, a value receiver is efficient and clear.
+</p>
+
+
<h3 id="new_and_make">
What's the difference between new and make?</h3>
garbage can have a huge effect).
</p>
+<p>
+In any case, Go can often be very competitive. See the blog post about
+<a href="http://blog.golang.org/2011/06/profiling-go-programs.html">profiling
+Go programs</a> for an informative example.
+
<h2 id="change_from_c">Changes from C</h2>
<h3 id="different_syntax">
</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.
+See the article about
+<a href="http://blog.golang.org/2010/07/gos-declaration-syntax.html">Go's Declaration Syntax</a>
+for more details.
</p>
<h3 id="no_pointer_arithmetic">
Finally, concurrency aside, garbage collection makes interfaces
simpler because they don't need to specify how memory is managed across them.
</p>
+
+<p>
+On the topic of performance, keep in mind that Go gives the programmer
+considerable control over memory layout and allocation, much more than
+is typical in garbage-collected languages. A careful programmer can reduce
+the garbage collection overhead dramatically by using the language well;
+see the article about
+<a href="http://blog.golang.org/2011/06/profiling-go-programs.html">profiling
+Go programs</a> for a worked example, including a demonstration of Go's
+profiling tools.
+</p>