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@ -145,17 +145,11 @@ Timeline (9/5/08):
<h2>Contents</h2> <h2>Contents</h2>
<p>
TODO: This should be autogenerated but that will take work to make h3s appear in this list
</p>
<ul> <ul>
<li>Introduction <li>Introduction
<ul>
<li>Guiding principles
<li>Program structure
<li>Modularity, identifiers and scopes
<li>Typing, polymorphism, and object-orientation
<li>Pointers and garbage collection
<li>Values and references
<li>Multithreading and channels
</ul>
<li>Notation <li>Notation
@ -288,171 +282,68 @@ Timeline (9/5/08):
<h2>Introduction</h2> <h2>Introduction</h2>
Go is a new systems programming language intended as an alternative to C++ at
Google. Its main purpose is to provide a productive and efficient programming
environment for compiled programs such as servers and distributed systems.
<h3>Guiding principles</h3>
The design of Go is motivated by the following goals (in no particular order):
<p> <p>
<ul> This is a reference manual for the Go programming language. For
<li>very fast compilation, instantaneous incremental compilation more information and other documents, see <a
<li>strongly typed href="/">the Go home page</a>.
<li>procedural </p>
<li>concise syntax avoiding repetition
<li>few, orthogonal, and general concepts
<li>support for threading and interprocess communication
<li>garbage collection
<li>container library written in Go
<li>efficient code, comparable to other compiled languages
</ul>
<h3>Program structure</h3>
A Go program consists of a number of ``packages''.
<p> <p>
A package is built from one or more source files, each of which consists Go is a general-purpose language designed with systems programming
of a package specifier followed by declarations. There are no statements at in mind. It is strongly typed and garbage-collected, and has explicit
the top level of a file. support for concurrent programming. Programs are constructed from
<i>packages</i>, whose properties allow efficient management of
dependencies. The existing implementations use a traditional
compile/link model to generate executable binaries.
</p>
<p> <p>
By convention, the package called "main" is the starting point for execution. The grammar is compact and regular, allowing for easy analysis by
It contains a function, also called "main", that is the first function invoked automatic tools such as integrated development environments.
by the run time system after initialization (if a source file within the program </p>
contains a function "init()", that function will be executed before "main.main()"
is called).
<p>
Source files can be compiled separately (without the source code of packages
they depend on), but not independently (the compiler does check dependencies
by consulting the symbol information in compiled packages).
<h3>Modularity, identifiers and scopes</h3>
A package is a collection of import, constant, type, variable, and function
declarations. Each declaration binds an ``identifier'' with a program entity
(such as a variable).
<p>
In particular, all identifiers occurring in a package are either declared
explicitly within the package, arise from an import declaration, or belong
to a small set of predeclared identifiers (such as "string").
<p>
Scoping follows the usual rules: The scope of an identifier declared within
a ``block'' generally extends from the declaration of the identifier to the
end of the block. An identifier shadows identifiers with the same name declared
in outer scopes. Within a scope, an identifier can be declared at most once.
<p>
Identifiers may be ``internal'' or ``exported''. Internal identifiers are only
accessible to files belonging to the package in which they are declared.
External identifiers are accessible to other packages.
<h3>Typing, polymorphism, and object-orientation</h3>
Go programs are strongly typed. Certain variables may be polymorphic.
The language provides mechanisms to make use of such polymorphic variables
type-safe.
<p>
Object-oriented programming is supported by interface types.
Different interface types are independent of each
other and no explicit hierarchy is required (such as single or
multiple inheritance explicitly specified through respective type
declarations). Interface types only define a set of methods that a
corresponding implementation must provide. Thus interface and
implementation are strictly separated.
<p>
An interface is implemented by associating methods with types. If a type
defines all methods of an interface, it implements that interface and thus
can be used where that interface is required. Unless used through a variable
of interface type, methods can always be statically bound (they are not
``virtual''), and invoking them incurs no extra run-time overhead compared
to ordinary functions.
<p>
Go has no explicit notion of classes, sub-classes, or inheritance.
These concepts are trivially modeled in Go through the use of
functions, structures, embedding of types, associated methods, and interfaces.
<p>
Go has no explicit notion of type parameters or templates. Instead,
containers (such as stacks, lists, etc.) are implemented through the
use of abstract operations on interface types.
<h3>Pointers and garbage collection</h3>
Variables may be allocated automatically (when entering the scope of
the variable) or explicitly on the heap. Pointers are used to refer
to heap-allocated variables. Pointers may also be used to point to
any other variable; such a pointer is obtained by "taking the
address" of that variable. Variables are automatically reclaimed when
they are no longer accessible. There is no pointer arithmetic in Go.
<h3>Values and references</h3>
Most data types have value semantics, but their contents may be accessed
through different pointers referring to the same object. However, some
data types have reference semantics to facilitate common usage patterns
and implementation.
<p>
For example, when calling a function with a struct, the struct is passed
by value, possibly by making a copy. To pass a reference, one must explicitly
pass a pointer to the struct. On the other hand, when calling a function with
a map, a reference to the map is passed implicitly without the need to pass a
pointer to the map; thus the map contents are not copied when a map is assigned
to a variable.
<h3>Multithreading and channels</h3>
Go supports multithreaded programming directly. A function may
be invoked as a parallel thread of execution. Communication and
synchronization are provided through channels and their associated
language support.
<hr>
<h2>Notation</h2> <h2>Notation</h2>
<p>
The syntax is specified using Extended Backus-Naur Form (EBNF): The syntax is specified using Extended Backus-Naur Form (EBNF):
</p>
<pre> <pre>
Production = production_name "=" Expression . Production = production_name "=" Expression .
Expression = Alternative { "|" Alternative } . Expression = Alternative { "|" Alternative } .
Alternative = Term { Term } . Alternative = Term { Term } .
Term = production_name | token [ "..." token ] | Group | Option | Repetition . Term = production_name | token [ "..." token ] | Group | Option | Repetition .
Group = "(" Expression ")" . Group = "(" Expression ")" .
Option = "[" Expression ")" . Option = "[" Expression ")" .
Repetition = "{" Expression "}" . Repetition = "{" Expression "}" .
</pre> </pre>
Productions are expressions constructed from terms and the following operators: <p>
Productions are expressions constructed from terms and the following
operators, in increasing precedence:
</p>
<pre> <pre>
| separates alternatives (least binding strength) | alternation
() groups () grouping
[] specifies an option (0 or 1 times) [] option (0 or 1 times)
{} specifies repetition (0 to n times) {} repetition (0 to n times)
</pre> </pre>
Lower-case production names are used to identify productions that cannot
be broken by white space or comments; they are tokens. Other production
names are in CamelCase.
<p> <p>
Tokens (lexical symbols) are enclosed in double quotes '''' (the Lower-case production names are used to identify lexical tokens.
double quote symbol is written as ''"''). Non-terminals are in CamelCase. Lexical symbols are enclosed in
double quotes <tt>""</tt> (the double quote symbol is written as
<tt>'"'</tt>).
</p>
<p> <p>
The form "a ... b" represents the set of characters from "a" through "b" as The form <tt>"a ... b"</tt> represents the set of characters from
alternatives. <tt>a</tt> through <tt>b</tt> as alternatives.
</p>
<p> <p>
Where possible, recursive productions are used to express evaluation order Where possible, recursive productions are used to express evaluation order
and operator precedence syntactically (for instance for expressions). and operator precedence syntactically.
<p> </p>
A production may be referenced from various places in this document
but is usually defined close to its first use. Productions and code
examples are indented.
<hr>
<h2>Source code representation</h2> <h2>Source code representation</h2>
@ -472,8 +363,9 @@ point is a single character in Go.
<h3>Characters</h3> <h3>Characters</h3>
The following terms are used to denote specific Unicode character classes:
<p> <p>
The following terms are used to denote specific Unicode character classes:
</p>
<ul> <ul>
<li>unicode_char an arbitrary Unicode code point <li>unicode_char an arbitrary Unicode code point
<li>unicode_letter a Unicode code point classified as "Letter" <li>unicode_letter a Unicode code point classified as "Letter"
@ -575,9 +467,10 @@ numbers.
<h3>Character and string literals</h3> <h3>Character and string literals</h3>
<p>
Character and string literals are almost the same as in C, with the Character and string literals are almost the same as in C, with the
following differences: following differences:
<p> </p>
<ul> <ul>
<li>The encoding is UTF-8 <li>The encoding is UTF-8
<li>`` strings exist; they do not interpret backslashes <li>`` strings exist; they do not interpret backslashes
@ -591,8 +484,9 @@ The rules are:
escaped_char = "\" ( "a" | "b" | "f" | "n" | "r" | "t" | "v" | "\" | "'" | """ ) . escaped_char = "\" ( "a" | "b" | "f" | "n" | "r" | "t" | "v" | "\" | "'" | """ ) .
</pre> </pre>
A unicode_value takes one of four forms:
<p> <p>
A unicode_value takes one of four forms:
</p>
<ul> <ul>
<li>The UTF-8 encoding of a Unicode code point. Since Go source <li>The UTF-8 encoding of a Unicode code point. Since Go source
text is in UTF-8, this is the obvious translation from input text is in UTF-8, this is the obvious translation from input
@ -1220,9 +1114,10 @@ available through the two predeclared constants, "true" and "false".
<h3>Strings</h3> <h3>Strings</h3>
<p>
The "string" type represents the set of string values (strings). The "string" type represents the set of string values (strings).
Strings behave like arrays of bytes, with the following properties: Strings behave like arrays of bytes, with the following properties:
<p> </p>
<ul> <ul>
<li>They are immutable: after creation, it is not possible to change the <li>They are immutable: after creation, it is not possible to change the
contents of a string. contents of a string.
@ -1789,6 +1684,7 @@ the same ValueType. They are equal if both values were created by the same
<h3>Type equality</h3> <h3>Type equality</h3>
<p>
Types may be ``different'', ``structurally equal'', or ``identical''. Types may be ``different'', ``structurally equal'', or ``identical''.
Go is a type-safe language; generally different types cannot be mixed Go is a type-safe language; generally different types cannot be mixed
in binary operations, and values cannot be assigned to variables of different in binary operations, and values cannot be assigned to variables of different
@ -1803,7 +1699,7 @@ are equal. Two type literals specify equal types if they have the same
literal structure and corresponding components have equal types. Loosely literal structure and corresponding components have equal types. Loosely
speaking, two types are equal if their values have the same layout in memory. speaking, two types are equal if their values have the same layout in memory.
More precisely: More precisely:
<p> </p>
<ul> <ul>
<li>Two array types are equal if they have equal element types and if they <li>Two array types are equal if they have equal element types and if they
have the same array length. have the same array length.
@ -1833,12 +1729,13 @@ More precisely:
<p> <p>
Type identity is defined by these rules: Type identity is defined by these rules:
</p>
<p> <p>
Two type names denote identical types if they originate in the same Two type names denote identical types if they originate in the same
type declaration. Two type literals specify identical types if they have the type declaration. Two type literals specify identical types if they have the
same literal structure and corresponding components have identical types. same literal structure and corresponding components have identical types.
More precisely: More precisely:
<p> </p>
<ul> <ul>
<li>Two array types are identical if they have identical element types and if <li>Two array types are identical if they have identical element types and if
they have the same array length. they have the same array length.
@ -2199,12 +2096,14 @@ A primary expression of the form
a[x] a[x]
</pre> </pre>
<p>
denotes the array or map element x. The value x is called the denotes the array or map element x. The value x is called the
``array index'' or ``map key'', respectively. The following ``array index'' or ``map key'', respectively. The following
rules apply: rules apply:
</p>
<p> <p>
For a of type A or *A where A is an array type (§Array types): For a of type A or *A where A is an array type (§Array types):
<p> </p>
<ul> <ul>
<li>x must be an integer value and 0 &lt;= x &lt; len(a) <li>x must be an integer value and 0 &lt;= x &lt; len(a)
<li>a[x] is the array element at index x and the type of a[x] <li>a[x] is the array element at index x and the type of a[x]
@ -2212,7 +2111,7 @@ For a of type A or *A where A is an array type (§Array types):
</ul> </ul>
<p> <p>
For a of type *M, where M is a map type (§Map types): For a of type *M, where M is a map type (§Map types):
<p> </p>
<ul> <ul>
<li>x must be of the same type as the key type of M <li>x must be of the same type as the key type of M
and the map must contain an entry with key x and the map must contain an entry with key x
@ -2434,8 +2333,9 @@ mul_op = "*" | "/" | "%" | "&lt;&lt;" | ">>" | "&amp;" .
unary_op = "+" | "-" | "!" | "^" | "*" | "&amp;" | "&lt;-" . unary_op = "+" | "-" | "!" | "^" | "*" | "&amp;" | "&lt;-" .
</pre> </pre>
The operand types in binary operations must be equal, with the following exceptions:
<p> <p>
The operand types in binary operations must be equal, with the following exceptions:
</p>
<ul> <ul>
<li>If one operand has numeric type and the other operand is <li>If one operand has numeric type and the other operand is
an ideal number, the ideal number is converted to match the type of an ideal number, the ideal number is converted to match the type of
@ -2849,8 +2749,9 @@ omitted in some cases as expressed by the OptSemicolon production.
StatementList = Statement { OptSemicolon Statement } . StatementList = Statement { OptSemicolon Statement } .
</pre> </pre>
A semicolon may be omitted immediately following:
<p> <p>
A semicolon may be omitted immediately following:
</p>
<ul> <ul>
<li>a closing parenthesis ")" ending a list of declarations (§Declarations and scope rules) <li>a closing parenthesis ")" ending a list of declarations (§Declarations and scope rules)
<li>a closing brace "}" ending a type declaration (§Type declarations) <li>a closing brace "}" ending a type declaration (§Type declarations)
@ -3538,7 +3439,6 @@ base type and may be forward-declared.
<h3>Predeclared functions</h3> <h3>Predeclared functions</h3>
<p>
<ul> <ul>
<li>cap <li>cap
<li>convert <li>convert
@ -3965,8 +3865,9 @@ uint32, int32, float32 4
uint64, int64, float64 8 uint64, int64, float64 8
</pre> </pre>
A Go compiler guarantees the following minimal alignment properties:
<p> <p>
A Go compiler guarantees the following minimal alignment properties:
</p>
<ol> <ol>
<li>For a variable "x" of any type: "1 <= unsafe.Alignof(x) <= unsafe.Maxalign". <li>For a variable "x" of any type: "1 <= unsafe.Alignof(x) <= unsafe.Maxalign".