go/doc/go_ref.html

<h2>Introduction</h2>

<p>
This is a reference manual for the Go programming language. For more information and other documents, see <a href="/">the Go home page</a>.
</p>

<p>
Go is a general-purpose language designed with systems programming in mind. It is strongly typed and garbage-collected, and has explicit 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>
The grammar is simple and regular, allowing for easy analysis by automatic tools such as integrated development environments.
</p>

<h2>Notation</h2>

<p>
The syntax is specified using Extended Backus-Naur Form (EBNF):
</p>

<pre>
Production  = production_name "=" Expression .
Expression  = Alternative { "|" Alternative } .
Alternative = Term { Term } .
Term        = production_name | token [ "..." token ] | Group | Option | Repetition .
Group       = "(" Expression ")" .
Option      = "[" Expression ")" .
Repetition  = "{" Expression "}" .
</pre>

<p>
Productions are expressions constructed from terms and the following operators, in increasing precedence:
</p>
<pre>
|   alternation
()  grouping
[]  option (0 or 1 times)
{}  repetition (0 to n times)
</pre>

<p>
Lower-case production names are used to identify lexical tokens. 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>
The form <tt>"a ... b"</tt> represents the set of characters from <tt>a</tt> through <tt>b</tt> as alternatives.
</p>

<p>
Where possible, recursive productions are used to express evaluation order
and operator precedence syntactically.
</p>


<h2>Lexical properties</h2>

<p>
A program is constructed from a set of <i>packages</i>.  Each package is defined by one or more source files compiled separately.  In processing the source text in each file, the input is divided into a sequence of <i>tokens</i>.
</p>

<h3>Unicode text</h3>

<p>
Go source text is a sequence of Unicode code points encoded in UTF-8.  The language processor does not canonicalize the input, so it will treat a single accented code point as distinct from the same character constructed from combining an accent and a letter; those are treated as two code points.  For simplicity, this document will use the term <i>character</i> to refer to a Unicode code point.
</p>
<p>
Each code point is distinct; for example, upper and lower case letters are different characters.
</p>

<h3>Tokens</h3>

<p>
There are four classes of tokens: identifiers, keywords, operators and delimiters, and literals.  <i>White space</i>, formed from blanks, tabs, and newlines, is ignored except as it separates tokens that would otherwise combine into a single token.  Comments, defined below, behave as white space.  While breaking the input into tokens, the next token is the longest sequence of characters that form a valid token.
</p>

<h3>Comments</h3>

<p>
There are two forms of comments.  The first starts at a the character sequence <tt>//</tt> and continues through the next newline.  The second starts at the character sequence <tt>/*</tt> and continues through the character sequence <tt>*/</tt>.  Comments do not nest.
</p>

<h3>Identifiers</h3>

<p>
An identifier is a sequence of one or more letters and digits. The meaning of <i>letter</i> and <i>digit</i> is defined by the Unicode properties for the corresponding characters, with the addition that the underscore character <tt>_</tt> (U+005F) is considered a letter.  The first character in an identifier must be a letter.
</p>

<pre>
letter        = unicode_letter | "_" .
identifier    = letter { letter | unicode_digit } .
</pre>

<h3>Keywords</h3>

<p>
The following keywords are reserved and may not be used as identifiers.
</p>
<pre>
break        default      func         interface    select
case         defer        go           map          struct
chan         else         goto         package      switch
const        fallthrough  if           range        type
continue     for          import       return       var
</pre>

<h3>Operators and Delimiters</h3>

<p>
The following character sequences are tokens representing operators, delimiters, and other special lexemes:
</p>
<pre>
+    &amp;     +=    &amp;=     &amp;&amp;    ==    !=    (    )
-    |     -=    |=     ||    &lt;     &lt;=    [    ]
*    ^     *=    ^=     &lt;-    &gt;     &gt;=    {    }
/    <<    /=    <<=    ++    =     :=    ,    ;
%    >>    %=    >>=    --    !     ...   .    :
</pre>

<h3>Literals</h3>

<h4>Integer literals</h4>

<p>
An integer literal is a sequence of one or more digits in the corresponding base, which may be 8, 10, or 16.  An optional prefix sets a non-decimal base: <tt>0</tt> for octal, <tt>0x</tt> or <tt>0X</tt> for hexadecimal.  In hexadecimal literals, letters <tt>a-f</tt> and <tt>A-F</tt> represent values 10 through 15.
</p>
<pre>
int_lit       = decimal_lit | octal_lit | hex_lit .
decimal_lit   = ( "1" ... "9" ) { decimal_digit } .
octal_lit     = "0" { octal_digit } .
hex_lit       = "0" ( "x" | "X" ) hex_digit { hex_digit } .
decimal_digit = "0" ... "9" .
octal_digit   = "0" ... "7" .
hex_digit     = "0" ... "9" | "A" ... "F" | "a" ... "f" .
</pre>

<p>
Integer literals represent values of arbitrary precision, or <i>ideal integers</i>; they have no implicit size or type.
</p>

<h4>Floating-point literals</h4>
<p>
A floating-point literal is a decimal representation of a floating-point number.  It has an integer part, a decimal point, a fractional part, and an exponent part.  The integer and fractional part comprise decimal digits; the exponent part is an <tt>e</TT> or <tt>E</tt> followed by an optionally signed decimal exponent.  One of the integer part or the fractional part may be elided; one of the decimal point or the exponent may be elided.
</p>
<pre>
float_lit    = decimals "." [ decimals ] [ exponent ] |
               decimals exponent |
               "." decimals [ exponent ] .
decimals = decimal_digit { decimal_digit } .
exponent = ( "e" | "E" ) [ "+" | "-" ] decimals .
</pre>

<p>
As with integers, floating-point literals represent values of arbitrary precision, or <i>ideal floats</i>.
</p>

<h4>Character literals</h4>

<p>
A character literal represents an integer value, typically a Unicode code point, as one or more characters enclosed in single quotes.  Within the quotes, any character may appear except single quote and newline; a quoted single character represents itself, while multi-character sequences beginning with a backslash encode values in various formats.
</p>
<p>
The simplest form represents the exact character within the quotes; since Go source text is Unicode characters encoded in UTF-8, multiple UTF-8-encoded bytes may represent a single integer value.  For instance, the literal <tt>'a'</tt> holds a single byte representing a literal <tt>a</tt>, Unicode U+0061, value <tt>0x61</tt>, while <tt>'ä'</tt> holds two bytes (<tt>0xc3</tt> <tt>0xa4</tt>) representing a literal <tt>a</tt>-dieresis, U+00E4, value <tt>0xe4</tt>.
</p>
<p>
Several backslash escapes allow arbitrary values to be represented as ASCII text.  There are four ways to represent the integer value as a numeric constant: <tt>\x</tt> followed by exactly two hexadecimal digits; <tt>\u</tt> followed by exactly four hexadecimal digits; <tt>\U</tt> followed by exactly eight hexadecimal digits, and a plain backslash <tt>\</tt> followed by exactly three octal digits.  In each case the value of the literal is the value represented by the digits in the appropriate base.
</p>
<p>
Although these representations all result in an integer, they have different valid ranges.  Octal escapes must represent a value between 0 and 255 inclusive.  (Hexadecimal escapes satisfy this condition by construction). The `Unicode' escapes <tt>\u</tt> and <tt>\U</tt> represent Unicode code points so within them some values are illegal, in particular those above <tt>0x10FFFF</tt> and surrogate halves.
</p>
<p>
After a backslash, certain single-character escapes represent special values:
</p>
<pre>
\a   U+0007 alert or bell
\b   U+0008 backspace
\f   U+000C form feed
\n   U+000A line feed or newline
\r   U+000D carriage return
\t   U+0009 horizontal tab
\v   U+000b vertical tab
\\   U+005c backslash
\'   U+0027 single quote  (legal within character literals only)
\"   U+0022 double quote  (legal within interpreted string literals only)
</pre>
<p>
All other sequences are illegal inside character literals.
</p>
<pre>
char_lit         = "'" ( unicode_value | byte_value ) "'" .
unicode_value    = unicode_char | little_u_value | big_u_value | escaped_char .
byte_value       = octal_byte_value | hex_byte_value .
octal_byte_value = "\" octal_digit octal_digit octal_digit .
hex_byte_value   = "\" "x" hex_digit hex_digit .
little_u_value   = "\" "u" hex_digit hex_digit hex_digit hex_digit .
big_u_value      = "\" "U" hex_digit hex_digit hex_digit hex_digit
                           hex_digit hex_digit hex_digit hex_digit .
escaped_char     = "\" ( "a" | "b" | "f" | "n" | "r" | "t" | "v" | "\" | "'" | """ ) .
</pre>

<p>
The value of a character literal is an ideal integer, just as with integer literals.
</p>

<h4>String literals</h4>

<p>
String literals represent constant values of type <tt>string</tt>.  There are two forms: raw string literals and interpreted string literals.
</p>
<p>
Raw string literals are character sequences between back quotes <tt>``</tt>.  Within the quotes, any character is legal except newline and back quote. The value of a raw string literal is the string composed of the uninterpreted bytes between the quotes.
</p>
<p>
Interpreted string literals are character sequences between double quotes <tt>&quot;&quot;</tt>. The text between the quotes forms the value of the literal, with backslash escapes interpreted as they are in character literals.  The three-digit octal (<tt>\000</tt>) and two-digit hexadecimal (<tt>\x00</tt>) escapes represent individual <i>bytes</i> of the resulting string; all other escapes represent the (possibly multi-byte) UTF-8 encoding of individual <i>characters</i>.  Thus inside a string literal <tt>\377</tt> and <tt>\xFF</tt> represent a single byte of value <tt>0xFF</tt>=255, while <tt>ÿ</tt>, <tt>\u00FF</tt>, <tt>\U000000FF</tt> and <tt>\xc3\xbf</tt> represent the two bytes <tt>0xc3 0xbf</tt> of the UTF-8 encoding of character U+00FF.
</p>

<pre>
string_lit             = raw_string_lit | interpreted_string_lit .
raw_string_lit         = "`" { unicode_char } "`" .
interpreted_string_lit = """ { unicode_value | byte_value } """ .
</pre>

<p>
During tokenization, two adjacent string literals separated only by the empty string, white space, or comments are implicitly combined into a single string literal whose value is the concatenated values of the literals.
</p>
<pre>
StringLit              = string_lit { string_lit } .
</pre>

<h2>Everything else</h2>

<p>
I don't believe this organization is complete or correct but it's here to be worked on and thought about.
</p>

<h2>Types</h2>

<h2>Constants</h2>

<h2>Expressions</h2>

<h2>Declarations</h2>

<h2>Control Structures</h2>

<h2>Program structure</h2>

<h2>Packages</h2>

<h2>Differences between this doc and implementation - TODO</h2>
<p>
<font color=red>
Current implementation accepts only ASCII digits for digits; doc says Unicode.
<br>
</font>
</p>
</div>

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