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go_spec.html: clarify rune and string literals

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No changes to the meaning, just clearer language and more
examples, including illegal rune and string literals.
In particular, "character literal" and "character constant"
are now called "rune literal" and "rune constant" and the
word "character" always refers to the source text, not
program values.

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/6448137
This commit is contained in:
Rob Pike 2012-08-29 14:46:57 -07:00
parent bdf6a43e23
commit 9dfc6f6427
1 changed files with 37 additions and 26 deletions
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63
doc/go_spec.html
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@ -1,6 +1,6 @@
<!--{
"Title": "The Go Programming Language Specification",
"Subtitle": "Version of August 17, 2012",
"Subtitle": "Version of August 29, 2012",
"Path": "/ref/spec"
}-->
@ -88,7 +88,8 @@ Source code is Unicode text encoded in
canonicalized, so a single accented code point is 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.
will use the unqualified term <i>character</i> to refer to a Unicode code point
in the source text.
</p>
<p>
Each code point is distinct; for instance, upper and lower case letters
@ -197,7 +198,7 @@ token is
<a href="#Integer_literals">integer</a>,
<a href="#Floating-point_literals">floating-point</a>,
<a href="#Imaginary_literals">imaginary</a>,
<a href="#Character_literals">character</a>, or
<a href="#Rune_literals">rune</a>, or
<a href="#String_literals">string</a> literal
</li>
@ -359,13 +360,15 @@ imaginary_lit = (decimals | float_lit) "i" .
</pre>
<h3 id="Character_literals">Character literals</h3>
<h3 id="Rune_literals">Rune literals</h3>
<p>
A character literal represents a <a href="#Constants">character constant</a>,
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 single quoted character represents itself,
A rune literal represents a <a href="#Constants">rune constant</a>,
an integer value identifying a Unicode code point.
A rune literal is expressed as one or more characters enclosed in single quotes.
Within the quotes, any character may appear except single
quote and newline. A single quoted character represents the Unicode value
of the character itself,
while multi-character sequences beginning with a backslash encode
values in various formats.
</p>
@ -379,7 +382,7 @@ a literal <code>a</code>, Unicode U+0061, value <code>0x61</code>, while
a literal <code>a</code>-dieresis, U+00E4, value <code>0xe4</code>.
</p>
<p>
Several backslash escapes allow arbitrary values to be represented
Several backslash escapes allow arbitrary values to be encoded as
as ASCII text. There are four ways to represent the integer value
as a numeric constant: <code>\x</code> followed by exactly two hexadecimal
digits; <code>\u</code> followed by exactly four hexadecimal digits;
@ -408,11 +411,11 @@ After a backslash, certain single-character escapes represent special values:
\t U+0009 horizontal tab
\v U+000b vertical tab
\\ U+005c backslash
\' U+0027 single quote (valid escape only within character literals)
\' U+0027 single quote (valid escape only within rune literals)
\" U+0022 double quote (valid escape only within string literals)
</pre>
<p>
All other sequences starting with a backslash are illegal inside character literals.
All other sequences starting with a backslash are illegal inside rune literals.
</p>
<pre class="ebnf">
char_lit = "'" ( unicode_value | byte_value ) "'" .
@ -438,6 +441,11 @@ escaped_char = `\` ( "a" | "b" | "f" | "n" | "r" | "t" | "v" | `\` | "'" | `
'\xff'
'\u12e4'
'\U00101234'
'aa' // illegal: too many characters
'\xa' // illegal: too few hexadecimal digits
'\0' // illegal: too few octal digits
'\uDFFF' // illegal: surrogate half
'\U00110000' // illegal: invalid Unicode code point
</pre>
@ -452,7 +460,8 @@ raw string literals and interpreted string literals.
Raw string literals are character sequences between back quotes
<code>``</code>. Within the quotes, any character is legal except
back quote. The value of a raw string literal is the
string composed of the uninterpreted characters between the quotes;
string composed of the uninterpreted (implicitly UTF-8-encoded) characters
between the quotes;
in particular, backslashes have no special meaning and the string may
contain newlines.
Carriage returns inside raw string literals
@ -463,8 +472,9 @@ Interpreted string literals are character sequences between double
quotes <code>&quot;&quot;</code>. The text between the quotes,
which may not contain newlines, forms the
value of the literal, with backslash escapes interpreted as they
are in character literals (except that <code>\'</code> is illegal and
<code>\"</code> is legal). The three-digit octal (<code>\</code><i>nnn</i>)
are in rune literals (except that <code>\'</code> is illegal and
<code>\"</code> is legal), with the same restrictions.
The three-digit octal (<code>\</code><i>nnn</i>)
and two-digit hexadecimal (<code>\x</code><i>nn</i>) 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>.
@ -491,6 +501,8 @@ interpreted_string_lit = `"` { unicode_value | byte_value } `"` .
"日本語"
"\u65e5本\U00008a9e"
"\xff\u00FF"
"\uD800" // illegal: surrogate half
"\U00110000" // illegal: invalid Unicode code point
</pre>
<p>
@ -500,15 +512,15 @@ These examples all represent the same string:
<pre>
"日本語" // UTF-8 input text
`日本語` // UTF-8 input text as a raw literal
"\u65e5\u672c\u8a9e" // The explicit Unicode code points
"\U000065e5\U0000672c\U00008a9e" // The explicit Unicode code points
"\xe6\x97\xa5\xe6\x9c\xac\xe8\xaa\x9e" // The explicit UTF-8 bytes
"\u65e5\u672c\u8a9e" // the explicit Unicode code points
"\U000065e5\U0000672c\U00008a9e" // the explicit Unicode code points
"\xe6\x97\xa5\xe6\x9c\xac\xe8\xaa\x9e" // the explicit UTF-8 bytes
</pre>
<p>
If the source code represents a character as two code points, such as
a combining form involving an accent and a letter, the result will be
an error if placed in a character literal (it is not a single code
an error if placed in a rune literal (it is not a single code
point), and will appear as two code points if placed in a string
literal.
</p>
@ -517,7 +529,7 @@ literal.
<h2 id="Constants">Constants</h2>
<p>There are <i>boolean constants</i>,
<i>character constants</i>,
<i>rune constants</i>,
<i>integer constants</i>,
<i>floating-point constants</i>, <i>complex constants</i>,
and <i>string constants</i>. Character, integer, floating-point,
@ -527,7 +539,7 @@ collectively called <i>numeric constants</i>.
<p>
A constant value is represented by a
<a href="#Character_literals">character</a>,
<a href="#Rune_literals">rune</a>,
<a href="#Integer_literals">integer</a>,
<a href="#Floating-point_literals">floating-point</a>,
<a href="#Imaginary_literals">imaginary</a>,
@ -2392,7 +2404,7 @@ In all other cases, <code>x.f</code> is illegal.
If <code>x</code> is of pointer or interface type and has the value
<code>nil</code>, assigning to, evaluating, or calling <code>x.f</code>
causes a <a href="#Run_time_panics">run-time panic</a>.
</i>
</li>
</ol>
<p>
@ -3586,7 +3598,7 @@ wherever it is legal to use an operand of boolean, numeric, or string type,
respectively.
Except for shift operations, if the operands of a binary operation are
different kinds of untyped constants, the operation and, for non-boolean operations, the result use
the kind that appears later in this list: integer, character, floating-point, complex.
the kind that appears later in this list: integer, rune, floating-point, complex.
For example, an untyped integer constant divided by an
untyped complex constant yields an untyped complex constant.
</p>
@ -3614,7 +3626,7 @@ const f = int32(1) &lt;&lt; 33 // f == 0 (type int32)
const g = float64(2) &gt;&gt; 1 // illegal (float64(2) is a typed floating-point constant)
const h = "foo" &gt; "bar" // h == true (untyped boolean constant)
const j = true // j == true (untyped boolean constant)
const k = 'w' + 1 // k == 'x' (untyped character constant)
const k = 'w' + 1 // k == 'x' (untyped rune constant)
const l = "hi" // l == "hi" (untyped string constant)
const m = string(k) // m == "x" (type string)
const Σ = 1 - 0.707i // (untyped complex constant)
@ -3624,7 +3636,7 @@ const Φ = iota*1i - 1/1i // (untyped complex constant)
<p>
Applying the built-in function <code>complex</code> to untyped
integer, character, or floating-point constants yields
integer, rune, or floating-point constants yields
an untyped complex constant.
</p>
@ -3960,7 +3972,7 @@ is assigned to a variable of interface type, the constant is <a href="#Conversio
to type <code>bool</code>, <code>rune</code>, <code>int</code>, <code>float64</code>,
<code>complex128</code> or <code>string</code>
respectively, depending on whether the value is a
boolean, character, integer, floating-point, complex, or string constant.
boolean, rune, integer, floating-point, complex, or string constant.
</p>
@ -5499,7 +5511,6 @@ uintptr(unsafe.Pointer(&amp;x)) % unsafe.Alignof(x) == 0
Calls to <code>Alignof</code>, <code>Offsetof</code>, and
<code>Sizeof</code> are compile-time constant expressions of type <code>uintptr</code>.
</p>
<p>
<h3 id="Size_and_alignment_guarantees">Size and alignment guarantees</h3>