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https://github.com/golang/go
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rewording around ideal and basic types
DELTA=355 (93 added, 85 deleted, 177 changed) OCL=34904 CL=34998
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doc/go_spec.html
469
doc/go_spec.html
@ -64,7 +64,8 @@ Open issues:
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- no mechanism to declare a local type name: type T P.T
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Todo's:
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Todo
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[ ] clarify: two equal lowercase identifiers from different packages denote different objects
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[ ] need language about function/method calls and parameter passing rules
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[ ] need to say something about "scope" of selectors?
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[ ] clarify what a field name is in struct declarations
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@ -78,7 +79,6 @@ Todo's:
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though obvious
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[ ] specify iteration direction for range clause
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[ ] review language on implicit dereferencing
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[ ] document T.m mechanism to obtain a function from a method
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-->
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@ -86,8 +86,7 @@ Todo's:
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<p>
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This is a reference manual for the Go programming language. For
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more information and other documents, see <a
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href="/">the Go home page</a>.
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more information and other documents, see <a href="http://go/go">go/go</a>.
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</p>
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<p>
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@ -258,9 +257,9 @@ The following character sequences represent <a href="#Operators">operators</a>,
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<h3 id="Integer_literals">Integer literals</h3>
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<p>
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An integer literal is a sequence of one or more digits in the
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corresponding base, which may be 8, 10, or 16. An optional prefix
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sets a non-decimal base: <code>0</code> for octal, <code>0x</code> or
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An integer literal is a sequence of digits representing an
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<a href="#Constants">integer constant</a>.
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An optional prefix sets a non-decimal base: <code>0</code> for octal, <code>0x</code> or
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<code>0X</code> for hexadecimal. In hexadecimal literals, letters
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<code>a-f</code> and <code>A-F</code> represent values 10 through 15.
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</p>
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@ -280,8 +279,9 @@ hex_lit = "0" ( "x" | "X" ) hex_digit { hex_digit } .
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<h3 id="Floating-point_literals">Floating-point literals</h3>
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<p>
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A floating-point literal is a decimal representation of a floating-point
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number. It has an integer part, a decimal point, a fractional part,
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A floating-point literal is a decimal representation of a
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<a href="#Constants">floating-point constant</a>.
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It has an integer part, a decimal point, a fractional part,
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and an exponent part. The integer and fractional part comprise
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decimal digits; the exponent part is an <code>e</code> or <code>E</code>
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followed by an optionally signed decimal exponent. One of the
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@ -306,28 +306,12 @@ exponent = ( "e" | "E" ) [ "+" | "-" ] decimals .
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.12345E+5
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</pre>
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<h3 id="Ideal_numbers">Ideal numbers</h3>
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<p>
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Integer literals represent values of arbitrary precision, or <i>ideal
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integers</i>. Similarly, floating-point literals represent values
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of arbitrary precision, or <i>ideal floats</i>. These <i>ideal
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numbers</i> have no size or named type and cannot overflow. However,
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when (used in an expression) assigned to a variable or typed constant,
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the destination must be able to represent the assigned value.
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</p>
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<p>
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Implementation restriction: A compiler may implement ideal numbers
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by choosing an internal representation with at least twice as many
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bits as any machine type; for floats, both the mantissa and exponent
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must be twice as large.
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</p>
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<h3 id="Character_literals">Character literals</h3>
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<p>
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A character literal represents an integer value, typically a
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Unicode code point, as one or more characters enclosed in single
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A character literal represents an <a href="#Constants">integer constant</a>,
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typically a Unicode code point, as one or more characters enclosed in single
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quotes. Within the quotes, any character may appear except single
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quote and newline. A single quoted character represents itself,
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while multi-character sequences beginning with a backslash encode
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@ -389,6 +373,7 @@ big_u_value = `\` "U" hex_digit hex_digit hex_digit hex_digit
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hex_digit hex_digit hex_digit hex_digit .
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escaped_char = `\` ( "a" | "b" | "f" | "n" | "r" | "t" | "v" | `\` | "'" | `"` ) .
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</pre>
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<pre>
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'a'
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'ä'
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@ -403,19 +388,13 @@ escaped_char = `\` ( "a" | "b" | "f" | "n" | "r" | "t" | "v" | `\` | "'" | `
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'\U00101234'
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</pre>
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<p>
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The value of a character literal is an ideal integer, just as with
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integer literals.
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</p>
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<h3 id="String_literals">String literals</h3>
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<p>
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String literals represent <i>ideal string</i> values. Ideal strings do not
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have a named type but they are compatible with type <code>string</code>
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(§<a href="#Type_identity_and_compatibility">Type identity and compatibility</a>).
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There are two forms: raw string literals and interpreted string
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literals.
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A string literal represents a <a href="#Constants">string constant</a>
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obtained from concatenating a sequence of characters. There are two forms:
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raw string literals and interpreted string literals.
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</p>
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<p>
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Raw string literals are character sequences between back quotes
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@ -486,14 +465,63 @@ point), and will appear as two code points if placed in a string
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literal.
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</p>
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<h3 id="Boolean_literals">Boolean literals</h3>
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<h2 id="Constants">Constants</h2>
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<p>There are <i>boolean constants</i>, <i>integer constants</i>, <i>floating-point constants</i>,
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and <i>string constants</i>. Integer and floating-point constants are
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collectively called <i>numeric constants</i>.
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</p>
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<p>
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A boolean literal is one of the predeclared constants
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<code>true</code> or <code>false</code>. The value of a boolean
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literal is an <i>ideal bool</i>.
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A constant value is represented by an
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<a href="#Integer_literals">integer</a>,
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<a href="#Floating-point_literals">floating-point</a>,
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<a href="#Character_literals">character</a>, or
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<a href="#String_literals">string</a> literal,
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an identifier denoting a constant,
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a <a href="#Constant_expressions">constant expression</a>, or
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the result value of some built-in functions such as <code>unsafe.Sizeof</code>
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and <code>cap</code> or <code>len</code> applied to an array,
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or <code>len</code> applied to a string constant.
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The boolean truth values are represented by the predeclared constants
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<code>true</code> and <code>false</code>. The predeclared identifier
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<a href="#Iota">iota</a> denotes an integer constant.
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</p>
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<p>
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Numeric constants represent values of arbitrary precision that
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have no size and cannot overflow.
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</p>
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<p>
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Constants may be <a href="#Types">typed</a> or untyped.
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Literal constants, <code>true</code>, <code>false</code>, <code>iota</code>,
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and certain <a href="#Constant_expressions">constant expressions</a>
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containing only untyped constant operands are untyped.
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</p>
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<p>
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A constant may be given a type explicitly by a <a href="#Constant_declarations">constant declaration</a>
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or <a href="#Conversions">conversion</a>, or implicitly when used in a
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<a href="#Variable_declarations">variable declaration</a> or an
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<a href="#Assignments">assignment</a> or as an
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operand in an <a href="#Expressions">expression</a>.
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It is an error if the constant value
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cannot be accurately represented as a value of the respective type.
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For instance, <code>3.0</code> can be given any integer type but also any
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floating-point type, while <code>-1e12</code> can be given the types
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<code>float32</code>, <code>float64</code>, or even <code>int64</code> but
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not <code>uint64</code> or <code>string</code>.
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</p>
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<p>
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Implementation restriction: A compiler may implement numeric constants by choosing
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an internal representation with at least twice as many bits as any machine type;
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for floating-point values, both the mantissa and exponent must be twice as large.
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</p>
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<h2 id="Types">Types</h2>
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<p>
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@ -511,16 +539,14 @@ TypeLit = ArrayType | StructType | PointerType | FunctionType | InterfaceType
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</pre>
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<p>
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<i>Basic types</i> such as <code>int</code> are predeclared (§<a href="#Predeclared_identifiers">Predeclared identifiers</a>).
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Other types may be constructed from these, recursively,
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including arrays, structs, pointers, functions, interfaces, slices, maps, and
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channels.
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Named instances of the boolean, numeric, and string types are <a href="#Predeclared_identifiers">predeclared</a>.
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Array, struct, pointer, function, interface, slice, map, and channel types may be constructed using type literals.
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</p>
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<p>
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A type may have a <i>method set</i> associated with it
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(§<a href="#Interface_types">Interface types</a>, §<a href="#Method_declarations">Method declarations</a>).
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The method set of an interface type (§<a href="#Interface_types">Interface types</a>) is its interface.
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The method set of an <a href="#Interface_types">interface type</a> is its interface.
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The method set of any other named type <code>T</code>
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consists of all methods with receiver
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type <code>T</code>.
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@ -532,23 +558,26 @@ Any other type has an empty method set.
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<p>
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The <i>static type</i> (or just <i>type</i>) of a variable is the
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type defined by its declaration. Variables of interface type
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(§<a href="#Interface_types">Interface types</a>) also have a distinct <i>dynamic type</i>, which
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also have a distinct <i>dynamic type</i>, which
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is the actual type of the value stored in the variable at run-time.
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The dynamic type may vary during execution but is always compatible
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with the static type of the interface variable. For non-interface
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The dynamic type may vary during execution but is always assignment compatible
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to the static type of the interface variable. For non-interface
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types, the dynamic type is always the static type.
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</p>
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<h3 id="Basic_types">Basic types</h3>
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<p>
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Basic types include traditional numeric types, booleans, and strings. All are predeclared.
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</p>
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<h3 id="Boolean_types">Boolean types</h3>
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A <i>boolean type</i> represents the set of Boolean truth values
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denoted by the predeclared constants <code>true</code>
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and <code>false</code>. The predeclared boolean type is <code>bool</code>.
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<h3 id="Numeric_types">Numeric types</h3>
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<p>
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The architecture-independent numeric types are:
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A <i>numeric type</i> represents sets of integer or floating-point values.
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The predeclared architecture-independent numeric types are:
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</p>
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<pre class="grammar">
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@ -562,8 +591,8 @@ int16 the set of all signed 16-bit integers (-32768 to 32767)
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int32 the set of all signed 32-bit integers (-2147483648 to 2147483647)
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int64 the set of all signed 64-bit integers (-9223372036854775808 to 9223372036854775807)
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float32 the set of all valid IEEE-754 32-bit floating point numbers
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float64 the set of all valid IEEE-754 64-bit floating point numbers
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float32 the set of all IEEE-754 32-bit floating-point numbers
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float64 the set of all IEEE-754 64-bit floating-point numbers
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byte familiar alias for uint8
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</pre>
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@ -575,7 +604,7 @@ as the two's complement of its absolute value.
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</p>
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<p>
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There is also a set of numeric types with implementation-specific sizes:
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There is also a set of predeclared numeric types with implementation-specific sizes:
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</p>
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<pre class="grammar">
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@ -595,19 +624,13 @@ are not the same type even though they may have the same size on a
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particular architecture.
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<h3 id="Booleans">Booleans</h3>
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The type <code>bool</code> comprises the Boolean truth values
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represented by the predeclared constants <code>true</code>
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and <code>false</code>.
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<h3 id="Strings">Strings</h3>
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<h3 id="String_types">String types</h3>
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<p>
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The <code>string</code> type represents the set of string values.
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A <i>string type</i> represents the set of string values.
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Strings behave like arrays of bytes but are immutable: once created,
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it is impossible to change the contents of a string.
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The predeclared string type is <code>string</code>.
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<p>
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The elements of strings have type <code>byte</code> and may be
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@ -616,7 +639,7 @@ illegal to take the address of such an element; if
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<code>s[i]</code> is the <i>i</i>th byte of a
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string, <code>&s[i]</code> is invalid. The length of string
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<code>s</code> can be discovered using the built-in function
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<code>len(s)</code>. The length is a compile-time constant if <code>s</code>
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<code>len</code>. The length is a compile-time constant if <code>s</code>
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is a string literal.
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</p>
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@ -1002,7 +1025,7 @@ A map value may be <code>nil</code>.
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</p>
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<pre class="ebnf">
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MapType = "map" "[" KeyType "]" ValueType .
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MapType = "map" "[" KeyType "]" ElementType .
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KeyType = Type .
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ValueType = Type .
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</pre>
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@ -1010,7 +1033,7 @@ ValueType = Type .
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<p>
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The comparison operators <code>==</code> and <code>!=</code>
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(§<a href="#Comparison_operators">Comparison operators</a>) must be fully defined for operands of the
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key type; thus the key type must be a basic, pointer, interface,
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key type; thus the key type must be a boolean, numeric, string, pointer, function, interface,
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map, or channel type. If the key type is an interface type, these
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comparison operators must be defined for the dynamic key values;
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failure will cause a run-time error.
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@ -1109,9 +1132,7 @@ received, <code>closed(c)</code> returns true.
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<p>
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Two types may be <i>identical</i>, <i>compatible</i>, or <i>incompatible</i>.
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Two identical types are always compatible, but two compatible types may not be identical.
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Go is <i>type safe</i>: a value of one type cannot be assigned to a variable of an
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incompatible type, and two values of incompatible types cannot be mixed in
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binary operations.</p>
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</p>
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<h3 id="Type_identity_and_compatibility">Type identity and compatibility</h3>
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@ -1212,34 +1233,46 @@ they have different field names.
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<h3 id="Assignment_compatibility">Assignment compatibility</h3>
|
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|
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<p>
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Values of any type may always be assigned to variables
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of compatible static type. Some types and values have conditions under which they may
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be assigned to otherwise incompatible types:
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A value <code>v</code> of static type <code>V</code> is <i>assignment compatible</i>
|
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with a type <code>T</code> if one of the following conditions applies:
|
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</p>
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<ul>
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<li>
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A value can be assigned to an interface variable if the static
|
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type of the value implements the interface.
|
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<code>V</code> is compatible with <code>T</code>.
|
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</li>
|
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<li>
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The predeclared constant <code>nil</code> can be assigned to any
|
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pointer, function, slice, map, channel, or interface variable.
|
||||
<li>
|
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A pointer <code>p</code> to an array can be assigned to a slice variable
|
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<code>v</code> with compatible element type
|
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if the type of <code>p</code> or <code>v</code> is unnamed.
|
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The slice variable then refers to the original array; the data is not copied.
|
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<code>T</code> is an interface type and
|
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<code>V</code> <a href="#Interface_types">implements</a> <code>T</code>.
|
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</li>
|
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<li>
|
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A bidirectional channel <code>c</code> can be assigned to a channel variable
|
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<code>v</code> with compatible channel value type
|
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if the type of <code>c</code> or <code>v</code> is unnamed.
|
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<code>V</code> is a pointer to an array and <code>T</code> is a slice type
|
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with compatible element type and at least one of <code>V</code> or <code>T</code> is unnamed.
|
||||
After assignment, the slice variable refers to the original array; the elements are not
|
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copied.
|
||||
</li>
|
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<li>
|
||||
A value can always be assigned to the <a href="#Blank_identifier">blank identifier</a>.
|
||||
<code>V</code> is a bidirectional channel and <code>T</code> is a channel type
|
||||
with compatible element type and at least one of <code>V</code> or <code>T</code> is unnamed.
|
||||
</li>
|
||||
</ul>
|
||||
|
||||
<p>
|
||||
An untyped <a href="#Constants">constant</a> <code>v</code>
|
||||
is assignment compatible with type <code>T</code> if <code>v</code>
|
||||
can be represented accurately as a value of type <code>T</code>.
|
||||
</p>
|
||||
|
||||
<p>
|
||||
The predeclared identifier <code>nil</code> is assignment compatible with any
|
||||
pointer, function, slice, map, channel, or interface type and
|
||||
represents the <a href="#The_zero_value">zero value<a> for that type.
|
||||
</p>
|
||||
|
||||
<p>
|
||||
Any value may be assigned to the <a href="#Blank_identifier">blank identifier</a>.
|
||||
</p>
|
||||
|
||||
<h3 id="Comparison_compatibility">Comparison compatibility</h3>
|
||||
|
||||
<p>
|
||||
@ -1416,7 +1449,10 @@ Architecture-specific convenience types:
|
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float int uint uintptr
|
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|
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Constants:
|
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true false iota nil
|
||||
true false iota
|
||||
|
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Zero value:
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nil
|
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|
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Functions:
|
||||
cap close closed len make new panic panicln print println
|
||||
@ -1448,7 +1484,7 @@ any other identifier but the declaration does not introduce a new binding.
|
||||
</p>
|
||||
|
||||
|
||||
<h3 id="Const_declarations">Const declarations</h3>
|
||||
<h3 id="Constant_declarations">Constant declarations</h3>
|
||||
|
||||
<p>
|
||||
A constant declaration binds a list of identifiers (the names of
|
||||
@ -1469,23 +1505,25 @@ ExpressionList = Expression { "," Expression } .
|
||||
</pre>
|
||||
|
||||
<p>
|
||||
If the type is present, all constants take the type specified, and
|
||||
the expressions must be <a href="#Assignment_compatibility">assignment compatible</a> with that type.
|
||||
If the type is omitted, the constants take the
|
||||
individual types of the corresponding expressions, which may be
|
||||
an <a href="#Ideal_numbers">ideal number</a>, <a href="#String_literals">ideal string</a>,
|
||||
or <a href="#Boolean_literals">ideal bool</a>.
|
||||
If the type is present, all constants take the type specified, and the types
|
||||
of all the expressions must be assignment-compatible
|
||||
with that type.
|
||||
individual types of the corresponding expressions.
|
||||
If the expression values are untyped <a href="#Constants">constants</a>,
|
||||
the declared constants remain untyped and the constant identifiers
|
||||
denote the constant values. For instance, if the expression is a
|
||||
floating-point literal, the constant identifier denotes a floating-point
|
||||
constant, even if the literal's fractional part is zero.
|
||||
</p>
|
||||
|
||||
<pre>
|
||||
const Pi float64 = 3.14159265358979323846
|
||||
const E = 2.718281828
|
||||
const zero = 0.0 // untyped floating-point constant
|
||||
const (
|
||||
size int64 = 1024;
|
||||
eof = -1;
|
||||
eof = -1; // untyped integer constant
|
||||
)
|
||||
const a, b, c = 3, 4, "foo" // a = 3, b = 4, c = "foo"
|
||||
const a, b, c = 3, 4, "foo" // a = 3, b = 4, c = "foo", untyped integer and string constants
|
||||
const u, v float = 0, 3 // u = 0.0, v = 3.0
|
||||
</pre>
|
||||
|
||||
@ -1519,9 +1557,9 @@ const (
|
||||
|
||||
<p>
|
||||
Within a constant declaration, the predeclared pseudo-constant
|
||||
<code>iota</code> represents successive integers. It is reset to 0
|
||||
whenever the reserved word <code>const</code> appears in the source
|
||||
and increments with each semicolon. It can be used to construct a
|
||||
<code>iota</code> represents successive untyped integer <a href="#Constants">
|
||||
constants</a>. It is reset to 0 whenever the reserved word <code>const</code>
|
||||
appears in the source and increments with each semicolon. It can be used to construct a
|
||||
set of related constants:
|
||||
</p>
|
||||
|
||||
@ -1539,9 +1577,9 @@ const (
|
||||
)
|
||||
|
||||
const (
|
||||
u = iota * 42; // u == 0 (ideal integer)
|
||||
v float = iota * 42; // v == 42.0 (float)
|
||||
w = iota * 42; // w == 84 (ideal integer)
|
||||
u = iota * 42; // u == 0 (untyped integer constant)
|
||||
v float = iota * 42; // v == 42.0 (float constant)
|
||||
w = iota * 42; // w == 84 (untyped integer constant)
|
||||
)
|
||||
|
||||
const x = iota; // x == 0 (iota has been reset)
|
||||
@ -1640,17 +1678,18 @@ of the expression list.
|
||||
</p>
|
||||
|
||||
<p>
|
||||
If the type is absent and the corresponding expression is a constant
|
||||
expression of ideal integer, float, string or bool type, the type of the
|
||||
declared variable is <code>int</code>, <code>float</code>,
|
||||
<code>string</code>, or <code>bool</code> respectively:
|
||||
If the type is absent and the corresponding expression evaluates to an
|
||||
untyped <a href="#Constants">constant</a>, the type of the declared variable
|
||||
is <code>bool</code>, <code>int</code>, <code>float</code>, or <code>string</code>
|
||||
respectively, depending on whether the value is a boolean, integer,
|
||||
floating-point, or string constant:
|
||||
</p>
|
||||
|
||||
<pre>
|
||||
var b = true // t has type bool
|
||||
var i = 0 // i has type int
|
||||
var f = 3.1415 // f has type float
|
||||
var f = 3.0 // f has type float
|
||||
var s = "OMDB" // s has type string
|
||||
var t = true // t has type bool
|
||||
</pre>
|
||||
|
||||
<h3 id="Short_variable_declarations">Short variable declarations</h3>
|
||||
@ -1792,8 +1831,7 @@ However, a function declared this way is not a method.
|
||||
|
||||
<p>
|
||||
An expression specifies the computation of a value by applying
|
||||
operators and functions to operands. An expression has a value
|
||||
and a type.
|
||||
operators and functions to operands.
|
||||
</p>
|
||||
|
||||
<h3 id="Operands">Operands</h3>
|
||||
@ -1807,17 +1845,6 @@ BasicLit = int_lit | float_lit | char_lit | StringLit .
|
||||
</pre>
|
||||
|
||||
|
||||
<h3 id="Constants">Constants</h3>
|
||||
|
||||
<p>
|
||||
A <i>constant</i> is a literal of a basic type
|
||||
(including the predeclared constants <code>true</code>, <code>false</code>
|
||||
and <code>nil</code>
|
||||
and values denoted by <code>iota</code>)
|
||||
or a constant expression (§<a href="#Constant_expressions">Constant expressions</a>).
|
||||
Constants have values that are known at compile time.
|
||||
</p>
|
||||
|
||||
<h3 id="Qualified_identifiers">Qualified identifiers</h3>
|
||||
|
||||
<p>
|
||||
@ -2220,7 +2247,7 @@ or for <code>a</code> of type <code>S</code> where <code>S</code> is a <a href="
|
||||
|
||||
<p>
|
||||
For <code>a</code> of type <code>T</code>
|
||||
where <code>T</code> is a <a href="#Strings">string type</a>:
|
||||
where <code>T</code> is a <a href="#String_types">string type</a>:
|
||||
</p>
|
||||
<ul>
|
||||
<li><code>x</code> must be an integer value and <code>0 <= x < len(a)</code>
|
||||
@ -2307,15 +2334,18 @@ s[1] == 3
|
||||
</pre>
|
||||
|
||||
<p>
|
||||
The slice length must be non-negative.
|
||||
The slice length must not be negative.
|
||||
For arrays or strings, the indexes
|
||||
<code>lo</code> and <code>hi</code> must satisfy
|
||||
0 <= <code>lo</code> <= <code>hi</code> <= length;
|
||||
for slices, the upper bound is the capacity rather than the length.
|
||||
</p>
|
||||
|
||||
<p>
|
||||
If the sliced operand is a string, the result of the slice operation is another, new
|
||||
<a href="#Strings">string</a>. If the sliced operand is an array or slice, the result
|
||||
of the slice operation is a <a href="#Slice_types">slice</a>.
|
||||
If the sliced operand is a string or slice, the result of the slice operation
|
||||
is a string or slice of the same type.
|
||||
If the sliced operand is an array, the result of the slice operation is a slice
|
||||
with the same element type as the array.
|
||||
</p>
|
||||
|
||||
|
||||
@ -2482,13 +2512,12 @@ unary_op = "+" | "-" | "!" | "^" | "*" | "&" | "<-" .
|
||||
</pre>
|
||||
|
||||
<p>
|
||||
Comparisons are discussed elsewhere
|
||||
(§<a href="#Comparison_compatibility">Comparison compatibility</a>).
|
||||
For other binary operators, the
|
||||
operand types must be identical
|
||||
Comparisons are discussed <a href="#Comparison_operators">elsewhere</a>.
|
||||
For other binary operators, the operand types must be identical
|
||||
(§<a href="#Properties_of_types_and_values">Properties of types and values</a>)
|
||||
unless the operation involves
|
||||
channels, shifts, or ideal constants.
|
||||
unless the operation involves channels, shifts, or untyped <a href="#Constants">constants</a>.
|
||||
For operations involving constants only, see the section on
|
||||
<a href="#Constant_expressions">constant expressions</a>.
|
||||
</p>
|
||||
|
||||
<p>
|
||||
@ -2498,24 +2527,20 @@ second is a value of the channel's element type.
|
||||
|
||||
<p>
|
||||
Except for shift operations,
|
||||
if one operand has ideal type and the other operand does not,
|
||||
the ideal operand is converted to match the type of
|
||||
the other operand (§<a href="#Expressions">Expressions</a>).
|
||||
If both operands are ideal numbers and one is an
|
||||
ideal float, the other is converted to ideal float
|
||||
(relevant for <code>/</code> and <code>%</code>).
|
||||
if one operand is an untyped <a href="#Constants">constant</a>
|
||||
and the other operand is not, the constant is <a href="#Conversions">converted</a>
|
||||
to the type of the other operand.
|
||||
</p>
|
||||
|
||||
<p>
|
||||
The right operand in a shift operation must have unsigned integer type
|
||||
or be an ideal number that can be converted to unsigned integer type
|
||||
(§<a href="#Arithmetic_operators">Arithmetic operators</a>).
|
||||
or be an untyped constant that can be converted to unsigned integer type.
|
||||
</p>
|
||||
|
||||
<p>
|
||||
If the left operand of a non-constant shift operation is an ideal number,
|
||||
the type of the ideal number
|
||||
is what it would be if the shift operation were replaced by the left operand alone.
|
||||
If the left operand of a non-constant shift operation is an untyped constant,
|
||||
the type of constant is what it would be if the shift operation were replaced by
|
||||
the left operand alone.
|
||||
</p>
|
||||
|
||||
<pre>
|
||||
@ -2568,11 +2593,11 @@ x == y+1 && <-chan_ptr > 0
|
||||
|
||||
<h3 id="Arithmetic_operators">Arithmetic operators</h3>
|
||||
<p>
|
||||
Arithmetic operators apply to numeric types and yield a result of the same
|
||||
Arithmetic operators apply to numeric values and yield a result of the same
|
||||
type as the first operand. The four standard arithmetic operators (<code>+</code>,
|
||||
<code>-</code>, <code>*</code>, <code>/</code>) apply both to integer and
|
||||
floating point types, while <code>+</code> applies also
|
||||
to strings; all other arithmetic operators apply to integers only.
|
||||
<code>-</code>, <code>*</code>, <code>/</code>) apply to integer and
|
||||
floating-point types; <code>+</code> also applies
|
||||
to strings. All other arithmetic operators apply to integers only.
|
||||
</p>
|
||||
|
||||
<pre class="grammar">
|
||||
@ -2663,7 +2688,7 @@ follows:
|
||||
</pre>
|
||||
|
||||
<p>
|
||||
For floating point numbers,
|
||||
For floating-point numbers,
|
||||
<code>+x</code> is the same as <code>x</code>,
|
||||
while <code>-x</code> is the negation of <code>x</code>.
|
||||
</p>
|
||||
@ -2692,11 +2717,10 @@ not occur. For instance, it may not assume that <code>x < x + 1</code> is alw
|
||||
<h3 id="Comparison_operators">Comparison operators</h3>
|
||||
|
||||
<p>
|
||||
Comparison operators yield a boolean result.
|
||||
Comparison operators yield a value of type <code>bool</code>.
|
||||
The operators <code>==</code> and <code>!=</code> apply, at least in some cases,
|
||||
to all types except arrays and structs.
|
||||
All other comparison operators apply only
|
||||
to basic types except <code>bool</code>.
|
||||
to operands of all types except arrays and structs.
|
||||
All other comparison operators apply only to numeric and string values.
|
||||
</p>
|
||||
|
||||
<pre class="grammar">
|
||||
@ -2709,13 +2733,14 @@ to basic types except <code>bool</code>.
|
||||
</pre>
|
||||
|
||||
<p>
|
||||
Numeric basic types are compared in the usual way.
|
||||
Operands of numeric type are compared in the usual way.
|
||||
</p>
|
||||
<p>
|
||||
Strings are compared byte-wise (lexically).
|
||||
Operands of string type are compared byte-wise (lexically).
|
||||
</p>
|
||||
<p>
|
||||
Booleans are equal if they are either both "true" or both "false".
|
||||
Operands of boolean type are equal if they are either both <code>true</code>
|
||||
or both <code>false</code>.
|
||||
</p>
|
||||
<p>
|
||||
The rules for comparison of composite types are described in the
|
||||
@ -2726,7 +2751,8 @@ section on §<a href="#Comparison_compatibility">Comparison compatibility</a>.
|
||||
<h3 id="Logical_operators">Logical operators</h3>
|
||||
|
||||
<p>
|
||||
Logical operators apply to boolean operands and yield a boolean result.
|
||||
Logical operators apply to <a href="#Boolean_types">boolean</a> values
|
||||
and yield a result of the same type as the operands.
|
||||
The right operand is evaluated conditionally.
|
||||
</p>
|
||||
|
||||
@ -2972,36 +2998,31 @@ The resulting function takes an explicit receiver of that interface type.
|
||||
<h3 id="Constant_expressions">Constant expressions</h3>
|
||||
|
||||
<p>
|
||||
Constant expressions may contain only constants, <code>iota</code>,
|
||||
numeric literals, string literals, and
|
||||
some constant-valued built-in functions such as <code>unsafe.Sizeof</code>
|
||||
and <code>len</code> applied to an array.
|
||||
In practice, constant expressions are those that can be evaluated at compile time.
|
||||
<p>
|
||||
The type of a constant expression is determined by the type of its
|
||||
elements. If it contains only numeric literals, its type is <i>ideal
|
||||
integer</i> or <i>ideal float</i> (§<a href="#Ideal_numbers">Ideal numbers</a>). Whether a literal
|
||||
is an integer or float depends on the syntax of the literals (123 vs. 123.0).
|
||||
The nature of the arithmetic
|
||||
operations within the expression depends, elementwise, on the values;
|
||||
for example, 3/2 is an integer division yielding 1, while 3./2. is
|
||||
a floating point division yielding 1.5. Thus
|
||||
</p>
|
||||
|
||||
<pre>
|
||||
const x = 3./2. + 3/2;
|
||||
</pre>
|
||||
|
||||
<p>
|
||||
yields a floating point constant of ideal float value 2.5 (1.5 +
|
||||
1); its constituent expressions are evaluated using distinct rules
|
||||
for division.
|
||||
Constant expressions may contain only <a href="#Constants">constant</a>
|
||||
operands and are evaluated at compile-time.
|
||||
</p>
|
||||
|
||||
<p>
|
||||
Intermediate values and the constants themselves
|
||||
may require precision significantly larger than any concrete type
|
||||
in the language. The following are legal declarations:
|
||||
Untyped boolean, numeric, and string constants may be used as operands
|
||||
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 an untyped integer constant and an untyped floating-point constant,
|
||||
the integer constant is converted to an untyped floating-point constant
|
||||
(relevant for <code>/</code> and <code>%</code>).
|
||||
<p>
|
||||
|
||||
</p>
|
||||
Applying an operator to untyped constants results in an untyped
|
||||
constant of the same kind (that is, a boolean, integer, floating-point, or
|
||||
string constant), except for
|
||||
<a href="#Comparison_operators">comparison operators</a> which result in
|
||||
a constant of type <code>bool</code>.
|
||||
</p>
|
||||
|
||||
<p>
|
||||
Constant expressions are always evaluated exactly; intermediate values and the
|
||||
constants themselves may require precision significantly larger than supported
|
||||
by any predeclared type in the language. The following are legal declarations:
|
||||
</p>
|
||||
|
||||
<pre>
|
||||
@ -3010,38 +3031,26 @@ const Four int8 = Huge >> 98;
|
||||
</pre>
|
||||
|
||||
<p>
|
||||
A constant expression may appear in any context, such as assignment
|
||||
to a variable of any numeric type, as long as the value of the
|
||||
expression can be represented accurately in that context.
|
||||
It is erroneous to assign a value with a non-zero fractional part
|
||||
to an integer, or if the assignment would overflow or underflow,
|
||||
or in general if the value cannot be represented by the type of
|
||||
the variable.
|
||||
For
|
||||
instance, <code>3</code> can be assigned to any integer variable but also to any
|
||||
floating point variable, while <code>-1e12</code> can be assigned to a
|
||||
<code>float32</code>, <code>float64</code>, or even <code>int64</code>
|
||||
but not <code>uint64</code> or <code>string</code>.
|
||||
</p>
|
||||
|
||||
<p>
|
||||
If a typed constant expression evaluates to a value that is not
|
||||
representable by that type, the compiler reports an error.
|
||||
The values of <i>typed</i> constants must always be accurately representable as values
|
||||
of the constant type. The following constant expressions are illegal:
|
||||
</p>
|
||||
|
||||
<pre>
|
||||
uint8(-1) // error, out of range
|
||||
uint8(100) * 100 // error, out of range
|
||||
uint(-1) // -1 overflows uint
|
||||
int(3.14) // 3.14 truncated to integer
|
||||
int64(Huge) // 1<<100 overflows int64
|
||||
Four * 300 // 300 overflows int8
|
||||
Four * 100 // 400 overflows int8
|
||||
</pre>
|
||||
|
||||
<p>
|
||||
The mask used by the unary bitwise complement operator matches
|
||||
The mask used by the unary bitwise complement operator <code>^</code> matches
|
||||
the rule for non-constants: the mask is all 1s for unsigned constants
|
||||
and -1 for signed and ideal constants.
|
||||
and -1 for signed and untyped constants.
|
||||
</p>
|
||||
|
||||
<pre>
|
||||
^1 // ideal constant, equal to -2
|
||||
^1 // untyped integer constant, equal to -2
|
||||
uint8(^1) // error, same as uint8(-2), out of range
|
||||
^uint8(1) // typed uint8 constant, same as 0xFF ^ uint8(1) = uint8(0xFE)
|
||||
int8(^1) // same as int8(-2)
|
||||
@ -3056,6 +3065,7 @@ overflow etc. errors being caught.
|
||||
</font>
|
||||
</p>
|
||||
|
||||
|
||||
<h3 id="Order_of_evaluation">Order of evaluation</h3>
|
||||
|
||||
<p>
|
||||
@ -3164,8 +3174,9 @@ f(x+y)
|
||||
|
||||
<p>
|
||||
The "++" and "--" statements increment or decrement their operands
|
||||
by the ideal numeric value 1. As with an assignment, the operand
|
||||
must be a variable, pointer indirection, field selector or index expression.
|
||||
by the untyped <a href="#Constants">constant</a> <code>1</code>.
|
||||
As with an assignment, the operand must be a variable, pointer indirection,
|
||||
field selector or index expression.
|
||||
</p>
|
||||
|
||||
<pre class="ebnf">
|
||||
@ -3259,9 +3270,13 @@ a, b = b, a // exchange a and b
|
||||
</pre>
|
||||
|
||||
<p>
|
||||
In assignments, the type of each value must be
|
||||
In assignments, each value must be
|
||||
<a href="#Assignment_compatibility">assignment compatible</a> with the type of the
|
||||
operand to which it is assigned.
|
||||
operand to which it is assigned. If an untyped <a href="#Constants">constant</a>
|
||||
is assigned to a variable of interface type, the constant is <a href="#Conversions">converted</a>
|
||||
to type <code>bool</code>, <code>int</code>, <code>float</code>, or <code>string</code>
|
||||
respectively, depending on whether the value is a boolean, integer, floating-point,
|
||||
or string constant.
|
||||
</p>
|
||||
|
||||
|
||||
@ -3599,7 +3614,7 @@ for i, s := range a {
|
||||
}
|
||||
|
||||
var key string;
|
||||
var val interface {}; // value type of m is assignment-compatible to val
|
||||
var val interface {}; // value type of m is assignment compatible to val
|
||||
for key, val = range m {
|
||||
h(key, val)
|
||||
}
|
||||
@ -3967,32 +3982,26 @@ The following conversion rules apply:
|
||||
</p>
|
||||
<ul>
|
||||
<li>
|
||||
1) The conversion succeeds if the value is assignment-compatible
|
||||
to a variable of type T.
|
||||
1) The conversion succeeds if the value is <a href="#Assignment_compatibility">assignment compatible</a>
|
||||
with type <code>T</code>.
|
||||
</li>
|
||||
<li>
|
||||
2) The conversion succeeds if the value would be assignment-compatible
|
||||
to a variable of type T if the value's type, or T, or any of their component
|
||||
types are unnamed (§<a href="#Type_identity_and_compatibility">Type identity and compatibility</a>).
|
||||
2) The conversion succeeds if the value would be assignment compatible
|
||||
with type <code>T</code> if the value's type, or <code>T</code>, or any of their component
|
||||
types were unnamed (§<a href="#Type_identity_and_compatibility">Type identity and compatibility</a>).
|
||||
</li>
|
||||
<li>
|
||||
3a) From an ideal number to an integer type.
|
||||
The ideal number must be representable in the result type; it must not overflow.
|
||||
For example, <code>uint8(0xFF)</code> is legal but <code>int8(0xFF)</code> is not.
|
||||
</li>
|
||||
<li>
|
||||
3b) From a non-ideal integer value to an integer type. If the value is a signed quantity, it is
|
||||
3) Between integer types. If the value is a signed quantity, it is
|
||||
sign extended to implicit infinite precision; otherwise it is zero
|
||||
extended. It is then truncated to fit in the result type's size.
|
||||
For example, if <code>x := uint16(0x10F0)</code>, then <code>uint32(int8(x)) == 0xFFFFFFF0</code>.
|
||||
The conversion always yields a valid value; there is no indication of overflow.
|
||||
</li>
|
||||
<li>
|
||||
4) Between integer and floating point types, or between floating point
|
||||
types.
|
||||
When converting a floating point number to an integer, the fraction is discarded
|
||||
4) Between integer and floating-point types, or between floating-point types.
|
||||
When converting a floating-point number to an integer, the fraction is discarded
|
||||
(truncation towards zero).
|
||||
In all conversions involving floating point, if the result type cannot represent the
|
||||
In all conversions involving floating-point values, if the result type cannot represent the
|
||||
value the conversion succeeds but the result value is unspecified.
|
||||
<font color=red>This behavior may change.</font>
|
||||
</li>
|
||||
@ -4029,9 +4038,9 @@ string([]byte{'h', 'e', 'l', 'l', 'o'}) // "hello"
|
||||
|
||||
<p>
|
||||
There is no linguistic mechanism to convert between pointers and integers.
|
||||
The <code>unsafe</code> package
|
||||
The package <a href="#Package_unsafe"><code>unsafe</code></a>
|
||||
implements this functionality under
|
||||
restricted circumstances (§<a href="#Package_unsafe">Package <code>unsafe</code></a>).
|
||||
restricted circumstances.
|
||||
</p>
|
||||
|
||||
|
||||
@ -4287,7 +4296,7 @@ and no explicit initialization is provided, the memory is
|
||||
given a default initialization. Each element of such a value is
|
||||
set to the <i>zero value</i> for its type: <code>false</code> for booleans,
|
||||
<code>0</code> for integers, <code>0.0</code> for floats, <code>""</code>
|
||||
for strings, and <code>nil</code> for pointers, interfaces, slices, channels, and maps.
|
||||
for strings, and <code>nil</code> for pointers, functions, interfaces, slices, channels, and maps.
|
||||
This initialization is done recursively, so for instance each element of an
|
||||
array of structs will have its fields zeroed if no value is specified.
|
||||
</p>
|
||||
|
Loading…
Reference in New Issue
Block a user