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Diffstat (limited to 'doc')
| -rw-r--r-- | doc/go_spec.html | 81 |
1 files changed, 48 insertions, 33 deletions
diff --git a/doc/go_spec.html b/doc/go_spec.html index de79f7ee4..7fa02e419 100644 --- a/doc/go_spec.html +++ b/doc/go_spec.html @@ -1,6 +1,6 @@ <!--{ "Title": "The Go Programming Language Specification", - "Subtitle": "Version of September 29, 2014", + "Subtitle": "Version of September 30, 2014", "Path": "/ref/spec" }--> @@ -577,7 +577,7 @@ Numeric constants represent values of arbitrary precision and do not overflow. </p> <p> -Constants may be <a href="#Types">typed</a> or untyped. +Constants may be <a href="#Types">typed</a> or <i>untyped</i>. Literal constants, <code>true</code>, <code>false</code>, <code>iota</code>, and certain <a href="#Constant_expressions">constant expressions</a> containing only untyped constant operands are untyped. @@ -598,6 +598,17 @@ not <code>int32</code> or <code>string</code>. </p> <p> +An untyped constant has a <i>default type</i> which is the type to which the +constant is implicitly converted in contexts where a typed value is required, +for instance, in a <a href="#Short_variable_declarations">short variable declaration</a> +such as <code>i := 0</code> where there is no explicit type. +The default type of an untyped constant is <code>bool</code>, <code>rune</code>, +<code>int</code>, <code>float64</code>, <code>complex128</code> or <code>string</code> +respectively, depending on whether it is a boolean, rune, integer, floating-point, +complex, or string constant. +</p> + +<p> There are no constants denoting the IEEE-754 infinity and not-a-number values, but the <a href="/pkg/math/"><code>math</code> package</a>'s <a href="/pkg/math/#Inf">Inf</a>, @@ -1882,9 +1893,10 @@ func (tz TimeZone) String() string { <h3 id="Variable_declarations">Variable declarations</h3> <p> -A variable declaration creates a variable, binds an identifier to it and -gives it a type and optionally an initial value. +A variable declaration creates one or more variables, binds corresponding +identifiers to them, and gives each a type and an initial value. </p> + <pre class="ebnf"> VarDecl = "var" ( VarSpec | "(" { VarSpec ";" } ")" ) . VarSpec = IdentifierList ( Type [ "=" ExpressionList ] | "=" ExpressionList ) . @@ -1905,22 +1917,27 @@ var _, found = entries[name] // map lookup; only interested in "found" <p> If a list of expressions is given, the variables are initialized -by <a href="#Assignments">assigning</a> the expressions to the variables -in order; all expressions must be consumed and all variables initialized from them. +with the expressions following the rules for <a href="#Assignments">assignments</a>. Otherwise, each variable is initialized to its <a href="#The_zero_value">zero value</a>. </p> <p> -If the type is present, each variable is given that type. -Otherwise, the types are deduced from the assignment -of the expression list. +If a type is present, each variable is given that type. +Otherwise, each variable is given the type of the corresponding +initialization value in the assignment. +If that value is an untyped constant, it is first +<a href="#Conversions">converted</a> to its <a href="#Constants">default type</a>; +if it is an untyped boolean value, it is first converted to type <code>bool</code>. +The predeclared value <code>nil</code> cannot be used to initialize a variable +with no explicit type. </p> -<p> -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 as described in ยง<a href="#Assignments">Assignments</a>. -</p> +<pre> +var d = math.Sin(0.5) // d is int64 +var i = 42 // i is int +var t, ok = x.(T) // t is T, ok is bool +var n = nil // illegal +</pre> <p> Implementation restriction: A compiler may make it illegal to declare a variable @@ -4318,7 +4335,7 @@ a[i] = 23 <p> An <i>assignment operation</i> <code>x</code> <i>op</i><code>=</code> -<code>y</code> where <i>op</i> is a binary arithmetic operation equivalent +<code>y</code> where <i>op</i> is a binary arithmetic operation is equivalent to <code>x</code> <code>=</code> <code>x</code> <i>op</i> <code>y</code> but evaluates <code>x</code> only once. The <i>op</i><code>=</code> construct is a single token. @@ -4336,8 +4353,8 @@ i &^= 1<<n A tuple assignment assigns the individual elements of a multi-valued operation to a list of variables. There are two forms. In the first, the right hand operand is a single multi-valued expression -such as a function evaluation or <a href="#Channel_types">channel</a> or -<a href="#Map_types">map</a> operation or a <a href="#Type_assertions">type assertion</a>. +such as a function call, a <a href="#Channel_types">channel</a> or +<a href="#Map_types">map</a> operation, or a <a href="#Type_assertions">type assertion</a>. The number of operands on the left hand side must match the number of values. For instance, if <code>f</code> is a function returning two values, @@ -4411,23 +4428,21 @@ to the type of the operand to which it is assigned, with the following special c </p> <ol> -<li><p> - If an untyped <a href="#Constants">constant</a> +<li> + Any typed value may be assigned to the blank identifier. +</li> + +<li> + If an untyped constant is assigned to a variable of interface type or the blank identifier, - the constant is first <a href="#Conversions">converted</a> 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, rune, integer, floating-point, complex, or - string constant. -</p></li> - -<li><p> - <!-- Note that the result of a comparison is an untyped bool that may not be constant. --> - If a left-hand side is the blank identifier, any typed or non-constant - value except for the predeclared identifier - <a href="#Predeclared_identifiers"><code>nil</code></a> - may be assigned to it. -</p></li> + the constant is first <a href="#Conversions">converted</a> to its + <a href="#Constants">default type</a>. +</li> + +<li> + If an untyped boolean value is assigned to a variable of interface type or + the blank identifier, it is first converted to type <code>bool</code>. +</li> </ol> <h3 id="If_statements">If statements</h3> |