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diff --git a/doc/go_spec.html b/doc/go_spec.html index e8bb35f0b..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 August 28, 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>, @@ -1154,11 +1165,11 @@ interface{} <p> Similarly, consider this interface specification, which appears within a <a href="#Type_declarations">type declaration</a> -to define an interface called <code>Lock</code>: +to define an interface called <code>Locker</code>: </p> <pre> -type Lock interface { +type Locker interface { Lock() Unlock() } @@ -1174,28 +1185,35 @@ func (p T) Unlock() { … } </pre> <p> -they implement the <code>Lock</code> interface as well +they implement the <code>Locker</code> interface as well as the <code>File</code> interface. </p> + <p> -An interface may use an interface type name <code>T</code> -in place of a method specification. -The effect, called embedding an interface, -is equivalent to enumerating the methods of <code>T</code> explicitly -in the interface. +An interface <code>T</code> may use a (possibly qualified) interface type +name <code>E</code> in place of a method specification. This is called +<i>embedding</i> interface <code>E</code> in <code>T</code>; it adds +all (exported and non-exported) methods of <code>E</code> to the interface +<code>T</code>. </p> <pre> -type ReadWrite interface { +type ReadWriter interface { Read(b Buffer) bool Write(b Buffer) bool } type File interface { - ReadWrite // same as enumerating the methods in ReadWrite - Lock // same as enumerating the methods in Lock + ReadWriter // same as adding the methods of ReadWriter + Locker // same as adding the methods of Locker Close() } + +type LockedFile interface { + Locker + File // illegal: Lock, Unlock not unique + Lock() // illegal: Lock not unique +} </pre> <p> @@ -1875,9 +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 ) . @@ -1898,22 +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 §<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 @@ -4311,7 +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. @@ -4329,8 +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, @@ -4404,23 +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> @@ -4675,6 +4697,7 @@ additionally it may specify an <i>init</i> and a <i>post</i> statement, such as an assignment, an increment or decrement statement. The init statement may be a <a href="#Short_variable_declarations">short variable declaration</a>, but the post statement must not. +Variables declared by the init statement are re-used in each iteration. </p> <pre class="ebnf"> @@ -4801,7 +4824,7 @@ The iteration variables may be declared by the "range" clause using a form of <a href="#Short_variable_declarations">short variable declaration</a> (<code>:=</code>). In this case their types are set to the types of the respective iteration values -and their <a href="#Declarations_and_scope">scope</a> ends at the end of the "for" +and their <a href="#Declarations_and_scope">scope</a> is the block of the "for" statement; they are re-used in each iteration. If the iteration variables are declared outside the "for" statement, after execution their values will be those of the last iteration. @@ -5243,13 +5266,16 @@ Calls of built-in functions are restricted as for </p> <p> -Each time the "defer" statement +Each time a "defer" statement executes, the function value and parameters to the call are <a href="#Calls">evaluated as usual</a> -and saved anew but the actual function body is not executed. -Instead, deferred functions are executed immediately before +and saved anew but the actual function is not invoked. +Instead, deferred functions are invoked immediately before the surrounding function returns, in the reverse order they were deferred. +If a deferred function value evaluates +to <code>nil</code>, execution <a href="#Handling_panics">panics</a> +when the function is invoked, not when the "defer" statement is executed. </p> <p> @@ -5916,20 +5942,42 @@ var t T </pre> <h3 id="Package_initialization">Package initialization</h3> + +<p> +Within a package, package-level variables are initialized in +<i>declaration order</i> but after any of the variables +they <i>depend</i> on. +</p> + +<p> +More precisely, a package-level variable is considered <i>ready for +initialization</i> if it is not yet initialized and either has +no <a href="#Variable_declarations">initialization expression</a> or +its initialization expression has no dependencies on uninitialized variables. +Initialization proceeds by repeatedly initializing the next package-level +variable that is earliest in declaration order and ready for initialization, +until there are no variables ready for initialization. +</p> + +<p> +If any variables are still uninitialized when this +process ends, those variables are part of one or more initialization cycles, +and the program is not valid. +</p> + <p> -Within a package, package-level variables are initialized according -to their <i>dependencies</i>: if a variable <code>x</code> depends on -a variable <code>y</code>, <code>x</code> will be initialized after -<code>y</code>. +The declaration order of variables declared in multiple files is determined +by the order in which the files are presented to the compiler: Variables +declared in the first file are declared before any of the variables declared +in the second file, and so on. </p> <p> Dependency analysis does not rely on the actual values of the variables, only on lexical <i>references</i> to them in the source, -analyzed transitively. For instance, a variable <code>x</code>'s -<a href="#Variable_declarations">initialization expression</a> -may refer to a function whose body refers to variable <code>y</code>; -if so, <code>x</code> depends on <code>y</code>. +analyzed transitively. For instance, if a variable <code>x</code>'s +initialization expression refers to a function whose body refers to +variable <code>y</code> then <code>x</code> depends on <code>y</code>. Specifically: </p> @@ -5962,11 +6010,6 @@ or to a function or method that depends on <code>y</code>. Dependency analysis is performed per package; only references referring to variables, functions, and methods declared in the current package are considered. -It is an error if variable dependencies form a cycle -(but dependency cycles containing no variables are permitted). -If two variables are independent of each other, -they are initialized in the order they are declared -in the source, possibly in multiple files, as presented to the compiler. </p> <p> @@ -5989,8 +6032,6 @@ func f() int { <p> the initialization order is <code>d</code>, <code>b</code>, <code>c</code>, <code>a</code>. -Since <code>b</code> and <code>c</code> are independent of each other, they are -initialized in declaration order (<code>b</code> before <code>c</code>). </p> <p> @@ -6033,6 +6074,12 @@ the <code>init</code> functions: it will not invoke the next one until the previous one has returned. </p> +<p> +To ensure reproducible initialization behavior, build systems are encouraged +to present multiple files belonging to the same package in lexical file name +order to a compiler. +</p> + <h3 id="Program_execution">Program execution</h3> <p> |