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https://github.com/golang/go
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- completed section on built-in functions
- moved Conversions section out of built-in functions and into expressions - fixed syntax of conversions (parens are not mandatory if the type is not a TypeName) (this is the only change to the Conversions section; the rest of the text is just moved; old line: 4043, new line: 3078) - fixed syntax of composite literals (parens are allowed around LiteralType) DELTA=239 (115 added, 98 deleted, 26 changed) OCL=35118 CL=35159
This commit is contained in:
Robert Griesemer
parent
cce01111a9
commit
164a7bceeb
259
doc/go_spec.html
259
doc/go_spec.html
@ -69,9 +69,6 @@ Todo
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(struct{T} vs struct {T T} vs struct {t T})
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[ ] need explicit language about the result type of operations
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[ ] may want to have some examples for the types of shift operations
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[ ] document illegality of package-external tuple assignments to structs
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w/ private fields: P.T(1, 2) illegal since same as P.T(a: 1, b: 2) for
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a T struct { a b int }.
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[ ] should probably write something about evaluation order of statements even
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though obvious
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[ ] specify iteration direction for range clause
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@ -1121,12 +1118,8 @@ or absent, the communication succeeds only when both a sender and receiver are r
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</p>
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<p>
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For a channel <code>c</code>, the predefined function <code>close(c)</code>
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marks the channel as unable to accept more
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values through a send operation. After any previously
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sent values have been received, receives will return
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the zero value for the channel's type. After at least one such zero value has been
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received, <code>closed(c)</code> returns true.
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A channel may be closed and tested for closure with the built-in functions
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<a href="#Close_and_closed"><code>close</code> and <code>closed</code></a>.
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</p>
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<h2 id="Properties_of_types_and_values">Properties of types and values</h2>
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@ -1946,7 +1939,7 @@ a single expression or a key-value pair.
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<pre class="ebnf">
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CompositeLit = LiteralType "{" [ ElementList ] "}" .
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LiteralType = StructType | ArrayType | "[" "..." "]" ElementType |
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SliceType | MapType | TypeName .
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SliceType | MapType | TypeName | "(" LiteralType ")" .
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ElementList = Element { "," Element } [ "," ] .
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Element = [ Key ":" ] Value .
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Key = FieldName | Index .
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@ -2134,10 +2127,15 @@ as they are accessible.
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<h3 id="Primary_expressions">Primary expressions</h3>
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<p>
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Primary expressions are the operands for unary and binary expressions.
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</p>
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<pre class="ebnf">
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PrimaryExpr =
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Operand |
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Conversion |
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BuiltinCall |
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PrimaryExpr Selector |
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PrimaryExpr Index |
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PrimaryExpr Slice |
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@ -3059,6 +3057,84 @@ It is legal to derive a function value from a method of an interface type.
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The resulting function takes an explicit receiver of that interface type.
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</p>
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<h3 id="Conversions">Conversions</h3>
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<p>
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Conversions are expressions of the form <code>T(x)</code>
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where <code>T</code> is a type and <code>x</code> is an expression
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that can be converted to type <code>T</code>.
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</p>
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<pre class="ebnf">
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Conversion = LiteralType "(" Expression ")" .
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</pre>
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<p>
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The following conversion rules apply:
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</p>
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<ul>
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<li>
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1) The conversion succeeds if the value is <a href="#Assignment_compatibility">assignment compatible</a>
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with type <code>T</code>.
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</li>
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<li>
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2) The conversion succeeds if the value would be assignment compatible
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with type <code>T</code> if the value's type, or <code>T</code>, or any of their component
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types were unnamed.
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</li>
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<li>
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3) Between integer types: If the value is a signed quantity, it is
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sign extended to implicit infinite precision; otherwise it is zero
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extended. It is then truncated to fit in the result type's size.
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For example, if <code>x := uint16(0x10F0)</code>, then <code>uint32(int8(x)) == 0xFFFFFFF0</code>.
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The conversion always yields a valid value; there is no indication of overflow.
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</li>
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<li>
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4) Between integer and floating-point types, or between floating-point types:
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When converting a floating-point number to an integer, the fraction is discarded
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(truncation towards zero).
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In all conversions involving floating-point values, if the result type cannot represent the
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value the conversion succeeds but the result value is unspecified.
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<font color=red>This behavior may change.</font>
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</li>
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<li>
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5) Strings permit three special conversions:
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</li>
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<li>
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5a) Converting an integer value yields a string containing the UTF-8
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representation of the integer.
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<pre>
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string(0x65e5) // "\u65e5" == "日" == "\xe6\x97\xa5"
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</pre>
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</li>
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<li>
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5b) Converting a slice of integers yields a string that is the
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concatenation of the individual integers converted to strings.
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If the slice value is <code>nil</code>, the result is the empty string.
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<pre>
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string([]int{0x767d, 0x9d6c, 0x7fd4}) // "\u767d\u9d6c\u7fd4" == "白鵬翔"</pre>
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</li>
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<li>
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5c) Converting a slice of bytes yields a string whose successive
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bytes are those of the slice. If the slice value is <code>nil</code>,
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the result is the empty string.
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<pre>
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string([]byte{'h', 'e', 'l', 'l', 'o'}) // "hello"
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</pre>
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</li>
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</ul>
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<p>
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There is no linguistic mechanism to convert between pointers and integers.
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The package <a href="#Package_unsafe"><code>unsafe</code></a>
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implements this functionality under
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restricted circumstances.
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</p>
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<h3 id="Constant_expressions">Constant expressions</h3>
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<p>
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@ -3978,30 +4054,47 @@ for i := 0; i <= 3; i++ {
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}
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</pre>
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<h2 id="Predeclared_functions">Predeclared functions</h2>
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<ul>
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<li>cap
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<li>close
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<li>closed
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<li>len
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<li>make
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<li>new
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<li>panic
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<li>panicln
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<li>print
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<li>println
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</ul>
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<h2 id="Built-in_functions">Built-in functions</h2>
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<p>
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A small number of built-in functions are
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<a href="#Predeclared_identifiers">predeclared</a>.
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They are called like any other function but some of them
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accept a type instead of an expression as the first argument.
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</p>
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<pre class="ebnf">
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BuiltinCall = identifier "(" [ BuiltinArgs ] ")" .
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BuiltinArgs = Type [ "," ExpressionList ] | ExpressionList .
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</pre>
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<h3 id="Close_and_closed">Close and closed</h3>
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<p>
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For a channel <code>c</code>, the predefined function <code>close(c)</code>
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marks the channel as unable to accept more
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values through a send operation. After any previously
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sent values have been received, receive operations will return
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the zero value for the channel's type. After at least one such zero value has been
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received, <code>closed(c)</code> returns true.
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</p>
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<h3 id="Length_and_capacity">Length and capacity</h3>
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<p>
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The built-in functions <code>len</code> and <code>cap</code> take arguments
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of various types and return a result of type <code>int</code>.
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The implementation guarantees that the result always fits into an <code>int</code>.
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</p>
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<pre class="grammar">
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Call Argument type Result
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len(s) string string length (in bytes)
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len(s) string type string length in bytes
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[n]T, *[n]T array length (== n)
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[]T slice length
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map[K]T map length (number of defined keys)
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chan T number of elements sent queued in channel buffer
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chan T number of elements queued in channel buffer
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cap(s) [n]T, *[n]T array length (== n)
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[]T slice capacity
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@ -4009,107 +4102,16 @@ cap(s) [n]T, *[n]T array length (== n)
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</pre>
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<p>
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The type of the result is always <code>int</code> and the
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implementation guarantees that
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the result always fits into an <code>int</code>.
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<p>
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The capacity of a slice or map is the number of elements for which there is
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space allocated in the underlying array (for a slice) or map. For a slice
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<code>s</code>, at any time the following relationship holds:
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The capacity of a slice is the number of elements for which there is
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space allocated in the underlying array.
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At any time the following relationship holds:
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</p>
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<pre>
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0 <= len(s) <= cap(s)
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</pre>
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<h3 id="Conversions">Conversions</h3>
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<p>
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Conversions look like function calls of the form
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</p>
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<pre>
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T(value)
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</pre>
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<p>
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where <code>T</code> is a type
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and <code>value</code> is an expression
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that can be converted to a value
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of result type <code>T</code>.
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</p>
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<pre class="ebnf">
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Conversion = ( TypeName | "(" Type ")" ) Expression .
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</pre>
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<p>
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The following conversion rules apply:
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</p>
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<ul>
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<li>
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1) The conversion succeeds if the value is <a href="#Assignment_compatibility">assignment compatible</a>
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with type <code>T</code>.
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</li>
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<li>
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2) The conversion succeeds if the value would be assignment compatible
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with type <code>T</code> if the value's type, or <code>T</code>, or any of their component
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types were unnamed (§<a href="#Type_identity_and_compatibility">Type identity and compatibility</a>).
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</li>
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<li>
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3) Between integer types. If the value is a signed quantity, it is
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sign extended to implicit infinite precision; otherwise it is zero
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extended. It is then truncated to fit in the result type's size.
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For example, if <code>x := uint16(0x10F0)</code>, then <code>uint32(int8(x)) == 0xFFFFFFF0</code>.
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The conversion always yields a valid value; there is no indication of overflow.
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</li>
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<li>
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4) Between integer and floating-point types, or between floating-point types.
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When converting a floating-point number to an integer, the fraction is discarded
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(truncation towards zero).
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In all conversions involving floating-point values, if the result type cannot represent the
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value the conversion succeeds but the result value is unspecified.
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<font color=red>This behavior may change.</font>
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</li>
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<li>
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5) Strings permit three special conversions:
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</li>
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<li>
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5a) Converting an integer value yields a string containing the UTF-8
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representation of the integer.
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<pre>
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string(0x65e5) // "\u65e5" == "日" == "\xe6\x97\xa5"
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</pre>
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</li>
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<li>
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5b) Converting a slice of integers yields a string that is the
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concatenation of the individual integers converted to strings.
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If the slice value is <code>nil</code>, the result is the empty string.
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<pre>
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string([]int{0x767d, 0x9d6c, 0x7fd4}) // "\u767d\u9d6c\u7fd4" == "白鵬翔"</pre>
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</li>
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<li>
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5c) Converting a slice of bytes yields a string whose successive
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bytes are those of the slice. If the slice value is <code>nil</code>,
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the result is the empty string.
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<pre>
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string([]byte{'h', 'e', 'l', 'l', 'o'}) // "hello"
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</pre>
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</li>
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</ul>
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<p>
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There is no linguistic mechanism to convert between pointers and integers.
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The package <a href="#Package_unsafe"><code>unsafe</code></a>
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implements this functionality under
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restricted circumstances.
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</p>
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<h3 id="Allocation">Allocation</h3>
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<p>
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@ -4180,10 +4182,25 @@ c := make(chan int, 10); # channel with a buffer size of 10
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m := make(map[string] int, 100); # map with initial space for 100 elements
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</pre>
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<h3 id="Bootstrapping">Bootstrapping</h3>
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<p>
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<font color=red>TODO: Need syntax that permits a type as first argument for built-ins.</font>
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Current implementations provide several built-in functions useful during
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bootstrapping. These functions are documented for completeness but are not
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guaranteed to stay in the language. They do not return a result.
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</p>
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<pre class="grammar">
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Call Behavior
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print prints all arguments; formatting of arguments is implementation-specific
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println like print but prints spaces between arguments and a newline at the end
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panic like print, aborts execution after printing
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panicln like println, aborts execution after printing
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</pre>
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<h2 id="Packages">Packages</h2>
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<p>
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