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spec: clarify index and selector expressions
(Replacement for CL 11884043.) 1) Explain a[i] and a[i:j] where a is of type *A as shortcut for (*a)[i] and (*a)[i:j], respectively. 2) Together with 1), because len() of nil slices is well defined, there's no need to special case nil operands anymore. 3) The result of indexing or slicing a constant string is always a non-constant byte or string value. 4) The result of slicing an untyped string is a value of type string. 5) If the operand of a valid slice a[i:j] is nil (i, j must be 0 for it to be valid - this already follows from the in-range rules), the result is a nil slice. Fixes #4913. Fixes #5951. R=r, rsc, iant, ken CC=golang-dev https://golang.org/cl/12198043
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@ -1,6 +1,6 @@
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<!--{
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"Title": "The Go Programming Language Specification",
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"Subtitle": "Version of July 25, 2013",
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"Subtitle": "Version of July 31, 2013",
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"Path": "/ref/spec"
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}-->
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@ -1909,7 +1909,7 @@ ShortVarDecl = IdentifierList ":=" ExpressionList .
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</pre>
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<p>
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It is a shorthand for a regular <a href="#Variable_declarations">variable declaration</a>
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It is shorthand for a regular <a href="#Variable_declarations">variable declaration</a>
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with initializer expressions but no types:
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</p>
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@ -2245,7 +2245,7 @@ element index plus one. A slice literal has the form
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</pre>
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<p>
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and is a shortcut for a slice operation applied to an array:
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and is shorthand for a slice operation applied to an array:
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</p>
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<pre>
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@ -2462,7 +2462,7 @@ is also a pointer to a struct, <code>x.y.z</code> is shorthand
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for <code>(*(*x).y).z</code>, and so on.
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If <code>x</code> contains an anonymous field of type <code>*A</code>,
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where <code>A</code> is also a struct type,
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<code>x.f</code> is a shortcut for <code>(*x.A).f</code>.
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<code>x.f</code> is shorthand for <code>(*x.A).f</code>.
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</p>
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<p>
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@ -2519,10 +2519,9 @@ a[x]
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</pre>
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<p>
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denotes the element of the array, slice, string or map <code>a</code> indexed by <code>x</code>.
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The value <code>x</code> is called the
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<i>index</i> or <i>map key</i>, respectively. The following
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rules apply:
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denotes the element of the array, pointer to array, slice, string or map <code>a</code> indexed by <code>x</code>.
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The value <code>x</code> is called the <i>index</i> or <i>map key</i>, respectively.
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The following rules apply:
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</p>
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<p>
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@ -2537,44 +2536,48 @@ If <code>a</code> is not a map:
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</ul>
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<p>
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For <code>a</code> of type <code>A</code> or <code>*A</code>
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where <code>A</code> is an <a href="#Array_types">array type</a>:
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For <code>a</code> of <a href="#Array_types">array type</a> <code>A</code>:
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</p>
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<ul>
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<li>a <a href="#Constants">constant</a> index must be in range</li>
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<li>if <code>a</code> is <code>nil</code> or if <code>x</code> is out of range at run time,
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<li>if <code>x</code> is out of range at run time,
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a <a href="#Run_time_panics">run-time panic</a> occurs</li>
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<li><code>a[x]</code> is the array element at index <code>x</code> and the type of
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<code>a[x]</code> is the element type of <code>A</code></li>
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</ul>
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<p>
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For <code>a</code> of type <code>S</code> where <code>S</code> is a <a href="#Slice_types">slice type</a>:
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For <code>a</code> of <a href="#Pointer_types">pointer</a> to array type:
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</p>
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<ul>
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<li>if the slice is <code>nil</code> or if <code>x</code> is out of range at run time,
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<li><code>a[x]</code> is shorthand for <code>(*a)[x]</code></li>
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</ul>
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<p>
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For <code>a</code> of <a href="#Slice_types">slice type</a> <code>S</code>:
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</p>
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<ul>
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<li>if <code>x</code> is out of range at run time,
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a <a href="#Run_time_panics">run-time panic</a> occurs</li>
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<li><code>a[x]</code> is the slice element at index <code>x</code> and the type of
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<code>a[x]</code> is the element type of <code>S</code></li>
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</ul>
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<p>
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For <code>a</code> of type <code>T</code>
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where <code>T</code> is a <a href="#String_types">string type</a>:
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For <code>a</code> of <a href="#String_types">string type</a>:
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</p>
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<ul>
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<li>a <a href="#Constants">constant</a> index must be in range
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if the string <code>a</code> is also constant</li>
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<li>if <code>x</code> is out of range at run time,
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a <a href="#Run_time_panics">run-time panic</a> occurs</li>
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<li><code>a[x]</code> is the byte at index <code>x</code> and the type of
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<li><code>a[x]</code> is the non-constant byte value at index <code>x</code> and the type of
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<code>a[x]</code> is <code>byte</code></li>
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<li><code>a[x]</code> may not be assigned to</li>
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</ul>
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<p>
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For <code>a</code> of type <code>M</code>
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where <code>M</code> is a <a href="#Map_types">map type</a>:
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For <code>a</code> of <a href="#Map_types">map type</a> <code>M</code>:
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</p>
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<ul>
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<li><code>x</code>'s type must be
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@ -2628,9 +2631,9 @@ a[low : high]
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</pre>
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<p>
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constructs a substring or slice. The indices <code>low</code> and
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<code>high</code> select which elements appear in the result. The result has
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indices starting at 0 and length equal to
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constructs a substring or slice. The <i>indices</i> <code>low</code> and
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<code>high</code> select which elements of operand <code>a</code> appear
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in the result. The result has indices starting at 0 and length equal to
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<code>high</code> - <code>low</code>.
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After slicing the array <code>a</code>
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</p>
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@ -2663,24 +2666,33 @@ a[:] // same as a[0 : len(a)]
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</pre>
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<p>
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For arrays or strings, the indices <code>low</code> and <code>high</code> are
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<i>in range</i> if <code>0</code> <= <code>low</code> <= <code>high</code> <= <code>len(a)</code>,
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If <code>a</code> is a pointer to an array, <code>a[low : high]</code> is shorthand for
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<code>(*a)[low : high]</code>.
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</p>
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<p>
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For arrays or strings, the indices are <i>in range</i> if
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<code>0</code> <= <code>low</code> <= <code>high</code> <= <code>len(a)</code>,
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otherwise they are <i>out of range</i>.
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For slices, the upper index bound is the slice capacity <code>cap(a)</code> rather than the length.
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A <a href="#Constants">constant</a> index must be non-negative and representable by a value of type
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<code>int</code>.
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If both indices
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are constant, they must satisfy <code>low <= high</code>. If <code>a</code> is <code>nil</code>
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or if the indices are out of range at run time, a <a href="#Run_time_panics">run-time panic</a> occurs.
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If both indices are constant, they must satisfy <code>low <= high</code>.
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If the indices are out of range at run time, a <a href="#Run_time_panics">run-time panic</a> occurs.
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</p>
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<p>
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If the sliced operand is a string or slice, the result of the slice operation
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is a string or slice of the same type.
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Except for <a href="#Constants">untyped strings</a>, if the sliced operand is a string or slice,
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the result of the slice operation is a non-constant value of the same type as the operand.
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For untyped string operands the result is a non-constant value of type <code>string</code>.
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If the sliced operand is an array, it must be <a href="#Address_operators">addressable</a>
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and the result of the slice operation is a slice with the same element type as the array.
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</p>
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<p>
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If the sliced operand of a valid slice expression is a <code>nil</code> slice, the result
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is a <code>nil</code> slice.
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<p>
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<h3 id="Type_assertions">Type assertions</h3>
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