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go spec: clarify semantics of range clause
This CL proposes some subtle language changes in an attempt to clarify the semantics of range clauses and simplify uses of maps. - nil maps behave like empty maps; but attempting to set a value in a nil map causes a run-time panic - nil channels are never ready for communication; sending or reading from a nil channel blocks forever - if there is only one index iteration variable in a range clause and len(range expression) would be a constant, the range expression is not evaluated. (was discrepancy with len/cap before) - the notion of what is a constant expression len(x) for (pointer to) arrays x has been generalized and simplified (can still be syntactically decided) (before: more restrictive syntactic rule that was not consistently implemented) Fixes #1713. R=r, rsc, iant, ken2, r2, bradfitz, rog CC=golang-dev https://golang.org/cl/4444050
This commit is contained in:
Robert Griesemer
parent
82d1a9dce7
commit
5473103666
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doc/go_spec.html
123
doc/go_spec.html
@ -1,5 +1,5 @@
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<!-- title The Go Programming Language Specification -->
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<!-- subtitle Version of May 4, 2011 -->
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<!-- subtitle Version of May 12, 2011 -->
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<!--
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TODO
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@ -1174,8 +1174,10 @@ make(map[string] int, 100)
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<p>
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The initial capacity does not bound its size:
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maps grow to accommodate the number of items
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stored in them.
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</p>
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stored in them, with the exception of <code>nil</code> maps.
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A <code>nil</code> map is equivalent to an empty map except that no elements
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may be added.
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</code>
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<h3 id="Channel_types">Channel types</h3>
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@ -1234,6 +1236,7 @@ succeed without blocking if the buffer is not full (sends) or not empty (receive
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and elements are received in the order they are sent.
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If the capacity is zero or absent, the communication succeeds only when both a sender and
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receiver are ready.
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A <code>nil</code> channel is never ready for communication.
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</p>
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<p>
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@ -1968,7 +1971,7 @@ package, which means that it must begin with a Unicode upper case letter.
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math.Sin
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</pre>
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<!---
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<!--
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<p>
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<span class="alert">TODO: Unify this section with Selectors - it's the same syntax.</span>
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</p>
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@ -2336,7 +2339,7 @@ p.M0 // ((*p).T0).M0
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</pre>
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<!---
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<!--
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<span class="alert">
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TODO: Specify what happens to receivers.
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</span>
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@ -2369,7 +2372,7 @@ or for <code>a</code> of type <code>S</code> where <code>S</code> is a <a href="
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<li><code>x</code> must be an integer value and <code>0 <= x < len(a)</code></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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<li>if the index <code>x</code> is out of range,
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<li>if <code>a</code> is <code>nil</code> or if the index <code>x</code> is out of range,
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a <a href="#Run_time_panics">run-time panic</a> occurs</li>
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</ul>
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@ -2397,7 +2400,7 @@ where <code>M</code> is a <a href="#Map_types">map type</a>:
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<li>if the map contains an entry with key <code>x</code>,
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<code>a[x]</code> is the map value with key <code>x</code>
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and the type of <code>a[x]</code> is the value type of <code>M</code></li>
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<li>if the map does not contain such an entry,
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<li>if the map is <code>nil</code> or does not contain such an entry,
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<code>a[x]</code> is the <a href="#The_zero_value">zero value</a>
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for the value type of <code>M</code></li>
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</ul>
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@ -2426,7 +2429,7 @@ where the result of the index expression is a pair of values with types
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</p>
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<p>
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Similarly, if an assignment to a map has the special form
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Similarly, if an assignment to a map element has the special form
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</p>
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<pre>
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@ -2440,6 +2443,11 @@ the entry for key <code>x</code> is deleted from the map; if
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a regular assignment to an element of the map.
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</p>
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<p>
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Assigning to an element of a <code>nil</code> map causes a
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<a href="#Run_time_panics">run-time panic</a>.
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</p>
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<h3 id="Slices">Slices</h3>
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@ -3043,6 +3051,7 @@ For an operand <code>ch</code> of <a href="#Channel_types">channel type</a>,
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the value of the receive operation <code><-ch</code> is the value received
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from the channel <code>ch</code>. The type of the value is the element type of
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the channel. The expression blocks until a value is available.
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Receiving from a <code>nil</code> channel blocks forever.
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</p>
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<pre>
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@ -3070,12 +3079,7 @@ or is a <a href="#The_zero_value">zero value</a> returned
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because the channel is closed and empty (<code>false</code>).
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</p>
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<p>
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Receiving from a <code>nil</code> channel causes a
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<a href="#Run_time_panics">run-time panic</a>.
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</p>
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<!---
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<!--
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<p>
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<span class="alert">TODO: Probably in a separate section, communication semantics
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need to be presented regarding send, receive, select, and goroutines.</span>
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@ -3449,7 +3453,7 @@ int8(^1) // same as int8(-2)
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^int8(1) // same as -1 ^ int8(1) = -2
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</pre>
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<!---
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<!--
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<p>
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<span class="alert">
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TODO: perhaps ^ should be disallowed on non-uints instead of assuming twos complement.
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@ -3575,17 +3579,13 @@ begins. Communication blocks until the send can proceed, at which point the
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value is transmitted on the channel.
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A send on an unbuffered channel can proceed if a receiver is ready.
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A send on a buffered channel can proceed if there is room in the buffer.
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A send on a <code>nil</code> channel blocks forever.
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</p>
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<pre>
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ch <- 3
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</pre>
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<p>
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Sending to a <code>nil</code> channel causes a
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<a href="#Run_time_panics">run-time panic</a>.
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</p>
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<h3 id="IncDec_statements">IncDec statements</h3>
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@ -3984,10 +3984,14 @@ As with an assignment, the operands on the left must be
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<a href="#Address_operators">addressable</a> or map index expressions; they
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denote the iteration variables. If the range expression is a channel, only
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one iteration variable is permitted, otherwise there may be one or two.
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If the second iteration variable is the <a href="#Blank_identifier">blank identifier</a>,
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the range clause is equivalent to the same clause with only the first variable present.
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</p>
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<p>
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The range expression is evaluated once before beginning the loop.
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The range expression is evaluated once before beginning the loop
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except if the expression is an array, in which case, depending on
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the expression, it might not be evaluated (see below).
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Function calls on the left are evaluated once per iteration.
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For each iteration, iteration values are produced as follows:
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</p>
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@ -4003,8 +4007,11 @@ channel c chan E element e E
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<ol>
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<li>
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For an array or slice value, the index iteration values are produced in
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increasing order, starting at element index 0.
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For an array, pointer to array, or slice value <code>a</code>, the index iteration
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values are produced in increasing order, starting at element index 0. As a special
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case, if only the first iteration variable is present, the range loop produces
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iteration values from 0 up to <code>len(a)</code> and does not index into the array
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or slice itself. For a <code>nil</code> slice, the number of iterations is 0.
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</li>
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<li>
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@ -4023,13 +4030,14 @@ The iteration order over maps is not specified.
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If map entries that have not yet been reached are deleted during iteration,
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the corresponding iteration values will not be produced. If map entries are
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inserted during iteration, the behavior is implementation-dependent, but the
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iteration values for each entry will be produced at most once.
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iteration values for each entry will be produced at most once. If the map
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is <code>nil</code>, the number of iterations is 0.
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</li>
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<li>
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For channels, the iteration values produced are the successive values sent on
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the channel until the channel is closed
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(§<a href="#Close"><code>close</code></a>).
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the channel until the channel is <a href="#Close">closed</a>. If the channel
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is <code>nil</code>, the range expression blocks forever.
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</li>
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</ol>
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@ -4048,9 +4056,17 @@ after execution their values will be those of the last iteration.
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</p>
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<pre>
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var testdata *struct {
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a *[7]int
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}
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for i, _ := range testdata.a {
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// testdata.a is never evaluated; len(testdata.a) is constant
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// i ranges from 0 to 6
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f(i)
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}
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var a [10]string
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m := map[string]int{"mon":0, "tue":1, "wed":2, "thu":3, "fri":4, "sat":5, "sun":6}
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for i, s := range a {
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// type of i is int
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// type of s is string
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@ -4065,6 +4081,11 @@ for key, val = range m {
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}
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// key == last map key encountered in iteration
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// val == map[key]
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var ch chan Work = producer()
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for w := range ch {
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doWork(w)
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}
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</pre>
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@ -4247,7 +4268,7 @@ func (devnull) Write(p []byte) (n int, _ os.Error) {
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Regardless of how they are declared, all the result values are initialized to the zero values for their type (§<a href="#The_zero_value">The zero value</a>) upon entry to the function.
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</p>
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<!---
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<!--
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<p>
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<span class="alert">
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TODO: Define when return is required.<br />
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@ -4332,7 +4353,7 @@ L:
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<p>
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is erroneous because the jump to label <code>L</code> skips
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the creation of <code>v</code>.
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<!---
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<!--
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(<span class="alert">TODO: Eliminate in favor of used and not set errors?</span>)
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-->
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</p>
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@ -4439,17 +4460,17 @@ The implementation guarantees that the result always fits into an <code>int</cod
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</p>
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<pre class="grammar">
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Call Argument type Result
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Call Argument type Result
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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 queued in channel buffer
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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 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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chan T channel buffer capacity
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cap(s) [n]T, *[n]T array length (== n)
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[]T slice capacity
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chan T channel buffer capacity
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</pre>
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<p>
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@ -4467,20 +4488,17 @@ The length and capacity of a <code>nil</code> slice, map, or channel are 0.
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</p>
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<p>
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The expression
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<code>len(s)</code> is a
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<a href="#Constants">constant</a> if <code>s</code> is a string constant.
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The expressions
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<code>len(s)</code> and
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<code>cap(s)</code> are
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constants if <code>s</code> is an (optionally parenthesized)
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identifier or
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<a href="#Qualified_identifiers">qualified identifier</a>
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denoting an array or pointer to array.
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Otherwise invocations of <code>len</code> and <code>cap</code> are not
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constant.
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The expression <code>len(s)</code> is <a href="#Constants">constant</a> if
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<code>s</code> is a string constant. The expressions <code>len(s)</code> and
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<code>cap(s)</code> are constants if the type of <code>s</code> is an array
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or pointer to an array and the expression <code>s</code> does not contain
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<a href="#Receive_operator">channel receives</a> or
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<a href="#Calls">function calls</a>; in this case <code>s</code> is not evaluated.
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Otherwise, invocations of <code>len</code> and <code>cap</code> are not
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constant and <code>s</code> is evaluated.
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</p>
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<h3 id="Allocation">Allocation</h3>
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<p>
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@ -5169,5 +5187,8 @@ The following minimal alignment properties are guaranteed:
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<h2 id="Implementation_differences"><span class="alert">Implementation differences - TODO</span></h2>
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<ul>
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<li><span class="alert">Implementation does not honor the restriction on goto statements and targets (no intervening declarations).</span></li>
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<li><span class="alert">The restriction on <code>goto</code> statements and targets (no intervening declarations) is not honored.</span></li>
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<li><span class="alert"><code>len(a)</code> is only a constant if <code>a</code> is a (qualified) identifier denoting an array or pointer to an array.</span></li>
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<li><span class="alert"><code>nil</code> maps are not treated like empty maps.</span></li>
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<li><span class="alert">Trying to send/receive from a <code>nil</code> channel causes a run-time panic.</span></li>
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</ul>
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