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spec: remove non-blocking channel operators
Add intended changes for close + closed, commented out. R=golang-dev, niemeyer, r, gri1 CC=golang-dev https://golang.org/cl/4013045
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@ -1,5 +1,5 @@
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<!-- title The Go Programming Language Specification -->
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<!-- subtitle Version of January 26, 2011 -->
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<!-- subtitle Version of January 27, 2011 -->
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<!--
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TODO
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@ -3027,28 +3027,7 @@ 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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</p>
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<p>
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If the send operation appears in an expression context, the value
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of the expression is a boolean and the operation is non-blocking.
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The value of the boolean reports true if the communication succeeded,
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false if it did not. (The channel and
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the expression to be sent are evaluated regardless.)
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These two examples are equivalent:
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</p>
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<pre>
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ok := ch <- 3
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if ok { print("sent") } else { print("not sent") }
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if ch <- 3 { print("sent") } else { print("not sent") }
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</pre>
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<p>
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In other words, if the program tests the value of a send operation,
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the send is non-blocking and the value of the expression is the
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success of the operation. If the program does not test the value,
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the operation blocks until it succeeds.
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</p>
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<p>
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The receive operation uses the prefix unary operator "<-".
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The value of the expression is the value received, whose type
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@ -3073,8 +3052,11 @@ f(<-ch)
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<-strobe // wait until clock pulse
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</pre>
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<!--
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TODO(rsc): Add after a release or two without any x,ok := <-c.
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<p>
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If a receive expression is used in an assignment or initialization of the form
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A receive expression used in an assignment or initialization of the form
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</p>
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<pre>
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@ -3084,14 +3066,13 @@ var x, ok = <-ch
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</pre>
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<p>
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the receive operation becomes non-blocking.
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If the operation can proceed, the boolean variable
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<code>ok</code> will be set to <code>true</code>
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and the value stored in <code>x</code>; otherwise
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<code>ok</code> is set
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to <code>false</code> and <code>x</code> is set to the
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zero value for its type (§<a href="#The_zero_value">The zero value</a>).
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yields an additional result.
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The boolean variable <code>ok</code> indicates whether
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the received value was sent on the channel (<code>true</code>)
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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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-->
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<p>
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Except in a communications clause of a <a href="#Select_statements">select statement</a>,
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@ -4097,6 +4078,9 @@ SelectStmt = "select" "{" { CommClause } "}" .
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CommClause = CommCase ":" { Statement ";" } .
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CommCase = "case" ( SendExpr | RecvExpr) | "default" .
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SendExpr = Expression "<-" Expression .
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<!-- TODO(rsc):
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RecvExpr = [ Expression [ "," Expression ] ( "=" | ":=" ) ] "<-" Expression .
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-->
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RecvExpr = [ Expression ( "=" | ":=" ) ] "<-" Expression .
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</pre>
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@ -4128,6 +4112,7 @@ in the "select" statement.
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If multiple cases can proceed, a pseudo-random fair choice is made to decide
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which single communication will execute.
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<p>
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<!-- TODO(rsc): s/variable/& or &s/ -->
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The receive case may declare a new variable using a
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<a href="#Short_variable_declarations">short variable declaration</a>.
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</p>
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@ -4140,6 +4125,14 @@ case i1 = <-c1:
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print("received ", i1, " from c1\n")
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case c2 <- i2:
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print("sent ", i2, " to c2\n")
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<!-- TODO(rsc): add , c3 to channel list above too
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case i3, ok := <-c3:
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if ok {
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print("received ", i3, " from c3\n")
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} else {
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print("c3 is closed\n")
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}
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-->
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default:
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print("no communication\n")
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}
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@ -4393,6 +4386,7 @@ BuiltinCall = identifier "(" [ BuiltinArgs [ "," ] ] ")" .
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BuiltinArgs = Type [ "," ExpressionList ] | ExpressionList .
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</pre>
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<!-- TODO(rsc): s/.and.closed//g -->
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<h3 id="Close_and_closed">Close and closed</h3>
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<p>
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@ -4402,6 +4396,11 @@ sending to or closing a closed channel causes a <a href="#Run_time_panics">run-t
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After calling <code>close</code>, and 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 without blocking.
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<!-- TODO(rsc): delete next sentence, replace with
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The multi-valued <a href="#Communication_operators">receive operation</a>
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returns a received value along with an indication of whether the channel is closed.
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-->
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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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