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spec: several clarifications to language on channels

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- A channel may be used between any number of goroutines,
  not just two.
- Replace "passing a value" (which is not further defined)
  by "sending and receiving a value".
- Made syntax production more symmetric.
- Talk about unbuffered channels before buffered channels.
- Clarify what the comma,ok receive values mean (issue 7785).

Not a language change.

Fixes #7785.

LGTM=rsc, r, iant
R=r, rsc, iant, ken
CC=golang-codereviews
https://golang.org/cl/94030045
This commit is contained in:
Robert Griesemer 2014-05-07 10:40:39 -07:00
parent dbe5f88804
commit 97aa90d251
1 changed files with 38 additions and 34 deletions
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72
doc/go_spec.html
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@ -1278,20 +1278,23 @@ may be added.
<h3 id="Channel_types">Channel types</h3>
<p>
A channel provides a mechanism for two concurrently executing functions
to synchronize execution and communicate by passing a value of a
specified element type.
A channel provides a mechanism for
<a href="#Go_statements">concurrently executing functions</a>
to communicate by
<a href="#Send_statements">sending</a> and
<a href="#Receive_operator">receiving</a>
values of a specified element type.
The value of an uninitialized channel is <code>nil</code>.
</p>
<pre class="ebnf">
ChannelType = ( "chan" [ "&lt;-" ] | "&lt;-" "chan" ) ElementType .
ChannelType = ( "chan" | "chan" "&lt;-" | "&lt;-" "chan" ) ElementType .
</pre>
<p>
The <code>&lt;-</code> operator specifies the channel <i>direction</i>,
The optional <code>&lt;-</code> operator specifies the channel <i>direction</i>,
<i>send</i> or <i>receive</i>. If no direction is given, the channel is
<i>bi-directional</i>.
<i>bidirectional</i>.
A channel may be constrained only to send or only to receive by
<a href="#Conversions">conversion</a> or <a href="#Assignments">assignment</a>.
</p>
@ -1318,7 +1321,7 @@ chan (&lt;-chan int)
A new, initialized channel
value can be made using the built-in function
<a href="#Making_slices_maps_and_channels"><code>make</code></a>,
which takes the channel type and an optional capacity as arguments:
which takes the channel type and an optional <i>capacity</i> as arguments:
</p>
<pre>
@ -1326,21 +1329,35 @@ make(chan int, 100)
</pre>
<p>
The capacity, in number of elements, sets the size of the buffer in the channel. If the
capacity is greater than zero, the channel is asynchronous: communication operations
succeed without blocking if the buffer is not full (sends) or not empty (receives),
and elements are received in the order they are sent.
If the capacity is zero or absent, the communication succeeds only when both a sender and
receiver are ready.
The capacity, in number of elements, sets the size of the buffer in the channel.
If the capacity is zero or absent, the channel is unbuffered and communication
succeeds only when both a sender and receiver are ready. Otherwise, the channel is
buffered and communication operations succeed without blocking if the buffer
is not full (sends) or not empty (receives).
A <code>nil</code> channel is never ready for communication.
</p>
<p>
A channel may be closed with the built-in function
<a href="#Close"><code>close</code></a>; the
multi-valued assignment form of the
<a href="#Close"><code>close</code></a>.
The multi-valued assignment form of the
<a href="#Receive_operator">receive operator</a>
tests whether a channel has been closed.
reports whether a received value was sent before
the channel was closed.
</p>
<p>
A single channel may be used in
<a href="#Send_statements">send statements</a>,
<a href="#Receive_operator">receive operations</a>,
and calls to the built-in functions
<a href="#Length_and_capacity"><code>cap</code></a> and
<a href="#Length_and_capacity"><code>len</code></a>
by any number of goroutines without further synchronization.
Channels act as first-in-first-out queues.
For example, if one goroutine sends values on a channel
and a second goroutine receives them, the values are
received in the order sent.
</p>
<h2 id="Properties_of_types_and_values">Properties of types and values</h2>
@ -3389,7 +3406,8 @@ and the type of the receive operation is the element type of the channel.
The expression blocks until a value is available.
Receiving from a <code>nil</code> channel blocks forever.
A receive operation on a <a href="#Close">closed</a> channel can always proceed
immediately, yielding the element type's <a href="#The_zero_value">zero value</a>.
immediately, yielding the element type's <a href="#The_zero_value">zero value</a>
after any previously sent values have been received.
</p>
<pre>
@ -4238,22 +4256,8 @@ A send on a closed channel proceeds by causing a <a href="#Run_time_panics">run-
A send on a <code>nil</code> channel blocks forever.
</p>
<p>
Channels act as first-in-first-out queues.
For example, if a single goroutine sends on a channel values
that are received by a single goroutine, the values are received in the order sent.
</p>
<p>
A single channel may be used for send and receive
operations and calls to the built-in functions
<a href="#Length_and_capacity"><code>cap</code></a> and
<a href="#Length_and_capacity"><code>len</code></a>
by any number of goroutines without further synchronization.
</p>
<pre>
ch &lt;- 3
ch &lt;- 3 // send value 3 to channel ch
</pre>
@ -5383,8 +5387,8 @@ make(T, n, m) slice slice of type T with length n and capacity m
make(T) map map of type T
make(T, n) map map of type T with initial space for n elements
make(T) channel synchronous channel of type T
make(T, n) channel asynchronous channel of type T, buffer size n
make(T) channel unbuffered channel of type T
make(T, n) channel buffered channel of type T, buffer size n
</pre>