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new channel syntax
select cleans up too SVN=127816
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Rob Pike
parent
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165
doc/go_lang.txt
165
doc/go_lang.txt
@ -4,7 +4,7 @@ The Go Programming Language (DRAFT)
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Robert Griesemer, Rob Pike, Ken Thompson
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----
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(July 8, 2008)
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(July 16, 2008)
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This document is a semi-formal specification/proposal for a new
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systems programming language. The document is under active
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@ -155,7 +155,7 @@ Multithreading and channels
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Go supports multithreaded programming directly. A function may
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be invoked as a parallel thread of execution. Communication and
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synchronization is provided through channels and their associated
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synchronization are provided through channels and their associated
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language support.
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@ -186,19 +186,19 @@ Here is a complete example Go program that implements a concurrent prime sieve:
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package main
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// Send the sequence 2, 3, 4, ... to channel 'ch'.
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func Generate(ch *chan> int) {
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func Generate(ch *chan-< int) {
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for i := 2; ; i++ {
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>ch = i // Send 'i' to channel 'ch'.
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ch -< i // Send 'i' to channel 'ch'.
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}
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}
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// Copy the values from channel 'in' to channel 'out',
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// removing those divisible by 'prime'.
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func Filter(in *chan< int, out *chan> int, prime int) {
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func Filter(in *chan<- int, out *chan-< int, prime int) {
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for {
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i := <in; // Receive value of new variable 'i' from 'in'.
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i := <-in; // Receive value of new variable 'i' from 'in'.
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if i % prime != 0 {
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>out = i // Send 'i' to channel 'out'.
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out -< i // Send 'i' to channel 'out'.
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}
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}
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}
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@ -208,7 +208,7 @@ Here is a complete example Go program that implements a concurrent prime sieve:
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ch := new(chan int); // Create a new channel.
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go Generate(ch); // Start Generate() as a subprocess.
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for {
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prime := <ch;
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prime := <-ch;
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printf("%d\n", prime);
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ch1 := new(chan int);
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go Filter(ch, ch1, prime);
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@ -728,12 +728,12 @@ By conversion or assignment, it may be restricted only to send or
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to receive; such a restricted channel
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is called a 'send channel' or a 'receive channel'.
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ChannelType = "chan" [ "<" | ">" ] ValueType .
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ChannelType = "chan" [ "<-" | "-<" ] ValueType .
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chan any // a generic channel
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chan int // a channel that can exchange only ints
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chan> float // a channel that can only be used to send floats
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chan< any // a channel that can receive (only) values of any type
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chan-< float // a channel that can only be used to send floats
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chan<- any // a channel that can receive (only) values of any type
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Channel variables always have type pointer to channel.
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It is an error to attempt to use a channel value and in
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@ -931,7 +931,7 @@ Types are structurally equivalent: Two types are equivalent (``equal'') if they
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are constructed the same way from equivalent types.
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For instance, all variables declared as "*int" have equivalent type,
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as do all variables declared as "map [string] chan int".
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as do all variables declared as "map [string] *chan int".
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More precisely, two struct types are equivalent if they have exactly the same fields
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in the same order, with equal field names and types. For all other composite types,
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@ -1215,13 +1215,14 @@ Expression syntax is based on that of C but with fewer precedence levels.
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ConversionType = TypeName | ArrayType | MapType | StructType | InterfaceType .
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Allocation = "new" "(" Type [ "," ExpressionList ] ")" .
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binary_op = log_op | rel_op | add_op | mul_op .
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binary_op = log_op | comm_op | rel_op | add_op | mul_op .
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log_op = "||" | "&&" .
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rel_op = "==" | "!=" | "<" | "<=" | ">" | ">=".
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add_op = "+" | "-" | "|" | "^".
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mul_op = "*" | "/" | "%" | "<<" | ">>" | "&".
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comm_op = "<-" | "-<" .
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rel_op = "==" | "!=" | "<" | "<=" | ">" | ">=" .
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add_op = "+" | "-" | "|" | "^" .
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mul_op = "*" | "/" | "%" | "<<" | ">>" | "&" .
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unary_op = "+" | "-" | "!" | "^" | "<" | ">" | "*" | "&" .
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unary_op = "+" | "-" | "!" | "^" | "*" | "&" | "<-" .
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Field selection and type assertions ('.') bind tightest, followed by indexing ('[]')
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and then calls and conversions. The remaining precedence levels are as follows
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@ -1230,10 +1231,11 @@ and then calls and conversions. The remaining precedence levels are as follows
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Precedence Operator
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1 ||
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2 &&
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3 == != < <= > >=
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4 + - | ^
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5 * / % << >> &
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6 + - ! ^ < > * & (unary)
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3 <- -<
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4 == != < <= > >=
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5 + - | ^
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6 * / % << >> &
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7 + - ! ^ * <- (unary) & (unary)
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For integer values, / and % satisfy the following relationship:
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@ -1291,7 +1293,7 @@ Examples of general expressions
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x <= f()
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^a >> b
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f() || g()
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x == y + 1 && <chan_ptr > 0
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x == y + 1 && <-chan_ptr > 0
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The nil value
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@ -1566,11 +1568,10 @@ Note that ++ and -- are not operators for expressions.
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Assignments
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----
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Assignment = SingleAssignment | TupleAssignment | Send .
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Assignment = SingleAssignment | TupleAssignment .
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SingleAssignment = PrimaryExpr assign_op Expression .
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TupleAssignment = PrimaryExprList assign_op ExpressionList .
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PrimaryExprList = PrimaryExpr { "," PrimaryExpr } .
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Send = ">" Expression "=" Expression .
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assign_op = [ add_op | mul_op ] "=" .
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@ -1580,6 +1581,7 @@ or an array index.
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x = 1
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*p = f()
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a[i] = 23
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k = <-ch
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As in C, arithmetic binary operators can be combined with assignments:
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@ -1607,21 +1609,87 @@ the value is assigned and the second, boolean variable is set to true. Otherwise
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the variable is unchanged, and the boolean value is set to false.
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value, present = map_var[key]
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Analogously, receiving a value from a channel can be written as a tuple assignment.
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value, success = <chan_var
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If the receive operation would block, the boolean is set to false.
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This provides a mechanism to avoid blocking on a receive operation.
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Sending on a channel is a form of assignment. The left hand side expression
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must denote a channel pointer value.
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>chan_ptr = value
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In assignments, the type of the expression must match the type of the left-hand side.
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Communication
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----
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The syntax presented above covers communication operations. This
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section describes their form and function.
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Here the term "channel" means "variable of type *chan".
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A channel is created by allocating it:
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ch := new(chan int)
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An optional argument to new() specifies a buffer size for an
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asynchronous channel; if absent or zero, the channel is synchronous:
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sync_chan := new(chan int)
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buffered_chan := new(chan int, 10)
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The send operator is the binary operator "-<", which operates on
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a channel and a value (expression):
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ch -< 3
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In this form, the send operation is an (expression) statement that
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blocks until the send can proceed, at which point the value is
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transmitted on the channel.
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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. These two examples are equivalent:
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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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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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The receive uses the binary operator "<-", analogous to send but
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with the channel on the right:
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v1 <- ch
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As with send operations, in expression context this form may
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be used as a boolean and makes the receive non-blocking:
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ok := e <- ch;
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if ok { print "received", e } else { print "did not receive" }
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The receive operator may also be used as a prefix unary operator
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on a channel.
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<- ch
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The expression blocks until a value is available, which then can
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be assigned to a variable or used like any other expression:
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v1 := <-ch
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v2 = <-ch
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f(<-ch)
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If the receive expression does not save the value, the value is
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discarded:
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<- strobe // wait until clock pulse
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Finally, as a special case unique to receive, the forms
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e, ok := <-ch
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e, ok = <-ch
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allow the operation to declare and/or assign the received value and
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the boolean indicating success. These two forms are always
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non-blocking.
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Go statements
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----
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@ -1635,7 +1703,7 @@ function to complete.
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go Server()
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go func(ch chan> bool) { for { sleep(10); >ch = true; }} (c)
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go func(ch chan-< bool) { for { sleep(10); ch -< true; }} (c)
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Return statements
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@ -1773,9 +1841,10 @@ cases all referring to communication operations.
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SelectStat = "select" "{" { CommClause } "}" .
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CommClause = CommCase [ StatementList [ ";" ] ] .
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CommCase = ( "default" | ( "case" ( SendCase | RecvCase) ) ) ":" .
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SendCase = Send .
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RecvCase = [ identifier "=" ] RecvExpr .
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RecvExpr = "<" Expression .
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SendCase = SendExpr .
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RecvCase = RecvExpr .
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SendExpr = Expression "-<" Expression .
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RecvExpr = [ identifier ] "<-" Expression .
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The select statement evaluates all the channel (pointers) involved.
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If any of the channels can proceed, the corresponding communication
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@ -1793,9 +1862,9 @@ which single communication will execute.
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var c, c1, c2 *chan int;
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select {
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case i1 = <c1:
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case i1 <-c1:
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printf("received %d from c1\n", i1);
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case >c2 = i2:
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case c2 -< i2:
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printf("sent %d to c2\n", i2);
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default:
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printf("no communication\n");
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@ -1803,8 +1872,8 @@ which single communication will execute.
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for { // send random sequence of bits to c
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select {
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case >c = 0: // note: no statement, no fallthrough, no folding of cases
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case >c = 1:
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case c -< 0: // note: no statement, no fallthrough, no folding of cases
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case c -< 1:
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}
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}
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@ -1812,13 +1881,13 @@ which single communication will execute.
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var i int;
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var f float;
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select {
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case i = <ca:
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case i <- ca:
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printf("received int %d from ca\n", i);
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case f = <ca:
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case f <- ca:
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printf("received float %f from ca\n", f);
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}
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TODO: do we allow case i := <c: ?
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TODO: do we allow case i := <-c: ?
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TODO: need to precise about all the details but this is not the right doc for that
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