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author | Martin Odersky <odersky@gmail.com> | 2004-01-09 14:33:04 +0000 |
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committer | Martin Odersky <odersky@gmail.com> | 2004-01-09 14:33:04 +0000 |
commit | 253a192ede3e994d6c832c124c7c02fb551d1006 (patch) | |
tree | c3fc90ddde9960bb4271ca758c58f92754dcc37b /doc/reference/ScalaByExample.tex | |
parent | 57bf1138b8fdaafc9c4ea1e883aba222ec8117cc (diff) | |
download | scala-253a192ede3e994d6c832c124c7c02fb551d1006.tar.gz scala-253a192ede3e994d6c832c124c7c02fb551d1006.tar.bz2 scala-253a192ede3e994d6c832c124c7c02fb551d1006.zip |
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Diffstat (limited to 'doc/reference/ScalaByExample.tex')
-rw-r--r-- | doc/reference/ScalaByExample.tex | 102 |
1 files changed, 59 insertions, 43 deletions
diff --git a/doc/reference/ScalaByExample.tex b/doc/reference/ScalaByExample.tex index 4885f7457f..850cc53eec 100644 --- a/doc/reference/ScalaByExample.tex +++ b/doc/reference/ScalaByExample.tex @@ -6381,14 +6381,19 @@ class ReadersWriters { \section{Asynchronous Channels} A fundamental way of interprocess communication is the asynchronous -channel. Its implementation makes use the following class for linked +channel. Its implementation makes use the following simple class for linked lists: \begin{lstlisting} -package scala.collection.mutable; -class LinkedList[A](head: A, tail: LinkedList[A]) - extends SingleLinkedList[A, LinkedList[A]] { - elem = head; - next = tail; +class LinkedList[a] { + var elem: a = _; + var next: LinkedList[a] = null; + def insert(elem: a): LinkedList[a] = { + val nx = new LinkedList[a]; + nx.elem = a; + nx.next = next; + next = nx; + next + } } \end{lstlisting} To facilitate insertion and deletion of elements into linked lists, @@ -6402,14 +6407,15 @@ wish to read from an empty channel, register their presence by incrementing the \code{nreaders} field and waiting to be notified. \begin{lstlisting} package scala.concurrent; + class Channel[a] with Monitor { - var dummy: a = _; - private var written = new LinkedList[a](dummy, null); + private var written = new LinkedList[a]; private var lastWritten = written; private var nreaders = 0; def write(x: a) = synchronized { - lastWritten.next = new LinkedList(x, null); + lastWritten.elem = x; + lastWritten.next = new LinkedList[a]; lastWritten = lastWritten.next; if (nreaders > 0) notify(); } @@ -6565,26 +6571,26 @@ class MailBox { def receiveWithin[a](msec: long)(f: PartialFunction[Any, a]): a; } \end{lstlisting} -The state of a message space consists of a multi-set of messages. -Messages are added to the space using the \code{send} method. Messages +The state of a mailbox consists of a multi-set of messages. +Messages are added to the mailbox the \code{send} method. Messages are removed using the \code{receive} method, which is passed a message processor \code{f} as argument, which is a partial function from messages to some arbitrary result type. Typically, this function is implemented as a pattern matching expression. The \code{receive} -method blocks until there is a message in the space for which its +method blocks until there is a message in the mailbox for which its message processor is defined. The matching message is then removed -from the space and the blocked thread is restarted by applying the +from the mailbox and the blocked thread is restarted by applying the message processor to the message. Both sent messages and receivers are ordered in time. A receiver $r$ is applied to a matching message $m$ only if there is no other (message, receiver) pair which precedes $(m, r)$ in the partial ordering on pairs that orders each component in time. -As a simple example of how message spaces are used, consider a +As a simple example of how mailboxes are used, consider a one-place buffer: \begin{lstlisting} class OnePlaceBuffer { - private val m = new MessageSpace; // An internal message space + private val m = new MailBox; // An internal milbox private case class Empty, Full(x: int); // Types of messages we deal with m send Empty; // Initialization def write(x: int): unit = @@ -6593,9 +6599,9 @@ class OnePlaceBuffer { m receive { case Full(x) => m send Empty ; x } } \end{lstlisting} -Here's how the message space class can be implemented: +Here's how the mailbox class can be implemented: \begin{lstlisting} -class MessageSpace { +class MailBox with Monitor { private abstract class Receiver extends Signal { def isDefined(msg: Any): boolean; var msg = null; @@ -6609,12 +6615,12 @@ receiver thread. When the receiver thread is woken up, the message it needs to be applied to is stored in the \code{msg} variable of \code{Receiver}. \begin{lstlisting} - private val sent = new LinkedList[Any](null) ; + private val sent = new LinkedList[Any]; private var lastSent = sent; - private var receivers = new LinkedList[Receiver](null); + private val receivers = new LinkedList[Receiver]; private var lastReceiver = receivers; \end{lstlisting} -The message space class maintains two linked lists, +The mailbox class maintains two linked lists, one for sent but unconsumed messages, the other for waiting receivers. \begin{lstlisting} def send(msg: Any): unit = synchronized { @@ -6625,30 +6631,28 @@ one for sent but unconsumed messages, the other for waiting receivers. if (r1 != null) { r.next = r1.next; r1.elem.msg = msg; r1.elem.notify; } else { - l = new LinkedList(msg); lastSent.next = l; lastSent = l; + lastSent = lastSent.insert(msg); } } \end{lstlisting} The \code{send} method first checks whether a waiting receiver is - applicable to the sent message. If yes, the receiver is notified. Otherwise, the message is appended to the linked list of sent messages. \begin{lstlisting} def receive[a](f: PartialFunction[Any, a]): a = { val msg: Any = synchronized { var s = sent, s1 = s.next; - while (s1 != null && !f.isDefined(s1.elem)) { + while (s1 != null && !f.isDefinedAt(s1.elem)) { s = s1; s1 = s1.next } if (s1 != null) { s.next = s1.next; s1.elem } else { - val r = new LinkedList( - new Receiver { - def isDefined(msg: Any) = f.isDefined(msg); - }); - lastReceiver.next = r; lastReceiver = r; - r.elem.wait; + val r = lastReceiver.insert(new Receiver { + def isDefined(msg: Any) = f.isDefinedAt(msg); + }); + lastReceiver = r; + r.elem.wait(); r.elem.msg } } @@ -6663,7 +6667,7 @@ and linked into the \code{receivers} list, and the thread waits for a notification on this receiver. Once the thread is woken up again, it continues by applying \code{f} to the message that was stored in the receiver. -The message space class also offers a method \code{receiveWithin} +The mailbox class also offers a method \code{receiveWithin} which blocks for only a specified maximal amount of time. If no message is received within the specified time interval (given in milliseconds), the message processor argument $f$ will be unblocked @@ -6673,18 +6677,16 @@ with the special \code{TIMEOUT} message. The implementation of def receiveWithin[a](msec: long)(f: PartialFunction[Any, a]): a = { val msg: Any = synchronized { var s = sent, s1 = s.next; - while (s1 != null && !f.isDefined(s1.elem)) { + while (s1 != null && !f.isDefinedAt(s1.elem)) { s = s1; s1 = s1.next ; } if (s1 != null) { s.next = s1.next; s1.elem } else { - val r = new LinkedList( - new Receiver { - def isDefined(msg: Any) = f.isDefined(msg); - } - ) - lastReceiver.next = r; lastReceiver = r; + val r = lastReceiver.insert(new Receiver { + def isDefined(msg: Any) = f.isDefinedAt(msg); + }); + lastReceiver = r; r.elem.wait(msec); if (r.elem.msg == null) r.elem.msg = TIMEOUT; r.elem.msg @@ -6692,7 +6694,7 @@ with the special \code{TIMEOUT} message. The implementation of } f(msg) } -} // end MessageSpace +} // end MailBox \end{lstlisting} The only differences are the timed call to \code{wait}, and the statement following it. @@ -6704,10 +6706,25 @@ Chapter~\ref{sec:ex-auction} sketched as a program example the implementation of an electronic auction service. This service was based on high-level actor processes, that work by inspecting messages in their mailbox using pattern matching. An actor is simply a thread -whose communication primitives are those of a message space. -Actors are therefore defined by a mixin composition of threads and message spaces. +whose communication primitives are those of a mailbox. Actors are +defined as an extension of Java's standard \code{Thread} class which +forwards all communication calls to an internal mailbox. \begin{lstlisting} -abstract class Actor extends Thread with MessageSpace; +abstract class Actor extends Thread { + + type Message = Any; + + private val mb = new MailBox; + + def send(msg: Message): unit = + mb.send(msg); + + protected def receive[a](f: PartialFunction[Message, a]): a = + mb.receive(f); + + protected def receiveWithin[a](msec: long)(f: PartialFunction[Message, a]): a = + mb.receiveWithin(msec)(f); +} \end{lstlisting} \comment{ @@ -7236,8 +7253,7 @@ The function \code{sumLeaves} sums up all the integer values in the leaves of a given tree \code{t}. It is is implemented by calling the \code{match} method of \code{t} with a {\em choice expression} as argument (\code{match} is a predefined method in class \code{Object}). -The choice expression consists of two cases which both -relate a pattern with an expression. The pattern of the first case, +The choice expression consists of two cases which botrelate a pattern with an expression. The pattern of the first case, \code{Branch(l, r)} matches all instances of class \code{Branch} and binds the {\em pattern variables} \code{l} and \code{r} to the constructor arguments, i.e.\ the left and right subtrees of the |