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remove atxt support
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Steinway Wu
@@ -1,215 +0,0 @@
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%{
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#include "./../ATEXT/atextfun.hats"
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%}
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
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"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<meta http-equiv="content-type" content="text/html; charset=UTF-8" />
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<title>EFFECTIVATS-DiningPhil2</title>
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#patscode_style()
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</head>
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<body>
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<h1>
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Effective ATS: Dining Philosophers
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</h1>
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In this article, I present an implementation of a slight variant of the
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famous problem of 5-Dining-Philosophers by Dijkstra that makes simple but
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convincing use of linear types.
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<h2>
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The Original Problem
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</h2>
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There are five philosophers sitting around a table and there are also 5
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forks placed on the table such that each fork is located between the left
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hand of a philosopher and the right hand of another philosopher. Each
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philosopher does the following routine repeatedly: thinking and dining. In
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order to dine, a philosopher needs to first acquire two forks: one located
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on his left-hand side and the other on his right-hand side. After
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finishing dining, a philosopher puts the two acquired forks onto the table:
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one on his left-hand side and the other on his right-hand side.
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<h2>
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A Variant of the Original Problem
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</h2>
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The following twist is added to the original version:
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<p>
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After a fork is used, it becomes a "dirty" fork and needs to be put in a
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tray for dirty forks. There is a cleaner who cleans dirty forks and then
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puts them back on the table.
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<h2>
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Channels for Communication
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</h2>
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A channel is just a shared queue of fixed capacity. The following two
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functions are for inserting an element into and taking an element out of a
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given channel:
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<pre
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class="patsyntax">
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#pats2xhtml_sats("\
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fun{a:vt0p} channel_insert (channel (a), a): void
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fun{a:vt0p} channel_takeout (chan: channel (a)): (a)
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")</pre>
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If [channel_insert] is called on a channel that is full, then the caller is
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blocked until an element is taken out of the channel. If [channel_takeout]
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is called on a channel that is empty, then the caller is blocked until an
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element is inserted into the channel.
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<h2>
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A Channel for Each Fork
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</h2>
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Forks are resources given a linear type. Each fork is initially stored in a
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channel, which can be obtained by calling the following function:
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<pre
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class="patsyntax">
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#pats2xhtml_sats("\
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fun fork_changet (n: nphil): channel(fork)
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")</pre>
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where the type [nphil] is defined to be [natLt(5)] (for natural numbers
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less than 5). The channels for storing forks are chosen to be of capacity
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2. The reason that channels of capacity 2 are chosen to store at most one
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element (in each of them) is to guarantee that these channels can never be
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full (so that there is no attempt made to send signals to awake callers
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supposedly being blocked due to channels being full).
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<h2>
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A Channel for the Fork Tray
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</h2>
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A tray for storing "dirty" forks is also a channel, which can be obtained
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by calling the following function:
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<pre
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class="patsyntax">
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#pats2xhtml_sats("\
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fun forktray_changet ((*void*)): channel(fork)
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")</pre>
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The capacity chosen for the channel is 6 (instead of 5) so that it can
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never become full (as there are only 5 forks in total).
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<h2>
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Philosopher Loop
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</h2>
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Each philosopher is implemented as a loop:
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<pre
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class="patsyntax">
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#pats2xhtml_dats('\
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implement
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phil_loop (n) = let
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//
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val () = phil_think (n)
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//
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val nl = phil_left (n) // = n
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val nr = phil_right (n) // = (n+1) % 5
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//
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val ch_lfork = fork_changet (nl)
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val ch_rfork = fork_changet (nr)
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//
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val lf = channel_takeout (ch_lfork)
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val () = println! ("phil_loop(", n, ") picks left fork")
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//
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val () = randsleep (2) // sleep up to 2 seconds
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//
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val rf = channel_takeout (ch_rfork)
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val () = println! ("phil_loop(", n, ") picks right fork")
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//
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val () = phil_dine (n, lf, rf)
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//
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val ch_forktray = forktray_changet ()
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val () = channel_insert (ch_forktray, lf) // left fork to dirty tray
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val () = channel_insert (ch_forktray, rf) // right fork to dirty tray
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//
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in
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phil_loop (n)
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end // end of [phil_loop]
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')</pre>
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It should be straighforward to follow the code for [phil_loop].
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<h2>
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Fork Cleaner Loop
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</h2>
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A cleaner is implemented as a loop:
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<pre
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class="patsyntax">
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#pats2xhtml_dats('\
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implement
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cleaner_loop () = let
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//
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val ch = forktray_changet ()
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val f0 = channel_takeout (ch) // [f0] is dirty
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//
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val () = cleaner_wash (f0) // washes dirty [f0]
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val () = cleaner_return (f0) // puts back cleaned [f0]
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//
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in
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cleaner_loop ()
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end // end of [cleaner_loop]
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')</pre>
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The function [cleaner_return] first finds out the number of a given fork
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and then uses the number to locate the channel for storing the fork. Its
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actual implementation is given as follows:
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<pre
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class="patsyntax">
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#pats2xhtml_dats('\
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implement
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cleaner_return (f) =
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{
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val n = fork_get_num (f)
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val ch = fork_changet (n)
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val () = channel_insert (ch, f)
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}
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')</pre>
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It should now be straighforward to follow the code for [cleaner_loop].
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<h2>
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Testing
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</h2>
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The entire code of this implementation is stored in the following files:
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<pre>
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DiningPhil2.sats
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DiningPhil2.dats
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DiningPhil2_fork.dats
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DiningPhil2_thread.dats
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</pre>
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There is also a Makefile available for compiling the ATS source code into
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an excutable for testing. One should be able to encounter a deadlock after
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running the simulation for a while.
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<hr size="2">
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This article is written by <a href="http://www.cs.bu.edu/~hwxi/">Hongwei Xi</a>.
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</body>
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</html>
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%{
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implement main () = fprint_filsub (stdout_ref, "main_atxt.txt")
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%}
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