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@ -2691,39 +2691,44 @@ handler. In this particular instance, it means that the $\nondet$ |
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handler is part of the extent. |
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% |
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We ultimately want to return just a list of $\alpha$s to ensure every |
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process has run to completion. To achieve this, we define an auxiliary |
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function which accepts as list of process states as input and |
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recursively runs any suspended computations under the $\nondet$ |
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handler. |
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process has run to completion. To achieve this, we need a function |
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that keeps track of the state of every process, and in particular it |
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must run each $\Suspended$-tagged computation under the $\nondet$ |
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handler to produce another list of process state, which must be |
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handled recursively. |
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% |
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\[ |
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\bl |
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\runNext : \List~(\Pstate~\alpha~\{\Fork:\Bool;\varepsilon\}) \to \List~\alpha \eff \varepsilon\\ |
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\runNext~ps \defas |
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\concatMap\,(\lambda p. |
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\schedule : \List~(\Pstate~\alpha~\{\Fork:\Bool;\varepsilon\}) \to \List~\alpha \eff \varepsilon\\ |
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\schedule~ps \defas |
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\ba[t]{@{}l} |
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\Case\;p\;\{ |
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\ba[t]{@{}l@{~}c@{~}l} |
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\Done~res &\mapsto& [res]\\ |
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\Suspended~m &\mapsto& \runNext\,(\nondet~m) \})\,ps |
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\ea |
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\Let\;run \revto |
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\Rec\;sched\,\Record{ps;done}.\\ |
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\qquad\Case\;ps\;\{ |
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\ba[t]{@{}r@{~}c@{~}l} |
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\nil &\mapsto& done\\ |
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(\Done~res) \cons ps' &\mapsto& sched\,\Record{ps';res \cons done}\\ |
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(\Suspended~m) \cons ps' &\mapsto& sched\,\Record{ps' \concat (\nondet~m);\, done} \} |
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\ea\\ |
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\In\;run\,\Record{ps;\nil} |
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\ea |
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\el |
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\] |
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% |
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The function $\runNext$ inspects each process in the input list $ps$ |
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to see whether it done or paused. It translates a $\Done$-tagged value |
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into a singleton list, whereas if it is a $\Suspended$-tagged |
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computation, then the computation is run under the $\nondet$ handler |
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to in order to handle invocations of $\Fork$, and $\runNext$ is |
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recursively applied to the result. |
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% |
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The function |
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$\concatMap : (\alpha \to \List~\beta) \to \List~\alpha \to |
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\List~\beta$ maps a function that produces a list of $\beta$s from an |
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$\alpha$ over a list of $\alpha$s, thus producing a list of list of |
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$\beta$s. The resulting list is flatten by using list concatenation to |
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combine the list members. |
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The function $\schedule$ implements a process scheduler. It takes as |
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input a list of process states, where $\Suspended$-tagged computations |
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may perform the $\Fork$ operation. Locally it defines a recursive |
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function $sched$ which carries a list of active processes $ps$ and the |
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results of completed processes $done$. The function inspects the |
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process list $ps$ to test whether it is empty or nonempty. If it is |
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empty it returns the list of results $done$. Otherwise, if the head is |
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$\Done$-tagged value, then the function is recursively invoked with |
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tail of processes $ps'$ and the list $done$ augmented with the value |
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$res$. If the head is a $\Suspended$-tagged computation $m$, then |
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$sched$ is recursively invoked with the process list $ps'$ |
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concatenated with the result of running $m$ under the $\nondet$ |
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handler. |
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% |
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Using the above machinery, we can define a function which adds |
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time-sharing capabilities to the system. |
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@ -2731,13 +2736,13 @@ time-sharing capabilities to the system. |
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\[ |
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\bl |
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\timeshare : (\UnitType \to \alpha \eff \Proc) \to \List~\alpha\\ |
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\timeshare~m \defas \runNext\,[\Suspended\,(\lambda\Unit.\reifyP~m)] |
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\timeshare~m \defas \schedule\,[\Suspended\,(\lambda\Unit.\reifyP~m)] |
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\el |
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\] |
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% |
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The function $\timeshare$ handles the invocations of $\Fork$ and |
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$\Interrupt$ in some computation $m$ by starting it in suspended state |
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under the $\reifyP$ handler. The $\runNext$ actually starts the |
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under the $\reifyP$ handler. The $\schedule$ actually starts the |
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computation, when it runs the computation under the $\nondet$ handler. |
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% |
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@ -2780,7 +2785,7 @@ invocation of $\Interrupt$ along side $\Write$. |
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\[ |
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\bl |
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\echo' : \String \to \UnitType \eff \{\Interrupt : \UnitType \opto \UnitType;\Write : \Record{\UFD;\String} \opto \UnitType\}\\ |
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\echo'~cs \defas \Do\;\Interrupt~\Unit;\,\Do\;\Write\,\Record{\stdout;cs} |
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\echo'~cs \defas \Do\;\Interrupt\,\Unit;\,\Do\;\Write\,\Record{\stdout;cs} |
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\el |
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\] |
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% |
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@ -2790,6 +2795,79 @@ backwards compatible, since the original definition of $\echo$ can be |
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used in a context that prohibits occurrences of $\Interrupt$. Clearly, |
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this alternative definition cannot be applied in such a context. |
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There is backwards-compatible way to bundle the two operations |
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together. We can implement a handler that \emph{intercepts} |
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invocations of $\Write$ and handles them by performing an interrupt |
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and, crucially, reperforming the intercepted write operation. |
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% |
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\[ |
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\bl |
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\dec{interruptWrite} : |
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\ba[t]{@{~}l@{~}l} |
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&(\UnitType \to \alpha \eff \{\Interrupt : \UnitType \opto \UnitType;\Write : \Record{\UFD;\String} \opto \UnitType\})\\ |
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\to& \alpha \eff \{\Interrupt : \UnitType \opto \UnitType;\Write : \Record{\UFD;\String} \opto \UnitType\} |
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\ea\\ |
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\dec{interruptWrite}~m \defas |
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\ba[t]{@{~}l} |
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\Handle\;m~\Unit\;\With\\ |
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~\ba{@{~}l@{~}c@{~}l} |
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\Return\;res &\mapsto& res\\ |
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\OpCase{\Write}{\Record{fd;cs}}{resume} &\mapsto& |
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\ba[t]{@{}l} |
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\interrupt\,\Unit;\\ |
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resume\,(\Do\;\Write~\Record{fd;cs}) |
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\ea |
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\ea |
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\ea |
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\el |
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\] |
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% |
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This handler is not `self-contained' as the other handlers we have |
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defined previously. It gives in some sense a `partial' interpretation |
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of $\Write$ as it leaves open the semantics of $\Interrupt$ and |
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$\Write$, i.e. this handler must be run in a suitable context of other |
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handlers. |
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Let us plug this handler into the previous example to see what |
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happens. |
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% |
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\[ |
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\ba{@{~}l@{~}l} |
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&\bl |
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\basicIO\,(\lambda\Unit.\\ |
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\qquad\timeshare\,(\lambda\Unit.\\ |
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\qquad\qquad\dec{interruptWrite}\,(\lambda\Unit.\\ |
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\qquad\qquad\qquad\sessionmgr\,\Record{\Root;\lambda\Unit.\\ |
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\qquad\qquad\qquad\qquad\status\,(\lambda\Unit. |
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\ba[t]{@{}l} |
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\If\;\fork~\Unit\;\Then\; |
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\su~\Alice;\, |
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\quoteRitchie~\Unit\\ |
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\Else\; |
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\su~\Bob;\, |
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\quoteHamlet~\Unit)}))) |
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\ea |
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\el \smallskip\\ |
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\reducesto^+& |
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\bl |
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\Record{ |
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\ba[t]{@{}l} |
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[0, 0]; |
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\texttt{"}\ba[t]{@{}l} |
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\texttt{UNIX is basically To be, or not to be,\nl{}}\\ |
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\texttt{a simple operating system, that is the question:\nl{}}\\ |
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\texttt{but Whether 'tis nobler in the mind to suffer\nl{}}\\ |
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\texttt{you have to be a genius to understand the simplicity.\nl{}"}} |
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\ea |
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\ea\\ |
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: \Record{\List~\Int; \UFile} |
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\el |
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\ea |
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\] |
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% |
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Evidently, each write operation has been interleaved, resulting in a |
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mishmash poetry of Shakespeare and \UNIX{}. |
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\subsection{State: file i/o} |
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\label{sec:tiny-unix-io} |
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