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@ -1663,7 +1663,7 @@ The term `pure' is heavily overloaded in the programming literature. |
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% back to 2016 when Richard Eisenberg asked me about how we do effect |
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% inference in Links.} |
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\chapter{Unary handlers} |
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\chapter{Programming with control via effect handlers} |
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\label{ch:unary-handlers} |
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% |
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In this chapter we study various flavours of unary effect |
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@ -2507,7 +2507,66 @@ The following handler implements this behaviour. |
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\el |
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\] |
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% |
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\dhil{This is an instance of non-blind backtracking} |
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The $\Return$-case returns a singleton list containing a result of |
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running $m$. |
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% |
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The $\Fork$-case invokes the provided resumption $resume$ twice. Each |
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invocation of $resume$ effectively copies $m$ and runs each copy to |
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completion. Each copy returns through the $\Return$-case, hence each |
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invocation of $resume$ returns a list of the possible results obtained |
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by interpreting $\Fork$ first as $\True$ and subsequently as |
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$\False$. The results are joined by list concatenation ($\concat$). |
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% |
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Thus the handler returns a list of all the possible results of $m$. |
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% |
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In fact, this handler is exactly the handler for nondeterministic |
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choice, and it satisfies the standard semi-lattice |
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equations~\cite{PlotkinP09,PlotkinP13}. |
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% \dhil{This is an instance of non-blind backtracking~\cite{FriedmanHK84}} |
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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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\quad\nondet\,(\lambda\Unit.\\ |
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\quad\quad\sessionmgr\,\Record{\Root;\lambda\Unit.\\ |
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\quad\quad\quad\status\,(\lambda\Unit.\\ |
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\quad\quad\quad\quad\ba[t]{@{}l} |
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\If\;\fork~\Unit\;\Then\\ |
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\quad\bl |
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\su~\Alice;\\ |
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\echo~\strlit{UNIX is basically a simple operating system,\nl};\\ |
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\echo~\strlit{but you have to be a genius }\\ |
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\echo~\strlit{to understand the simplicity.\nl} |
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\el\\ |
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\Else\\ |
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\quad\bl |
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\su~\Bob;\\ |
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\echo~\strlit{To be, or not to be, that is the question:\nl};\\ |
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\echo~\strlit{Whether 'tis nobler in the mind to suffer\nl};\\ |
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\echo~\strlit{The slings and arrows of outrageous fortune,\nl};\\ |
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\echo~\strlit{Or to take arms against a sea of troubles\nl};\\ |
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\echo~\strlit{And by opposing end them.\nl} )})) |
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\el |
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\ea |
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\el \smallskip\\ |
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\reducesto^+& |
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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 a simple operating system,}\\ |
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\texttt{but you have to be a genius to understand the simplicity.}\\ |
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\texttt{To be, or not to be, that is the question:}\\ |
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\texttt{Whether 'tis nobler in the mind to suffer}\\ |
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\texttt{The slings and arrows of outrageous fortune,}\\ |
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\texttt{Or to take arms against a sea of troubles}\\ |
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\texttt{And by opposing end them."}} : \Record{\List~\Int; \UFile} |
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\ea |
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\ea |
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\ea |
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\] |
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% |
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% |
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\[ |
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\bl |
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@ -2521,7 +2580,7 @@ The following handler implements this behaviour. |
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\Pstate~\alpha~\varepsilon \defas \forall \theta. |
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\ba[t]{@{}l@{}l} |
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[&\Done:\alpha;\\ |
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&\Suspended:\UnitType \to \Pstate~\alpha~\varepsilon \eff \{\Interrupt:\theta';\varepsilon\} ] |
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&\Suspended:\UnitType \to \Pstate~\alpha~\varepsilon \eff \{\Interrupt:\theta;\varepsilon\} ] |
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\ea |
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\] |
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% |
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