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First stab at streamlining the notation for the higher-order uncurried CPS translation for deep handlers. Also first stab at stating its correctness.

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Daniel Hillerström 5 years ago
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  1. 21
      macros.tex
  2. 458
      thesis.tex

21
macros.tex
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@ -80,6 +80,7 @@
\newcommand{\FV}{\ensuremath{\mathrm{FV}}} \newcommand{\FV}{\ensuremath{\mathrm{FV}}}
\newcommand{\reducesto}[0]{\ensuremath{\leadsto}} \newcommand{\reducesto}[0]{\ensuremath{\leadsto}}
\newcommand{\areducesto}{\ensuremath{\reducesto_{\textrm{a}}}}
\newcommand{\stepsto}[0]{\ensuremath{\longrightarrow}} \newcommand{\stepsto}[0]{\ensuremath{\longrightarrow}}
\newcommand{\EC}{\ensuremath{\mathcal{E}}} \newcommand{\EC}{\ensuremath{\mathcal{E}}}
@ -96,6 +97,8 @@
%\newcommand{\hex}{H^{\mathrm{ex}}} %\newcommand{\hex}{H^{\mathrm{ex}}}
\newcommand{\hell}{H^{\ell}} \newcommand{\hell}{H^{\ell}}
\newcommand{\depth}{\delta}
\newcommand{\alertbox}[2]{{\par\noindent\small\color{red} \framebox{\parbox{\dimexpr\linewidth-2\fboxsep-2\fboxrule}{\textbf{#1:} #2}}}} \newcommand{\alertbox}[2]{{\par\noindent\small\color{red} \framebox{\parbox{\dimexpr\linewidth-2\fboxsep-2\fboxrule}{\textbf{#1:} #2}}}}
\newcommand{\todo}[1]{\alertbox{TODO}{#1}} \newcommand{\todo}[1]{\alertbox{TODO}{#1}}
\newcommand{\dhil}[1]{\alertbox{Daniel}{#1}} \newcommand{\dhil}[1]{\alertbox{Daniel}{#1}}
@ -225,6 +228,9 @@
\newcommand{\hforward}{M_{\textrm{forward}}} \newcommand{\hforward}{M_{\textrm{forward}}}
\newcommand{\hid}{V_{\textrm{id}}} \newcommand{\hid}{V_{\textrm{id}}}
% continuation application
\newcommand{\kapp}{\keyw{app}}
%%% Continuation names %%% Continuation names
%%% The following set of macros are a bit more consistent with those %%% The following set of macros are a bit more consistent with those
%%% currently used by the abstract machine, and don't use the plural %%% currently used by the abstract machine, and don't use the plural
@ -246,9 +252,12 @@
\newcommand{\shk}{\kappa} % handler continuations \newcommand{\shk}{\kappa} % handler continuations
\newcommand{\sLet}{\static{\Let}} \newcommand{\sLet}{\static{\Let}}
\newcommand{\sIn}{\static{\In}} \newcommand{\sIn}{\static{\In}}
\newcommand{\sk}{\kappa}
\newcommand{\sks}{\mathit{\kappa s}}
\newcommand{\sh}{\eta}
% \newcommand{\sk}{\kappa}
% \newcommand{\sks}{\mathit{\kappa s}}
\newcommand{\sks}{\kappa}
% \newcommand{\sh}{\eta}
\newcommand{\sk}{\theta}
\newcommand{\sh}{\chi}
\newcommand{\sx}{\varepsilon} \newcommand{\sx}{\varepsilon}
\newcommand{\sP}{\mathcal{P}} \newcommand{\sP}{\mathcal{P}}
@ -256,3 +265,9 @@
\newcommand{\Vmap}{\keyw{vmap}} \newcommand{\Vmap}{\keyw{vmap}}
\newcommand{\Vmapsnd}{\keyw{vmapsnd}} \newcommand{\Vmapsnd}{\keyw{vmapsnd}}
\newcommand{\Fun}{\keyw{fun}} \newcommand{\Fun}{\keyw{fun}}
% Canonical variables for handler components
\newcommand{\vhret}{h^{\mathrm{ret}}}
\newcommand{\vhops}{h^{\mathrm{ops}}}
\newcommand{\svhret}{\chi^{\mathrm{ret}}}
\newcommand{\svhops}{\chi^{\mathrm{ops}}}

458
thesis.tex
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@ -2006,13 +2006,14 @@ consume stack space.
The syntax, semantics, and syntactic sugar for the target calculus The syntax, semantics, and syntactic sugar for the target calculus
$\UCalc$ is given in Figure~\ref{fig:cps-cbv-target}. The calculus $\UCalc$ is given in Figure~\ref{fig:cps-cbv-target}. The calculus
largely amounts to an untyped variation of $\BCalcRec$, specifically
largely amounts to an untyped variation of $\BCalc$, specifically
we retain the syntactic distinction between values ($V$) and we retain the syntactic distinction between values ($V$) and
computations ($M$). computations ($M$).
% %
The values ($V$) comprise lambda abstractions ($\lambda x.M$), The values ($V$) comprise lambda abstractions ($\lambda x.M$),
recursive functions ($\Rec\,g\,x.M$), empty tuples ($\Record{}$),
pairs ($\Record{V,W}$), and first-class labels ($\ell$).
% recursive functions ($\Rec\,g\,x.M$),
empty tuples ($\Record{}$), pairs ($\Record{V,W}$), and first-class
labels ($\ell$).
% %
Computations ($M$) comprise values ($V$), applications ($M~V$), pair Computations ($M$) comprise values ($V$), applications ($M~V$), pair
elimination ($\Let\; \Record{x, y} = V \;\In\; N$), label elimination elimination ($\Let\; \Record{x, y} = V \;\In\; N$), label elimination
@ -2038,7 +2039,8 @@ term) to cope with the case of pattern matching failure.
\flushleft \flushleft
\textbf{Syntax} \textbf{Syntax}
\begin{syntax} \begin{syntax}
\slab{Values} &U, V, W \in \UValCat &::= & x \mid \lambda x.M \mid \Rec\,g\,x.M \mid \Record{} \mid \Record{V, W} \mid \ell
\slab{Values} &U, V, W \in \UValCat &::= & x \mid \lambda x.M \mid % \Rec\,g\,x.M
\mid \Record{} \mid \Record{V, W} \mid \ell
\smallskip \\ \smallskip \\
\slab{Computations} &M,N \in \UCompCat &::= & V \mid M\,W \mid \Let\; \Record{x,y} = V \; \In \; N\\ \slab{Computations} &M,N \in \UCompCat &::= & V \mid M\,W \mid \Let\; \Record{x,y} = V \; \In \; N\\
& &\mid& \Case\; V\, \{\ell \mapsto M; y \mapsto N\} \mid \Absurd\,V & &\mid& \Case\; V\, \{\ell \mapsto M; y \mapsto N\} \mid \Absurd\,V
@ -2049,7 +2051,7 @@ term) to cope with the case of pattern matching failure.
\textbf{Reductions} \textbf{Reductions}
\begin{reductions} \begin{reductions}
\usemlab{App} & (\lambda x . \, M) V &\reducesto& M[V/x] \\ \usemlab{App} & (\lambda x . \, M) V &\reducesto& M[V/x] \\
\usemlab{Rec} & (\Rec\,g\,x.M) V &\reducesto& M[\Rec\,g\,x.M/g,V/x]\\
% \usemlab{Rec} & (\Rec\,g\,x.M) V &\reducesto& M[\Rec\,g\,x.M/g,V/x]\\
\usemlab{Split} & \Let \; \Record{x,y} = \Record{V,W} \; \In \; N &\reducesto& N[V/x,W/y] \\ \usemlab{Split} & \Let \; \Record{x,y} = \Record{V,W} \; \In \; N &\reducesto& N[V/x,W/y] \\
\usemlab{Case_1} & \usemlab{Case_1} &
\Case \; \ell \; \{ \ell \mapsto M; y \mapsto N\} &\reducesto& M \\ \Case \; \ell \; \{ \ell \mapsto M; y \mapsto N\} &\reducesto& M \\
@ -2088,7 +2090,7 @@ term) to cope with the case of pattern matching failure.
\section{CPS transform for fine-grain call-by-value} \section{CPS transform for fine-grain call-by-value}
\label{sec:cps-cbv} \label{sec:cps-cbv}
We start by giving a CPS translation of $\BCalcRec$ in
We start by giving a CPS translation of $\BCalc$ in
Figure~\ref{fig:cps-cbv}. Fine-grain call-by-value admits a Figure~\ref{fig:cps-cbv}. Fine-grain call-by-value admits a
particularly simple CPS translation due to the separation of values particularly simple CPS translation due to the separation of values
and computations. All constructs from the source language are and computations. All constructs from the source language are
@ -2114,7 +2116,7 @@ translate to value terms in the target.
\cps{x} &=& x \\ \cps{x} &=& x \\
\cps{\lambda x.M} &=& \lambda x.\cps{M} \\ \cps{\lambda x.M} &=& \lambda x.\cps{M} \\
\cps{\Lambda \alpha.M} &=& \lambda k.\cps{M}~k \\ \cps{\Lambda \alpha.M} &=& \lambda k.\cps{M}~k \\
\cps{\Rec\,g\,x.M} &=& \Rec\,g\,x.\cps{M}\\
% \cps{\Rec\,g\,x.M} &=& \Rec\,g\,x.\cps{M}\\
\cps{\Record{}} &=& \Record{} \\ \cps{\Record{}} &=& \Record{} \\
\cps{\Record{\ell = V; W}} &=& \Record{\ell = \cps{V}; \cps{W}} \\ \cps{\Record{\ell = V; W}} &=& \Record{\ell = \cps{V}; \cps{W}} \\
\cps{\ell~V} &=& \ell~\cps{V} \\ \cps{\ell~V} &=& \ell~\cps{V} \\
@ -2137,7 +2139,7 @@ translate to value terms in the target.
\end{eqs} \end{eqs}
\el \el
\] \]
\caption{First-order CPS translation of fine-grain call-by-value.}
\caption{First-order CPS translation of $\BCalc$.}
\label{fig:cps-cbv} \label{fig:cps-cbv}
\end{figure} \end{figure}
@ -2523,8 +2525,8 @@ resumptions.
y \mapsto \hforward((y,p,rs),ks) \} \\ y \mapsto \hforward((y,p,rs),ks) \} \\
\el \\ \el \\
\el \\ \el \\
\hforward((y,p,r),ks)
&\defas&\Let\; (k' \cons h' \cons ks') = ks \;\In\; h'\,\Record{y,\Record{p,h'::k'::r}} \,ks'
\hforward((y,p,rs),ks)
&\defas&\Let\; (k' \cons h' \cons ks') = ks \;\In\; h'\,\Record{y,\Record{p,h' \cons k' \cons rs}} \,ks'
\end{equations} \end{equations}
% %
The translation of $\Do$ constructs an initial resumption stack, The translation of $\Do$ constructs an initial resumption stack,
@ -2540,19 +2542,19 @@ resumption stack with the current continuation pair.
% %
\begin{figure} \begin{figure}
% %
\textbf{Static patterns and static lists}
%
\begin{syntax}
\slab{Static patterns} &\sP& ::= & \sk \scons \sP \mid \sks \\
\slab{Static lists} &\VS& ::= & V \scons \VS \mid \reflect V \\
\end{syntax}
%
\textbf{Reification}
%
\begin{equations}
\reify (V \scons \VS) &\defas& V \dcons \reify \VS \\
\reify \reflect V &\defas& V \\
\end{equations}
% \textbf{Static patterns and static lists}
% %
% \begin{syntax}
% \slab{Static patterns} &\sP& ::= & \sk \scons \sP \mid \sks \\
% \slab{Static lists} &\VS& ::= & V \scons \VS \mid \reflect V \\
% \end{syntax}
% %
% \textbf{Reification}
% %
% \begin{equations}
% \reify (V \scons \VS) &\defas& V \dcons \reify \VS \\
% \reify \reflect V &\defas& V \\
% \end{equations}
% %
\textbf{Values} \textbf{Values}
% %
@ -2582,8 +2584,7 @@ resumption stack with the current continuation pair.
\cps{\Let~x \revto M~\In~N} &\defas& \slam \sk \scons \sks.\cps{M} \sapp ((\dlam x\,ks.\cps{N} \sapp (\sk \scons \reflect ks)) \scons \sks) \\ \cps{\Let~x \revto M~\In~N} &\defas& \slam \sk \scons \sks.\cps{M} \sapp ((\dlam x\,ks.\cps{N} \sapp (\sk \scons \reflect ks)) \scons \sks) \\
\cps{\Do\;\ell\;V} &\defas& \slam \sk \scons \sh \scons \sks.\sh \dapp \cps{\Do\;\ell\;V} &\defas& \slam \sk \scons \sh \scons \sks.\sh \dapp
(\ell~\Record{\cps{V}, \sh \dcons \sk \dcons \dnil}) \dapp \reify \sks \\ (\ell~\Record{\cps{V}, \sh \dcons \sk \dcons \dnil}) \dapp \reify \sks \\
\cps{\Handle \; M \; \With \; H} &\defas& \slam \sks . \cps{M} \sapp (\cps{\hret} \scons \cps{\hops} \scons \sks),~\text{where}
\smallskip \\
\cps{\Handle \; M \; \With \; H} &\defas& \slam \sks . \cps{M} \sapp (\cps{\hret} \scons \cps{\hops} \scons \sks)
% %
\end{equations} \end{equations}
% %
@ -2591,92 +2592,250 @@ resumption stack with the current continuation pair.
% %
\begin{equations} \begin{equations}
\cps{-} &:& \HandlerCat \to \UCompCat\\ \cps{-} &:& \HandlerCat \to \UCompCat\\
\cps{\{\Return \; x \mapsto N\}} &\defas& \dlam x\, ks.\dLet\; (h \dcons ks') = ks \;\dIn\; \cps{N} \sapp \reflect ks'
\cps{\{\Return \; x \mapsto N\}} &\defas& \dlam x\, ks.\Let\; (h \dcons ks') = ks \;\In\; \cps{N} \sapp \reflect ks'
\\ \\
% \cps{\{(\ell \; p \; r \mapsto N_\ell)_{\ell \in \mathcal{L}}\}}
% &\defas&
% \bl
% \dlam z \, ks.\Case \;z\; \{
% \bl
% (\ell~\Record{p, s} \mapsto \dLet\;r=\Fun\,s \;\dIn\; \cps{N_{\ell}} \sapp \reflect ks)_{\ell \in \mathcal{L}};\,
% y \mapsto \hforward \} \\
% \el \\
% \el \\
% \hforward &\defas& \bl
% \dLet\; (k' \dcons h' \dcons ks') = ks \;\dIn\; \\
% \Vmap\,(\dlam\Record{p, s}\,(k \dcons ks).k\,\Record{p, h' \dcons k' \dcons s}\,ks)\,y\,(h' \dcons ks') \\
% \el\\
\cps{\{(\ell \; p \; r \mapsto N_\ell)_{\ell \in \mathcal{L}}\}} \cps{\{(\ell \; p \; r \mapsto N_\ell)_{\ell \in \mathcal{L}}\}}
&\defas&
\bl
\dlam z \, ks.\Case \;z\; \{
\bl
(\ell~\Record{p, s} \mapsto \dLet\;r=\Fun\,s \;\dIn\; \cps{N_{\ell}} \sapp \reflect ks)_{\ell \in \mathcal{L}};\,
y \mapsto \hforward \} \\
\el \\
\el \\
\hforward &\defas& \bl
\dLet\; (k' \dcons h' \dcons ks') = ks \;\dIn\; \\
\Vmap\,(\dlam\Record{p, s}\,(k \dcons ks).k\,\Record{p, h' \dcons k' \dcons s}\,ks)\,y\,(h' \dcons ks') \\
\el
&\defas& \bl
\dlam \Record{z,\Record{p,rs}}\,ks.\Case \;z\; \{
\bl
(\ell \mapsto \Let\;r=\Res\;rs \;\In\; \cps{N_{\ell}} \sapp \reflect ks)_{\ell \in \mathcal{L}};\,\\
y \mapsto \hforward((y,p,rs),ks) \} \\
\el \\
\el \\
\hforward((y,p,rs),ks)
&\defas&\Let\; (k' \dcons h' \dcons ks') = ks \;\In\; h'\,\Record{y,\Record{p,h' \dcons k' \dcons rs}} \,ks'
\end{equations} \end{equations}
%
\textbf{Top level program} \textbf{Top level program}
% %
\begin{equations} \begin{equations}
\pcps{M} &=& \cps{M} \sapp ((\dlam x\,ks.x) \scons (\dlam z\,ks.\Absurd~z) \scons \snil) \\ \pcps{M} &=& \cps{M} \sapp ((\dlam x\,ks.x) \scons (\dlam z\,ks.\Absurd~z) \scons \snil) \\
\end{equations} \end{equations}
\vspace{-1em}
\caption{Higher-order uncurried CPS translation of $\HCalc$}
\caption{Higher-order uncurried CPS translation of $\HCalc$.}
\label{fig:cps-higher-order-uncurried} \label{fig:cps-higher-order-uncurried}
\end{figure} \end{figure}
% %
We now adapt our uncurried CPS translation to a higher-order one-pass
CPS translation~\cite{DanvyF90} that partially evaluates
administrative redexes at translation time.
%
Following Danvy and Nielsen~\cite{DanvyN03}, we adopt a two-level
lambda calculus notation to distinguish between \emph{static} lambda
abstraction and application in the meta language and \emph{dynamic}
lambda abstraction and application in the target language.
% \begin{figure}[t]
% \flushleft
% Values
% \begin{equations}
% \cps{x} &\defas& x \\
% \cps{\lambda x.M} &\defas& \dlam x\,\dhk.\Let\;\dRecord{fs,\dRecord{\vhret,\vhops}}\dcons \dhk' = \dhk\;\In\;\cps{M} \sapp (\sRecord{\reflect fs, \sRecord{\reflect \vhret, \reflect \vhops}} \scons \reflect \dhk') \\
% \cps{\Lambda \alpha.M} &\defas& \dlam z\,\dhk.\Let\;\dRecord{fs,\dRecord{\vhret,\vhops}}\dcons \dhk' = \dhk\;\In\;\cps{M} \sapp (\sRecord{\reflect fs, \sRecord{\reflect \vhret, \reflect \vhops}} \scons \reflect \dhk') \\
% \cps{\Rec\,g\,x.M} &\defas& \Rec\,g\,x\,\dhk.\Let\;\dRecord{fs,\dRecord{\vhret,\vhops}}\dcons \dhk' = \dhk\;\In\;\cps{M} \sapp (\sRecord{\reflect fs, \sRecord{\reflect \vhret, \reflect \vhops}} \scons \reflect \dhk') \\
% \multicolumn{3}{c}{
% \cps{\Record{}} \defas \Record{}
% \qquad
% \cps{\Record{\ell = \!\!V; W}} \defas \Record{\ell = \!\cps{V}; \cps{W}}
% \qquad
% \cps{\ell\,V} \defas \ell\,\cps{V}
% }
% \end{equations}
% % \begin{displaymath}
% % \end{displaymath}
% Computations
% \begin{equations}
% \cps{V\,W} &\defas& \slam \shk.\cps{V} \dapp \cps{W} \dapp \reify \shk \\
% \cps{V\,T} &\defas& \slam \shk.\cps{V} \dapp \Record{} \dapp \reify \shk \\
% \cps{\Let\; \Record{\ell=x;y} = V \; \In \; N} &\defas& \slam \shk.\Let\; \Record{\ell=x;y} = \cps{V} \; \In \; \cps{N} \sapp \shk \\
% \cps{\Case~V~\{\ell~x \mapsto M; y \mapsto N\}} &\defas&
% \slam \shk.\Case~\cps{V}~\{\ell~x \mapsto \cps{M} \sapp \shk; y \mapsto \cps{N} \sapp \shk\} \\
% \cps{\Absurd~V} &\defas& \slam \shk.\Absurd~\cps{V} \\
% \end{equations}
% \begin{equations}
% \cps{\Return\,V} &=& \slam \shk.\kapp\;(\reify \shk)\;\cps{V} \\
% \cps{\Let~x \revto M~\In~N} &=&
% \bl\slam \sRecord{\shf, \sRecord{\svhret, \svhops}} \scons \shk.\\
% \qquad \cps{M} \sapp (\bl\sRecord{\reflect((\dlam x\,\dhk.\bl\Let\;\dRecord{fs,\dRecord{\vhret,\vhops}}\dcons k' = k\;\In\\
% \cps{N} \sapp (\sRecord{\reflect fs, \sRecord{\reflect \vhret, \reflect \vhops}} \scons \reflect k')) \dcons \reify\shf), \el\\
% \sRecord{\svhret, \svhops}} \scons \shk)\el
% \el\\
% \cps{\Do\;\ell\;V} &\defas&
% \slam \sRecord{\shf, \sRecord{\svhret, \svhops}} \scons \shk.\,
% \reify\svhops \dapp \dRecord{\ell,\dRecord{\cps{V}, \dRecord{\reify \shf, \dRecord{\reify\svhret, \reify\svhops}} \dcons \dnil}} \dapp \reify \shk \\
% \cps{\Handle^\depth \, M \; \With \; H} &\defas&
% \slam \shk . \cps{M} \sapp (\sRecord{\snil, \cps{H}^\depth} \scons \shk) \\
% \cps{H}^\depth &=& \sRecord{\reflect \cps{\hret}, \reflect \cps{\hops}^\depth}\\
% \cps{\{\Return \; x \mapsto N\}} &\defas& \dlam x\,\dhk.\bl\Let\;\dRecord{fs,\dRecord{\vhret,\vhops}} \dcons k' = k\;\In\\\cps{N} \sapp (\sRecord{\reflect fs, \sRecord{\reflect \vhret, \reflect \vhops}} \scons \reflect k')\el
% \\
% \cps{\{(\ell \; p \; r \mapsto N_\ell)_{\ell \in \mathcal{L}}\}}^\depth
% &\defas&
% \bl
% \dlam \dRecord{z,\dRecord{p,\dhkr}}\,\dhk.\bl
% \Case \;z\; \{
% \begin{eqs}
% (\ell &\mapsto& \Let\;r=\Res^\depth\,\dhkr\;\In\; \\
% & & \Let\;\dRecord{fs,\dRecord{\vhret, \vhops}}\dcons k'=k\;\In\\
% & & \cps{N_{\ell}} \sapp (\sRecord{\reflect fs, \sRecord{\reflect \vhret, \reflect \vhops}} \scons \reflect \dhk'))_{\ell \in \mathcal{L}}\\
% y &\mapsto& \hforward((y, p, \dhkr), \dhk) \} \\
% \end{eqs} \\
% \el \\
% \el \\
% \hforward((y, p, \dhkr), \dhk) &\defas& \bl
% \Let\; \dRecord{fs, \dRecord{\vhret, \vhops}} \dcons \dhk' = \dhk \;\In \\
% \Let\; rk' = \dRecord{fs, \dRecord{\vhret, \vhops}} \dcons \dhkr\;\In\\
% \vhops \dapp \dRecord{y,\dRecord{p, rk'}} \dapp \dhk' \\
% \el
% \end{equations}
% Top-level program
% \begin{equations}
% \pcps{M} &=& \cps{M} \sapp (\sRecord{\snil, \sRecord{\reflect \dlam x\,\dhk. x, \reflect \dlam \dRecord{z,\dRecord{p,rk}}\,\dhk.\Absurd~z}} \scons \snil) \\
% \end{equations}
% \caption{Higher-Order Uncurried CPS Translation of $\HCalc$}
% \label{fig:cps-higher-order-uncurried}
% \end{figure}
%
% We now adapt our uncurried CPS translation to a higher-order one-pass
% CPS translation~\cite{DanvyF90} that partially evaluates
% administrative redexes at translation time.
% %
% Following Danvy and Nielsen~\cite{DanvyN03}, we adopt a two-level
% lambda calculus notation to distinguish between \emph{static} lambda
% abstraction and application in the meta language and \emph{dynamic}
% lambda abstraction and application in the target language.
% %
% The idea is that redexes marked as static are reduced as part of the
% translation (at compile time), whereas those marked as dynamic are
% reduced at runtime.
% %
% The CPS translation is given in
% Figure~\ref{fig:cps-higher-order-uncurried}.
We now adapt the translation of Section~\ref{sec:pure-as-stack} to a
higher-order one-pass CPS translation~\citep{DanvyF90} that partially
evaluates administrative redexes at translation time.
%
Following \citet{DanvyN03}, we adopt a two-level lambda calculus
notation to distinguish between \emph{static} lambda abstraction and
application in the meta language and \emph{dynamic} lambda abstraction
and application in the target language:
{\color{blue}$\overline{\text{overline}}$} denotes a static syntax
constructor; {\color{red}$\underline{\text{underline}}$} denotes a
dynamic syntax constructor. The principal idea is that redexes marked
as static are reduced as part of the translation (at compile time),
whereas those marked as dynamic are reduced at runtime. To facilitate
this notation we write application in both calculi with an infix
``at'' symbol ($@$).
\paragraph{Static terms}
Static constructs are marked in
{\color{blue}$\overline{\text{static blue}}$}, and their redexes are
reduced as part of the translation (at compile time). We make use of
static lambda abstractions, pairs, and lists. Reflection of dynamic
language values into the static language is written as $\reflect V$.
%
We use $\shk$ for variables representing statically known
continuations (frame stacks), $\shf$ for variables representing pure
frame stacks, and $\chi$ for variables representing handlers.
%
We let $\sV, \sW$ range over meta language values,
$\sM$ range over static language expressions,
and $\sP, \sQ$ over static language patterns.
%
We use list and record pattern matching in the meta language, which
behaves as follows:
% %
The idea is that redexes marked as static are reduced as part of the
translation (at compile time), whereas those marked as dynamic are
reduced at runtime.
\begin{displaymath}
\ba{@{~}l@{~}l}
&(\slam \sRecord{\sP, \sQ}.\,\sM) \sapp \sRecord{\sV, \sW}\\
= &(\slam \sP.\,\slam \sQ.\,\sM) \sapp \sV \sapp \sW\\
= &(\slam (\sP \scons \sQ).\,\sM) \sapp (\sV \scons \sW)
\ea
\end{displaymath}
% %
The CPS translation is given in
Figure~\ref{fig:cps-higher-order-uncurried}.
An overline denotes a static syntax constructor and an underline
denotes a dynamic syntax constructor. In order to facilitate this
notation we write application explicitly as an infix ``at'' symbol
($@$). We assume the meta language is pure and hence respects the
usual $\beta$ and $\eta$ equivalences. We extend the overline and
underline notation to distinguish between static and dynamic let
bindings.
The reify operator $\reify$ maps static lists to dynamic ones and the
reflect constructor $\reflect$ allows dynamic lists to be treated as
static.
Static language values, comprised of reflected values, pairs, and list
conses, are reified as dynamic language values $\reify \sV$ by
induction on their structure:
% %
We use list pattern matching in the meta language.
\[
\ba{@{}l@{\qquad}c@{\qquad}r}
\reify \reflect V \defas V
&\reify (\sV \scons \sW) \defas \reify \sV \dcons \reify \sW
&\reify \sRecord{\sV, \sW} \defas \dRecord{\reify \sV, \reify \sW}
\ea
\]
% %
\begin{equations}
(\slam (\sk \scons \sP).\sM) \sapp (V \scons \VS)
&=& \sLet\; \sk = V \;\sIn\; (\slam \sP.\sM) \sapp \VS \\
(\slam (\sk \scons \sP).\sM) \sapp \reflect V
&=& \dLet\; (k \dcons ks) = V \;\dIn\;
\sLet\; \sk = k \;\sIn\; (\slam \sP.\sM) \sapp \reflect ks \\
\end{equations}
Here we let $\sM$ range over meta language expressions.
We assume the static language is pure and hence respects the usual
$\beta$ and $\eta$ equivalences.
Now the target calculus is refined so that all lambda abstractions and
applications take two arguments, the $\usemlab{Lift}$ rule is removed,
and the $\usemlab{App}$ rule is replaced by the following reduction
rule.
%
\begin{reductions}
\usemlab{AppTwo} & (\dlam x\,ks . \, M) \dapp V \dapp W &\reducesto& M[V/x, W/ks] \\
\end{reductions}
\paragraph{Higher-order translation}
The CPS translation is given in
Figure~\ref{fig:cps-higher-order-uncurried}. In essence, it is the
same as the CPS translation for deep and shallow handlers we described
in Section~\ref{sec:pure-as-stack}, albeit separated into static and
dynamic parts. A major difference that has a large cosmetic effect on
the presentation of the translation is that we maintain the invariant
that the statically known stack ($\sk$) always contains at least one
frame, consisting of a triple
$\sRecord{\reflect V_{fs}, \sRecord{\reflect V_{ret}, \reflect
V_{ops}}}$ of reflected dynamic pure frame stacks, return
handlers, and operation handlers. Maintaining this invariant ensures
that all translations are uniform in whether they appear statically
within the scope of a handler or not, and this simplifies our
correctness proof. To maintain the invariant, any place where a
dynamically known stack is passed in (as a continuation parameter
$k$), it is immediately decomposed using a dynamic language $\Let$ and
repackaged as a static value with reflected variable
names. Unfortunately, this does add some clutter to the translation
definition, as compared to the translations above. However, there is a
payoff in the removal of administrative reductions at run time.
\subsubsection{Correctness}
\label{sec:higher-order-cps-deep-handlers-correctness}
To prove the correctness of our CPS translation
(Theorem~\ref{thm:ho-simulation}), we first state several lemmas
describing how translated terms behave. In view of the invariant of
the translation that we described above, we state each of these lemmas
in terms of static continuation stacks where the shape of the top
element is always known statically, i.e., it is of the form
$\sRecord{\sV_{fs}, \sRecord{\sV_{ret},\sV_{ops}}} \scons
\sW$. Moreover, the static values $\sV_{fs}$, $\sV_{ret}$, and
$\sV_{ops}$ are all reflected dynamic terms (i.e., of the form
$\reflect V$). This fact is used implicitly in our proofs, which are
given in Appendix~\ref{sec:proofs}.
First, the higher-order CPS translation commutes with substitution:
\begin{lemma}[Substitution]\label{lem:subst}
%
The CPS translation $\cps{-}$ commutes with substitution in value terms
%
\[
\cps{W}[\cps{V}/x] = \cps{W[V/x]},
\]
%
and with substitution in computation terms
\[
(\cps{M} \sapp (\sk \scons \sh \scons \sks))[\cps{V}/x]
= \cps{M[V/x]} \sapp (\sk \scons \sh \scons \sks)[\cps{V}/x].
\]
%
\end{lemma}
% %
We add an extra dummy argument to the translation of type lambda
abstractions and applications in order to ensure that all dynamic
functions take exactly two arguments.
\begin{proof}
TODO\dots
\end{proof}
% %
The single argument lambdas and applications from the first-order
uncurried translation are still present, but now they are all static.
In order to reason about the behaviour of the \semlab{Handle-op} rule, In order to reason about the behaviour of the \semlab{Handle-op} rule,
which is defined in terms of an evaluation context, we extend the CPS which is defined in terms of an evaluation context, we extend the CPS
translation to evaluation contexts.
translation to evaluation contexts:
% %
\begin{displaymath} \begin{displaymath}
\ba{@{}r@{~}c@{~}r@{~}l@{}} \ba{@{}r@{~}c@{~}r@{~}l@{}}
@ -2685,12 +2844,131 @@ translation to evaluation contexts.
\cps{\Handle\; \EC \;\With\; H} &=& \slam \sks. &\cps{\EC} \sapp (\cps{\hret} \scons \cps{\hops} \scons \sks) \\ \cps{\Handle\; \EC \;\With\; H} &=& \slam \sks. &\cps{\EC} \sapp (\cps{\hret} \scons \cps{\hops} \scons \sks) \\
\ea \ea
\end{displaymath} \end{displaymath}
%
The following lemma is the characteristic property of the CPS The following lemma is the characteristic property of the CPS
translation on evaluation contexts. translation on evaluation contexts.
% %
This allows us to focus on the computation contained within This allows us to focus on the computation contained within
an evaluation context. an evaluation context.
\begin{lemma}[Decomposition]
\label{lem:decomposition}
%
\begin{equations}
\cps{\EC[M]} \sapp (\sV \scons \sW) &=& \cps{M} \sapp (\cps{\EC} \sapp (\sV \scons \sW)) \\
\end{equations}
%
\end{lemma}
%
\begin{proof}
TODO\dots
\end{proof}
%
Though we have eliminated the static administrative redexes, we are
still left with one form of administrative redex that cannot be
eliminated statically because it only appears at run-time. These arise
from pattern matching against a reified stack of continuations and are
given by the $\usemlab{SplitList}$ rule.
\begin{reductions}
\usemlab{SplitList} & \Let\; (k \dcons ks) = V \dcons W \;\In\; M &\reducesto& M[V/k, W/ks] \\
\end{reductions}
%
This is isomorphic to the \usemlab{Split} rule, but we now treat lists
and \usemlab{SplitList} as distinct from pairs, unit, and
\usemlab{Split} in the higher-order translation so that we can
properly account for administrative reduction.
%
We write $\areducesto$ for the compatible closure of
\usemlab{SplitList}.
By definition, $\reify \reflect V = V$, but we also need to reason
about the inverse composition.
%
\begin{lemma}[Reflect after reify]
\label{lem:reflect-after-reify}
$\cps{M} \sapp (V_1 \scons \dots V_n \scons \reflect \reify \VS)
\areducesto^*
\cps{M} \sapp (V_1 \scons \dots V_n \scons \VS)$
\end{lemma}
%
\begin{proof}
Proof is by induction on the structure of $M$.
\end{proof}
%
We next observe that the CPS translation simulates forwarding.
%
\begin{lemma}[Forwarding]
\label{lem:forwarding}
If $\ell \notin dom(H_1)$ then:
%
\begin{displaymath}
\cps{\hops_1} \dapp \ell\,\Record{U, V} \dapp (V_2 \dcons \cps{\hops_2} \dcons W)
\reducesto^+
\cps{\hops_2} \dapp \ell\,\Record{U, \cps{\hops_2} \dcons V_2 \dcons V} \dapp W
\end{displaymath}
\end{lemma}
%
\begin{proof}
TODO\dots
\end{proof}
%
Now we show that the translation simulates the \semlab{Handle-op}
rule.
%
\begin{lemma}[Handling]
\label{lem:handle-op}
If $\ell \notin BL(\EC)$ and $\hell = \{\ell\,p\,r \mapsto N_\ell\}$ then:
%
\begin{displaymath}
\bl
\cps{\Do\;\ell\;V} \sapp (\cps{\EC} \sapp (\cps{\hret} \scons \cps{\hops} \scons \VS)) \reducesto^+\areducesto^* \\
\quad
(\cps{N_\ell} \sapp \VS)[\cps{V}/p, (\lambda y\,ks.\cps{\Return\;y} \sapp (\cps{\EC} \sapp (\cps{\hret} \scons \cps{\hops} \scons \reflect ks)))/r] \\
\el
\end{displaymath}
\end{lemma}
%
%
\begin{proof}
Follows from Lemmas~\ref{lem:decomposition},
\ref{lem:reflect-after-reify}, and \ref{lem:forwarding}.
\end{proof}
%
We now turn to our main result which is a simulation result in style
of Plotkin~\cite{Plotkin75}. The theorem shows that the only extra
behaviour exhibited by a translated term is the bureaucracy of
deconstructing the continuation stack.
%
%
\begin{theorem}[Simulation]
\label{thm:ho-simulation}
If $M \reducesto N$ then $\pcps{M} \reducesto^+ \areducesto^* \pcps{N}$.
\end{theorem}
%
\begin{proof}
TODO\dots
\end{proof}
%
The proof is by case analysis on the reduction relation using Lemmas
\ref{lem:decomposition}--\ref{lem:handle-op}. The \semlab{Handle-op}
case follows from Lemma~\ref{lem:handle-op}.
In common with most CPS translations, full abstraction does not
hold. However, as our semantics is deterministic it is straightforward
to show a backward simulation result.
%
\begin{corollary}[Backwards simulation]
If $\pcps{M} \reducesto^+ \areducesto^* V$ then there exists $W$ such that
$M \reducesto^* W$ and $\pcps{W} = V$.
\end{corollary}
%
\begin{proof}
TODO\dots
\end{proof}
%
\chapter{Abstract machine semantics} \chapter{Abstract machine semantics}
\part{Expressiveness} \part{Expressiveness}

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