@ -3732,7 +3732,7 @@ that \citet{DybvigJS07} present a typed embedding of control and
prompts in Haskell (actually, they present an entire general monadic
framework for implementing control operators based on the idea of
\emph { multi-prompts} , which are a slight generalisation of prompts ---
we will revisit multi-prompts when we discuss cupto).
we will revisit multi-prompts when we discuss splitter and cupto).
%
% \dhil { Mention Yonezawa and Kameyama's type system.}
% %
@ -3972,19 +3972,19 @@ dynamic semantics as well.
%
\begin { reductions}
\slab { Value} & \reset { V} & \reducesto & V\\
\slab { Capture} & \reset { \EC [\shift\;k.M] } & \reducesto & \reset { M[\qq { \cont _ { \EC } } /k]} , \text { where $ \EC $ contains no $ \reset { - } $ } \\
\slab { Capture} & \reset { \EC [\shift\;k.M] } & \reducesto & \reset { M[\qq { \cont _ { \reset { \ EC } } } /k]} , \text { where $ \EC $ contains no $ \reset { - } $ } \\
% \slab { Resume} & \reset { \EC [\Continue~\cont_{\reset{\EC'}}~V] } & \reducesto & \reset { \EC [\reset{\EC'[V] } ]} \\
\slab { Resume} & \Continue ~\cont _ { \EC } ~V & \reducesto & \reset { \EC [V] } \\
\slab { Resume} & \Continue ~\cont _ { \reset { \ EC } } ~V & \reducesto & \reset { \EC [V] } \\
\end { reductions}
%
The $ \slab { Value } $ and $ \slab { Capture } $ rules are the same as for
control/prompt (modulo the syntactic differences). The static nature
of shift/reset manifests operationally in the $ \slab { Resume } $ rule,
where the reinstall ed context $ \EC $ gets enclosed in a ne w reset. T he
extra reset has ramifications for the operational behaviour of
subsequent occurrences of $ \shift $ in $ \EC $ . To put this into
perspective, let us revisit the second control/prompt example with
shift/reset instead.
The key difference between \citeauthor { Felleisen88} 's control/prompt
and shift/reset is that the $ \slab { Capture } $ rule for the latter
includes a copy of the delimiter in the reified continuation. This
delimiter gets installed along with the captur ed context $ \EC $ when
the continuation object is resumed. The extra reset has ramifications
for the operational behaviour of subsequent occurrences of $ \shift $ in
$ \EC $ . To put this into
control/prompt example with shift/reset instead.
%
\begin { derivation}
& 1 + \reset { 2 + (\shift \; k.3 + k\, 0) + (\shift \; k'.4)} \\
@ -4015,9 +4015,60 @@ translation in Standard ML New Jersey~\cite{AppelM91}, using
\citeauthor { Filinski94} 's encoding of shift/reset in terms of callcc
and state~\cite { Filinski94} .
%
As with control and prompt there exist various variation of shift and
reset. \citet { DanvyF89} also considered $ \shiftz $ and
$ \resetz { - } $ . The operational difference between $ \shiftz $ /$ \resetz { - } $
and $ \shift $ /$ \reset { - } $ manifests in the capture rule.
%
\begin { reductions}
\slab { Capture_ 0} & \resetz { \EC [\shiftz\,k.M] } & \reducesto & M[\qq { \cont _ { \resetz { \EC } } } /k], \text { where $ \EC $ contains no $ \resetz { - } $ } \\
\end { reductions}
%
The control reifier captures the continuation up to and including its
delimiter, however, unlike $ \shift $ , it removes the control delimiter
from the current evaluation context. Thus $ \shiftz $ /$ \resetz { - } $ are
`dynamic' variations on $ \shift $ /$ \reset { - } $ . \citet { MaterzokB12}
introduced $ \dollarz { - } { - } $ (pronounced ``dollar0'') as an
alternative control delimiter for $ \shiftz $ .
\begin { reductions}
\slab { Value_ { \$ _ 0} } & \dollarz { x.N} { V} & \reducesto & N[V/x]\\
\slab { Capture_ { \$ _ 0} } & \dollarz { x.N} { \EC [\shiftz\,k.M] } & \reducesto & M[\qq { \cont _ { (\dollarzh { x.N} { \EC } )} } /k],\\
& & & \quad \text { where $ \EC $ contains no $ \reset { - \mid - } $ } \\
\slab { Resume_ { \$ _ 0} } & \Continue ~\cont _ { (\dollarz { x.N} { \EC } )} ~V & \reducesto & \dollarz { x.N} { \EC [V] } \\
\end { reductions}
%
The intuition here is that $ \dollarz { x.N } { M } $ evaluates $ M $ to some
value $ V $ in a fresh context, and then continues as $ N $ with $ x $ bound
to $ V $ . Thus it builds in a form of ``success continuation'' that
makes it possible to post-process the result of a reset0 term. In
fact, reset0 is macro-expressible in terms of
dollar0~\cite { MaterzokB12} .
%
\[
\sembr { \resetz { M} } \defas \dollarz { x.x} { \sembr { M} } \\
\]
%
By taking the success continuation to be the identity function dollar0
becomes operationally equivalent to reset0. As it turns out reset0 and
shift0 (together) can macro-express dollar0~\cite { MaterzokB12} .
%
\dhil { Maybe mention the implication is that control/prompt has CPS semantics.}
\dhil { Mention shift0/reset0, dollar0\dots }
\[
\sembr { \dollarz { x.N} { M} } \defas (\lambda k.\resetz { (\lambda x.\shiftz ~z.k~x)\, \sembr { M} } )\, (\lambda x.\sembr { N} )
\]
%
This translation is a little more involved. The basic idea is to first
explicit pass in the success continuation, then evaluate $ M $ under a
reset to yield value which gets bound to $ x $ , and then subsequently
uninstall the reset by invoking $ \shiftz $ and throwing away the
captured continuation, afterwards we invoke the success continuation
with the value $ x $ .
% Even though the two constructs are equi-expressive (in the sense of macro-expressiveness) there are good reason for preferring dollar0 over reset0 Since the two constructs are equi-expressive, the curious reader might
% wonder why \citet { MaterzokB12} were
% \dhil { Maybe mention the implication is that control/prompt has CPS semantics.}
% \dhil { Mention shift0/reset0, dollar0\dots }
% \begin { reductions}
% % \slab { Value} & \reset { V} & \reducesto & V\\
% \slab { Capture} & \reset { \EC [\shift\,k.M] } & \reducesto & M[\cont _ { \reset { \EC } } /k]\\
@ -4026,18 +4077,18 @@ and state~\cite{Filinski94}.
%
\paragraph { \citeauthor { QueinnecS91} 's splitter} The `splitter' control
operator reconciles abortive undelimited control and composable
delimited control . It was introduced by \citet { QueinnecS91} in
1991. The name `splitter' is derived from it operational behaviour, as
an application of `splitter' marks evaluation context in order for it
to be split into two parts, where the context outside the mark
represents the rest of computation, and the context inside the mark
may be reified into a delimited continuation. The operator supports
two operations `abort' and `calldc' to control the splitting of
evaluation contexts. The former has the effect of escaping to the
outer context, whilst the latter reifies the inner context as a
delimited continuation (the operation name is short for ``call with
delimited continuation'').
operator reconciles abortive continuations and composable
continuations . It was introduced by \citet { QueinnecS91} in 1991. The
name `splitter' is derived from it operational behaviour, as an
application of `splitter' marks evaluation context in order for it to
be split into two parts, where the context outside the mark represents
the rest of computation, and the context inside the mark may be
reified into a delimited continuation. The operator supports two
operations `abort' and `calldc' to control the splitting of evaluation
contexts. The former has the effect of escaping to the outer context,
whilst the latter reifies the inner context as a delimited
continuation (the operation name is short for ``call with delimited
continuation'').
Splitter and the two operations abort and calldc are value forms.
%
@ -4111,8 +4162,8 @@ track of which prompt names have already been allocated.
\begin { reductions}
\slab { AppSplitter} & \splitter ~V,\rho & \reducesto & \Prompt _ p~V\, p,\rho \uplus \{ p\} \\
\slab { Value} & \Prompt _ p~V,\rho & \reducesto & V,\rho \\
\slab { Abort} & \Prompt _ p~\EC [\abort~ \Record{p;V}] ,\rho & \reducesto & V\, \Unit ,\rho ,\quad \text { where $ \EC $ contains no $ \Prompt _ p $ } \\
\slab { Capture} & \Prompt _ p~\EC [\calldc~V ] & \reducesto & V~\qq { \cont _ { \EC } } ,\rho \\
\slab { Abort} & \Prompt _ p~\EC [\abort\, \Record{p;V}] ,\rho & \reducesto & V\, \Unit ,\rho \\ % \quad \text { where $ \EC $ contains no $ \Prompt _ p $ } \\
\slab { Capture} & \Prompt _ p~\EC [\calldc\,\Record{p;V} ] & \reducesto & V~\qq { \cont _ { \EC } } ,\rho \\
\slab { Resume} & \Continue ~\cont _ { \EC } ~V,\rho & \reducesto & \EC [V] ,\rho
\end { reductions}
%
@ -4133,13 +4184,58 @@ operation the prompt is removed. % Thus, $\calldc$ behaves as a
% delimited variation of $ \Callcc $ .
%
It is clear by the prompt semantics that invocation of either $ \abort $
and $ \calldc $ is only well-defined within the dynamic extent of
$ \splitter $ . Since the prompt is eliminated after use of either
It is clear by the prompt semantics that an invocation of either
$ \abort $ $ \calldc $ is only well-defined within the dynamic extent
of $ \splitter $ . Since the prompt is eliminated after use of either
operation subsequent operation invocations must be guarded by a new
instance of $ \splitter $ .
%
\dhil { Show an example}
\begin { derivation}
& 2 + \splitter \, (\lambda p.2 + \splitter \, (\lambda p'.3 + \calldc \, \Record { p;\lambda k. k~0 + \abort \, \Record { p';\lambda \Unit .k~1} } )),\emptyset \\
\reducesto & \reason { \slab { AppSplitter} } \\
& 2 + \Prompt _ p~2 + \splitter \, (\lambda p'.3 + \calldc \, \Record { p;\lambda k. k~0 + \abort \, \Record { p';\lambda \Unit .k~1} } ), \{ p\} \\
\reducesto & \reason { \slab { AppSplitter} } \\
& 2 + \Prompt _ p~2 + \Prompt _ { p'} ~3 + \calldc \, \Record { p;\lambda k. k~0 + \abort \, \Record { p';\lambda \Unit .k~1} } , \{ p,p'\} \\
\reducesto & \reason { \slab { Capture} $ \EC = 2 + \Prompt _ { p' } ~ 3 + [ \, ] $ } \\
& 2 + k~0 + \abort \, \Record { p';\lambda \Unit .k~1} , \{ p,p'\} \\
\reducesto & \reason { \slab { Resume} $ \EC $ with $ 0 $ } \\
& 2 + 2 + \Prompt _ { p'} ~3 + \abort \, \Record { p';\lambda \Unit .\qq { \cont _ { \EC } } \, 1} , \{ p,p'\} \\
\reducesto ^ +& \reason { \slab { Abort} } \\
& 4 + \qq { \cont _ { \EC } } \, 1, \{ p,p'\} \\
\reducesto & \reason { \slab { Resume} $ \EC $ with $ 1 $ } \\
& 4 + 2 + \Prompt _ { p'} ~3 + 1, \{ p,p'\} \\
\reducesto ^ +& \reason { \slab { Value} } \\
& 6 + 4, \{ p,p'\} \reducesto 10, \{ p,p'\}
\end { derivation}
%
The important thing to observe here is that the application of
$ \calldc $ skips over the inner prompt and reifies it as part of the
continuation. The first application of $ k $ restores the context with
the prompt. The $ \abort $ application erases the evaluation context up
to this prompt, however, the body of the functional argument to
$ \abort $ reinvokes the continuation $ k $ which restores the prompt
context once again.
% \begin { derivation}
% & 1 + \splitter \, (\lambda p.2 + \splitter \, (\lambda p'.3 + \calldc \, \Record { p';\lambda k. k\, 0 + \calldc \, \Record { p';\lambda k'. k\, (k'\, 1)} } ))), \emptyset \\
% \reducesto & \reason { \slab { AppSplitter} } \\
% & 1 + \Prompt _ p~2 + \splitter \, (\lambda p'.3 + \calldc \, \Record { p;\lambda k.k~1 + \calldc \, \Record { p';\lambda k'. k\, (k'~1)} } )), \{ p\} \\
% \reducesto & \reason { \slab { AppSplitter} } \\
% & 1 + \Prompt _ p~2 + \Prompt _ { p'} 3 + \calldc \, \Record { p';\lambda k.k~0 + \calldc \, \Record { p';\lambda k'. k\, (k'~1)} } , \{ p,p'\} \\
% \reducesto & \reason { \slab { Capture} $ \EC = 2 + \Prompt _ { p' } ~ 3 + [ \, ] $ } , \{ p,p'\} \\
% & 1 + ((\lambda k.k~0 + \calldc \, \Record { p';\lambda k'. k\, (k'~1)} )\, \qq { \cont _ \EC } ),\{ p,p'\} \\
% \reducesto ^ +& \reason { \slab { Resume} $ \EC $ with $ 0 $ } \\
% & 1 + 2 + \Prompt _ { p'} ~3 + 0 + \calldc \, \Record { p';\lambda k'. \qq { \cont _ \EC } \, (k'~1)} \\
% \reducesto ^ +& \reason { \slab { Capture} $ \EC ' = 3 + [ \, ] $ } \\
% & 3 + (\lambda k'. \qq { \cont _ \EC } \, (k'~1)\, \qq { \cont _ { \EC '} } )\\
% \reducesto ^ +& \reason { \slab { Resume} $ \EC ' $ with $ 1 $ } \\
% & 3 + \qq { \cont _ \EC } \, (3 + 1)\\
% \reducesto ^ +& \reason { \slab { Resume} $ \EC $ with $ 4 $ } \\
% & 3 + 2 + \Prompt _ { p'} ~3 + 4\\
% \reducesto ^ +& \reason { \slab { Value} } \\
% & 5 + 7 \reducesto 13
% \end { derivation}
%
% \begin { reductions}
% \slab { Value} & \splitter ~abort~calldc.V & \reducesto & V\\
% \slab { Throw} & \splitter ~abort~calldc.\EC [\,abort~V] & \reducesto & V~\Unit \\
