13,694 research outputs found
A Rational Deconstruction of Landin's SECD Machine with the J Operator
Landin's SECD machine was the first abstract machine for applicative
expressions, i.e., functional programs. Landin's J operator was the first
control operator for functional languages, and was specified by an extension of
the SECD machine. We present a family of evaluation functions corresponding to
this extension of the SECD machine, using a series of elementary
transformations (transformation into continu-ation-passing style (CPS) and
defunctionalization, chiefly) and their left inverses (transformation into
direct style and refunctionalization). To this end, we modernize the SECD
machine into a bisimilar one that operates in lockstep with the original one
but that (1) does not use a data stack and (2) uses the caller-save rather than
the callee-save convention for environments. We also identify that the dump
component of the SECD machine is managed in a callee-save way. The caller-save
counterpart of the modernized SECD machine precisely corresponds to Thielecke's
double-barrelled continuations and to Felleisen's encoding of J in terms of
call/cc. We then variously characterize the J operator in terms of CPS and in
terms of delimited-control operators in the CPS hierarchy. As a byproduct, we
also present several reduction semantics for applicative expressions with the J
operator, based on Curien's original calculus of explicit substitutions. These
reduction semantics mechanically correspond to the modernized versions of the
SECD machine and to the best of our knowledge, they provide the first syntactic
theories of applicative expressions with the J operator
Lazy Evaluation and Delimited Control
The call-by-need lambda calculus provides an equational framework for
reasoning syntactically about lazy evaluation. This paper examines its
operational characteristics. By a series of reasoning steps, we systematically
unpack the standard-order reduction relation of the calculus and discover a
novel abstract machine definition which, like the calculus, goes "under
lambdas." We prove that machine evaluation is equivalent to standard-order
evaluation. Unlike traditional abstract machines, delimited control plays a
significant role in the machine's behavior. In particular, the machine replaces
the manipulation of a heap using store-based effects with disciplined
management of the evaluation stack using control-based effects. In short, state
is replaced with control. To further articulate this observation, we present a
simulation of call-by-need in a call-by-value language using delimited control
operations
Profile blunting and flow blockage in a yield stress fluid: A molecular dynamics study
The flow of a simple glass forming system (a 80:20 binary Lennard-Jones
mixture) through a planar channel is studied via molecular dynamics
simulations. The flow is driven by an external body force similar to gravity.
Previous studies show that the model exhibits both a static [Varnik et al. J.
Chem. Phys. 120, 2788 (2004)] and a dynamic [F. Varnik and O. Henrich Phys.
Rev. B 73, 174209 (2006)] yield stress in the glassy phase. \blue{These
observations are corroborated by the present work, where we investigate how the
presence of a yield stress may affect the system behavior in a Poiseuille-type
flow geometry.} In particular, we observe a blunted velocity profile across the
channel: A relatively wide region in the channel center flows with a constant
velocity (zero shear rate) followed by a non linear change of the shear rate as
the walls are approached. The observed velocity gradients are compared to those
obtained from the knowledge of the shear stress across the channel and the
flow-curves (stress versus shear rate), the latter being determined in our
previous simulations of homogeneous shear flow. Furthermore, using the value of
the (dynamic) yield stress known from previous simulations, we estimate the
threshold body force for a complete arrest of the flow. Indeed, a blockage is
observed as the imposed force falls below this threshold value. Small but
finite shear rates are observed at stresses above the dynamic but below the
static yield stress. We discuss the possible role of the \blue{stick-slip like
motion} for this observation.Comment: 22 pages, 8 figure
Comparative statics for a three player differential game in resource economics - the case of exhaustible resources and varying allocations of initial stocks
Differential games combine strategic interactions between agents and optimization concerning time. Decisions made in the past determine the present and even the future .in pay off as well as in the opportunities available . for oneself and for the rival players, eventually too. Unfortunately, due to high complexity it is hard to find a Nash-equilibrium within a differential game and it is even harder to get some results in comparative statics. It is the purpose of the paper at hand to present findings concerning comparative statics in a differential game discussed by Wacker and Blank (1999). Comparative statics become available due to a routine solving for the open-loop Nash equilibrium for each parameter combination under consideration. A description of the routine . a 4 step simulation run which approximates the equilibrium numerically . was presented in an earlier Working Paper. In the earlier Paper Excel was applied as it is a wild spread tool. Here again Excel, its Solver and Macros constitute the main instruments; they are used to get repeated simulation runs for varying parameter constellations. The findings presented here concern varying allocations in initial stocks. Generalization to comparative statics in further parameters is in progress.
Full Scale Proton Beam Impact Testing of new CERN Collimators and Validation of a Numerical Approach for Future Operation
New collimators are being produced at CERN in the framework of a large
particle accelerator upgrade project to protect beam lines against stray
particles. Their movable jaws hold low density absorbers with tight geometric
requirements, while being able to withstand direct proton beam impacts. Such
events induce considerable thermo-mechanical loads, leading to complex
structural responses, which make the numerical analysis challenging. Hence, an
experiment has been developed to validate the jaw design under representative
conditions and to acquire online results to enhance the numerical models. Two
jaws have been impacted by high-intensity proton beams in a dedicated facility
at CERN and have recreated the worst possible scenario in future operation. The
analysis of online results coupled to post-irradiation examinations have
demonstrated that the jaw response remains in the elastic domain. However, they
have also highlighted how sensitive the jaw geometry is to its mounting support
inside the collimator. Proton beam impacts, as well as handling activities, may
alter the jaw flatness tolerance value by 70 m, whereas the
flatness tolerance requirement is 200 m. In spite of having validated
the jaw design for this application, the study points out numerical limitations
caused by the difficulties in describing complex geometries and boundary
conditions with such unprecedented requirements.Comment: 22 pages, 17 figures, Prepared for submission to JINS
Broadcast Strategies with Probabilistic Delivery Guarantee in Multi-Channel Multi-Interface Wireless Mesh Networks
Multi-channel multi-interface Wireless Mesh Networks permit to spread the
load across orthogonal channels to improve network capacity. Although broadcast
is vital for many layer-3 protocols, proposals for taking advantage of multiple
channels mostly focus on unicast transmissions. In this paper, we propose
broadcast algorithms that fit any channel and interface assignment strategy.
They guarantee that a broadcast packet is delivered with a minimum probability
to all neighbors. Our simulations show that the proposed algorithms efficiently
limit the overhead
On the Static and Dynamic Extents of Delimited Continuations
We show that breadth-first traversal exploits the difference between the static delimited-control operator shift (alias S) and the dynamic delimited-control operator control (alias F). For the last 15 years, this difference has been repeatedly mentioned in the literature but it has only been illustrated with one-line toy examples. Breadth-first traversal fills this vacuum. We also point out where static delimited continuations naturally give rise to the notion of control stack whereas dynamic delimited continuations can be made to account for a notion of `control queue.'
A Dynamic Continuation-Passing Style for Dynamic Delimited Continuations
We present a new abstract machine that accounts for dynamic delimited continuations. We prove the correctness of this new abstract machine with respect to a pre-existing, definitional abstract machine. Unlike this definitional abstract machine, the new abstract machine is in defunctionalized form, which makes it possible to state the corresponding higher-order evaluator. This evaluator is in continuation+state passing style and threads a trail of delimited continuations and a meta-continuation. Since this style accounts for dynamic delimited continuations, we refer to it as `dynamic continuation-passing style.' We show that the new machine operates more efficiently than the definitional one and that the notion of computation induced by the corresponding evaluator takes the form of a monad. We also present new examples and a new simulation of dynamic delimited continuations in terms of static ones
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