Lasse Nielsen

Defunctionalization at work

Reynoldss defunctionalization technique is a whole-program transformation from higher-order to first-order functional programs. We study practical applications of this transformation and uncover new connections between seemingly unrelated higher-order and first-order specifications and between their correctness proofs. Defunctionalization therefore appearsboth as a springboard for rev ealing new connections and as a bridge for transferring existing results between the first-order world and the higher-order world.

A first-order one-pass CPS transformation

We present a new transformation of λ-terms into continuation-passing style (CPS). This transformation operates in one pass and is both compositional and first-order. Previous CPS transformations only enjoyed two out of the three properties of being first-order, one-pass, and compositional, but the new transformation enjoys all three properties. It is proved correct directly by structural induction over source terms instead of indirectly with a colon translation, as in Plotkins original proof. Similarly, it makes it possible to reason about CPS-transformed terms by structural induction over source terms, directly.The new CPS transformation connects separately published approaches to the CPS transformation. It has already been used to state a new and simpler correctness proof of a direct-style transformation, and to develop a new and simpler CPS transformation of control-flow information.

A Selective CPS Transformation

Abstract The CPS transformation makes all functions continuation-passing, uniformly. Not all functions, however, need continuations: they only do if their evaluation includes computational effects. In this paper we focus on control operations, in particular “call with current continuation” and “throw”. We characterize this involvement as a control effect and we present a selective CPS transformation that makes functions and expressions continuation-passing if they have a control effect, and that leaves the rest of the program in direct style. We formalize this selective CPS transformation with an operational semantics and a simulation theorem a la Plotkin.

On one-pass CPS transformations

We bridge two distinct approaches to one-pass CPS transformations, i.e, CPS transformations that reduce administrative redexes at transformation time instead of in a post-processing phase. One approach is compositional and higher-order, and is independently due to Appel, Danvy and Filinski, and Wand, building on Plotkins seminal work. The other is non-compositional and based on a reduction semantics for the lambda-calculus, and is due to Sabry and Felleisen. To relate the two approaches, we use three tools: Reynoldss defunctionalization and its left inverse, refunctionalization; a special case of fold–unfold fusion due to Ohori and Sasano, fixed-point promotion; and an implementation technique for reduction semantics due to Danvy and Nielsen, refocusing. This work is directly applicable to transforming programs into monadic normal form.

CPS transformation of beta-redexes

The extra compaction of the most compacting CPS transformation in existence, which is due to Sabry and Felleisen, is generally attributed to (1) making continuations occur first in CPS terms and (2) classifying more redexes as administrative. We show that this extra compaction is actually independent of the relative positions of values and continuations and furthermore that it is solely due to a context-sensitive transformation of beta-redexes. We stage the more compact CPS transformation into a first-order uncurrying phase and a context-insensitive CPS transformation. We also define a context-insensitive CPS transformation that provides the extra compaction. This CPS transformation operates in one pass and is dependently typed.

Capabilitarian Sufficiency: Capabilities and Social Justice

Abstract This paper suggests an account of sufficientarianism—that is, that justice is fulfilled when everyone has enough—laid out within a general framework of the capability approach. In doing so, it seeks to show that sufficiency is especially plausible as an ideal of social justice when constructed around key capabilitarian insights such as freedom, pluralism, and attention to empirical interconnections between central capabilities. Correspondingly, we elaborate on how a framework for evaluating social justice would look when constructed in this way and give reasons for why capabilitarians should embrace sufficientarianism. We do this by elaborating on how capabilitarian values underpin sufficiency. On this basis, we identify three categories of central capabilities; those related to biological and physical needs, those to fundamental interests of a human agent, and those to fundamental interests of a social being. In each category, we argue, achieving sufficiency requires different distributional patterns depending on how the capabilities themselves work and interrelate. This argument adds a new dimension to the way capabilitarians think about social justice and changes how we should target instances of social justice from social-political viewpoint.

Taking health needs seriously: against a luck egalitarian approach to justice in health

In recent works, Shlomi Segall suggests and defends a luck egalitarian approach to justice in health. Concurring with G. A. Cohen’s mature position he defends the idea that people should be compensated for “brute luck”, i.e. the outcome of actions that it would be unreasonable to expect them to avoid. In his defense of the luck egalitarian approach he seeks to rebut the criticism raised by Norman Daniels that luck egalitarianism is in some way too narrow and in another too wide to uphold justice in health and health care distribution. He points out that a pluralistic outline of luck egalitarianism taking into account the moral requirement of meeting everyone’s basic needs can avoid this line of criticism. In this article I argue against the application of such pluralistic luck egalitarianism in matters of health distribution. First of all, Segall has not shown that luck egalitarianism handles well health distributions above a threshold of basic needs. Secondly, his way of avoiding Elizabeth Anderson’s abandonment objection is theoretically problematic. Finally, I argue that luck egalitarianism in general fails to acknowledge the moral foundation of health and health care as a basic human entitlement. Thus I conclude that luck egalitarianism fails to take health needs seriously and that it cannot therefore uphold justice in health.

Playing for social equality

This article claims that the protection of children’s capability for play is a central social-political goal. It provides the following three-premise argument in defense of this claim: (i) we have strong and wide-ranging normative reasons to be concerned with clusters of social deficiency; (ii) particular fertile functionings play a key role for tackling clusters of social deficiency; and finally (iii) the capability for childhood play is a crucial, ontogenetic prerequisite for the development of those particular fertile functionings. Thus, in so far as we consider it a central political goal to tackle social deficiency, we should be concerned with protection of childhood play capability. This conclusion raises new insights on the importance – for global development policy as well as for welfare states’ aim to secure social justice – of protecting children’s capability to engage in playful activities.

Pluralism and Objectivism: Cornerstones for Interpersonal Comparisons

Abstract How to make interpersonal comparisons is one of the most important questions to address in the discussion of distributive justice. This paper discusses two of the most relevant dividing lines in the literature of interpersonal comparison: between a monistic and pluralistic approach to interpersonal comparison, and between a subjectivist and objectivist standard of interpersonal comparison. The paper provides a normative argument for pluralism and objectivism with regard to interpersonal comparison, and it suggests that the Capability Approach as developed by Amartya Sen and Martha Nussbaum fits these criteria.