46 research outputs found
Strict Ideal Completions of the Lambda Calculus
The infinitary lambda calculi pioneered by Kennaway et al. extend the basic
lambda calculus by metric completion to infinite terms and reductions.
Depending on the chosen metric, the resulting infinitary calculi exhibit
different notions of strictness. To obtain infinitary normalisation and
infinitary confluence properties for these calculi, Kennaway et al. extend
-reduction with infinitely many `-rules', which contract
meaningless terms directly to . Three of the resulting B\"ohm reduction
calculi have unique infinitary normal forms corresponding to B\"ohm-like trees.
In this paper we develop a corresponding theory of infinitary lambda calculi
based on ideal completion instead of metric completion. We show that each of
our calculi conservatively extends the corresponding metric-based calculus.
Three of our calculi are infinitarily normalising and confluent; their unique
infinitary normal forms are exactly the B\"ohm-like trees of the corresponding
metric-based calculi. Our calculi dispense with the infinitely many
-rules of the metric-based calculi. The fully non-strict calculus (called
) consists of only -reduction, while the other two calculi (called
and ) require two additional rules that precisely state their
strictness properties: (for ) and (for and )
Confluence of nearly orthogonal infinitary term rewriting systems
We give a relatively simple coinductive proof of confluence, modulo
equivalence of root-active terms, of nearly orthogonal infinitary
term rewriting systems. Nearly orthogonal systems allow certain root
overlaps, but no non-root overlaps. Using a slightly more complicated method we also show confluence modulo equivalence of
hypercollapsing terms. The condition we impose on root overlaps is
similar to the condition used by Toyama in the context of finitary
rewriting
On Undefined and Meaningless in Lambda Definability
We distinguish between undefined terms as used in lambda definability
of partial recursive functions and meaningless terms as used in
infinite lambda calculus for the infinitary terms models that
generalise the Bohm model. While there are uncountable many known
sets of meaningless terms, there are four known sets of undefined
terms. Two of these four are sets of meaningless terms.
In this paper we first present set of sufficient conditions for a set
of lambda terms to serve as set of undefined terms in lambda
definability of partial functions. The four known sets of undefined
terms satisfy these conditions.
Next we locate the smallest set of meaningless terms satisfying these
conditions. This set sits very low in the lattice of all sets of
meaningless terms. Any larger set of meaningless terms than this
smallest set is a set of undefined terms. Thus we find uncountably
many new sets of undefined terms.
As an unexpected bonus of our careful analysis of lambda definability
we obtain a natural modification, strict lambda-definability, which
allows for a Barendregt style of proof in which the representation of
composition is truly the composition of representations
Glueability of Resource Proof-Structures: Inverting the Taylor Expansion
A Multiplicative-Exponential Linear Logic (MELL) proof-structure can be expanded into a set of resource proof-structures: its Taylor expansion. We introduce a new criterion characterizing those sets of resource proof-structures that are part of the Taylor expansion of some MELL proof-structure, through a rewriting system acting both on resource and MELL proof-structures