Every accessible category is proved to be sketchable by a sketch with finite colimits. In contrast, a finitely accessible category is presented that cannot be sketched by a finitary sketch, i.e., a sketch with finite limits and finite colimits. Also, a category sketchable by a finitary sketch is found that is not finitely accessible.

We propose a new framework for representing logics, called LF+, which is based on the Edinburgh Logical Framework. The new framework allows us to give, apparently for the first time, general definitions that capture how well a logic has been represented. These definitions are possible because we are able to distinguish in a generic way that part of the LF+ entailment corresponding to the underlying logic. This distinction does not seem to be possible with other frameworks. Using our definitions, we show that, for example, natural deduction first-order logic can be well-represented in LF+, whereas linear and relevant logics cannot. We also show that our syntactic definitions of representation have a simple formulation as indexed isomorphisms, which both confirms that our approach is a natural one and provides a link between type-theoretic and categorical approaches to frameworks.

A new correctness criterion for discriminating Proof Nets among Proof Structures of Multiplicative Linear Logic with the MIX rule is provided. This criterion is inspired by an original interpretation of Proof Structures as distributed systems, and logical formulae as processes. The computation inside a system corresponds to the logical flow of information inside a proof, that is, roughly speaking, a distributed version of Girard's token trip. Proof Nets are then characterised as deadlock free Proof Structures (deadlock free distributed systems). This result follows by explicitly considering the causal dependencies among logical formulae inside proofs, and it provides a new understanding of notions such as acyclicity, chains and empires in terms of concurrent computations.

Tries, a form of string-indexed look-up structure, are generalized, in a manner first discovered by Wadsworth, to permit indexing by terms built according to an arbitrary signature. The construction is parametric with respect to the type of data to be stored as values; this is essential, because the recursion that defines tries appeals from one value type to others. ‘Trie’ (for any fixed signature) is then a functor, and the corresponding look-up function is a natural isomorphism.

The trie functor is in principle definable by the ‘initial fixed point’ semantics of Smyth and Plotkin. We simplify the construction, however, by introducing the ‘category-cpo’, a class of category within which calculations can retain some domain-theoretic flavor. Our construction of tries extends easily to many-sorted signatures.

We study a type system with a notion of subtyping that involves a largest type ⊤, a smallest type ⊥, atomic coercions between base types, and the usual ordering of function types. We prove that any λ-term typable in this system is strongly normalizing, which solves an open problem of Thatte. We also prove that the fragment without ⊥ types has strictly fewer terms. This demonstrates that ⊥ adds power to a type system.

A notion of bilattice was first proposed by Ginsberg as a general framework for many applications. Related notions were further investigated and applied for various goals by Fitting. In the present paper a general definition of bilattices is proposed, which covers all particular cases that have actually been used in the literature. It is shown also that in the finite case every bilattice in our sense is graphically representable by a special type of a two-dimensional diagram.