Learning in the limit with lattice-structured hypothesis spaces

We define a collection of language classes which are TxtEx-learnable (learnable in the limit from positive data). The learners map any data input to an element of a fixed lattice, and keep the least upper bound of all lattice elements thus obtained as the current hypothesis. Each element of the lattice is a grammar for a language, and the learner climbs the lattice searching for the right element. We call these classes in our collection lattice classes. We provide several characterizations of lattice classes and their learners, which suggests they are very natural. In particular, we show that any class of languages is a lattice class iff it is TxtEx-learnable consistently, conservatively, set-drivenly, and strongly monotonically. We show several language classes previously discussed in the literature to be lattice classes, including the locally k-testable classes, the piecewise k-testable classes, the k-reversible languages and the pattern languages. We also show that lattice classes contain three previously known collections of language classes: string extension language classes, function-distinguishable language classes, and closed-set systems. Finally, the lattice perspective helps analyze the learning of these classes. Illustrations include query-learning results in dependence on the lattice structure, characterizations of closure properties and the VC-dimension of lattice classes in terms of lattice properties.