Profinite trees, through monads and the lambda-calculus

In its simplest form, the theory of regular languages is the study of sets of finite words recognized by finite monoids. The finiteness condition on monoids gives rise to a topological space whose points, called profinite words, encode the limiting behavior of words with respect to finite monoids. Yet, some aspects of the theory of regular languages are not particular to monoids and can be described in a general setting. On the one hand, Boja\'{n}czyk has shown how to use monads to generalize the theory of regular languages and has given an abstract definition of the free profinite structure, defined by codensity, given a fixed monad and a notion of finite structure. On the other hand, Salvati has introduced the notion of language of $\lambda$-terms, using denotational semantics, which generalizes the case of words and trees through the Church encoding. In recent work, the author and collaborators defined the notion of profinite $\lambda$-term using semantics in finite sets and functions, which extend the Church encoding to profinite words. In this article, we prove that these two generalizations, based on monads and denotational semantics, coincide in the case of trees. To do so, we consider the monad of abstract clones which, when applied to a ranked alphabet, gives the associated clone of ranked trees. This induces a notion of free profinite clone, and hence of profinite trees. The main contribution is a categorical proof that the free profinite clone on a ranked alphabet is isomorphic, as a Stone-enriched clone, to the clone of profinite $\lambda$-terms of Church type. Moreover, we also prove a parametricity theorem on families of semantic elements which provides another equivalent formulation of profinite trees in terms of Reynolds parametricity

The Safe Lambda Calculus

Safety is a syntactic condition of higher-order grammars that constrains occurrences of variables in the production rules according to their type-theoretic order. In this paper, we introduce the safe lambda calculus, which is obtained by transposing (and generalizing) the safety condition to the setting of the simply-typed lambda calculus. In contrast to the original definition of safety, our calculus does not constrain types (to be homogeneous). We show that in the safe lambda calculus, there is no need to rename bound variables when performing substitution, as variable capture is guaranteed not to happen. We also propose an adequate notion of beta-reduction that preserves safety. In the same vein as Schwichtenberg's 1976 characterization of the simply-typed lambda calculus, we show that the numeric functions representable in the safe lambda calculus are exactly the multivariate polynomials; thus conditional is not definable. We also give a characterization of representable word functions. We then study the complexity of deciding beta-eta equality of two safe simply-typed terms and show that this problem is PSPACE-hard. Finally we give a game-semantic analysis of safety: We show that safe terms are denoted by `P-incrementally justified strategies'. Consequently pointers in the game semantics of safe lambda-terms are only necessary from order 4 onwards