On Synergies between Type Inference, Generation and Normalization of SK-Combinator Trees

The S and K combinator expressions form a well-known Turing-complete subset of the lambda calculus. Using Prolog as a meta-language, we specify evaluation, type inference and combinatorial generation algorithms for SK-combinator trees. In the process, we unravel properties shedding new light on interesting aspects of their structure and distribution. We study the proportion of well-typed terms among size-limited SK-expressions as well as the type-directed generation of terms of sizes smaller than the size of their simple types. We also introduce the well-typed frontier of an untypable term and we use it to design a simplification algorithm for untypable terms taking advantage of the fact that well-typed terms are normalizable.