Many synthesizers implicitly benefit from using polymorphic types, since parametric polymorphism reduces the search space. Additional synthesis constraints may interfere with parametricity. In particular, a polymorphic type may cause otherwise feasible input-output examples to contradict each other. We present Taxi (type-and-example based inferencer), a tool for efficiently reasoning about the feasibility of polymorphic programs specified by input-output examples, and Driver, a tactic language for top-down program synthesis that uses feasibility reasoning to prune the search space. Taxi guarantees that every search state corresponds to a correct (albeit possibly partial) implementation. In addition, it allows for shortcutting the synthesis when a subspecification covers all cases. We show that these techniques have the potential to speed up top-down enumerative type-and-example driven synthesizers.

Parametricity states that polymorphic functions behave the same regardless of how they are instantiated. When developing polymorphic programs, Wadler’s free theorems can serve as free specifications, which can turn otherwise partial specifications into total ones, and can make otherwise realizable specifications unrealizable. This is of particular interest to the field of program synthesis, where the unrealizability of a specification can be used to prune the search space. In this paper, we focus on the interaction between parametricity, input-output examples, and sketches. Unfortunately, free theorems introduce universally quantified functions that make automated reasoning difficult. Container morphisms provide an alternative representation for polymorphic functions that captures parametricity in a more manageable way. By using a translation to the container setting, we show how reasoning about the realizability of polymorphic programs with input-output examples can be automated.