Generating Well-Typed Terms That Are Not “Useless”

Random generation of well-typed terms lies at the core of effective random testing of compilers for functional languages. Existing techniques have had success following a top-down type-oriented approach to generation that makes choices locally, which suffers from an inherent limitation: the type of an expression is often generated independently from the expression itself. Such generation frequently yields functions with argument types that cannot be used to produce a result in a meaningful way, leaving those arguments unused. Such "use-less" functions can hinder both performance, as the argument generation code is dead but still needs to be compiled, and effectiveness, as a lot of interesting optimizations are tested less frequently. In this paper, we introduce a novel algorithm that is significantly more effective at generating functions that use their arguments. We formalize both the "local" and the "nonlocal" algorithms as step-relations in an extension of the simply-typed lambda calculus with type and arguments holes, showing how delaying the generation of types for subexpressions by allowing nonlocal generation steps leads to "useful" functions.