Programs under construction routinely contain incomplete code paths, yet no testing framework—to our knowledge—distinguishes incompleteness from failure. We propose incremental property-based testing (IPBT): a framework can be both tolerant of incompleteness, so partial code does not produce spurious failures, and informative about it, reporting which incomplete cases are exercised and how often, supporting prioritization of next implementation steps.

IPBT requires no language extension and only slight adjustments to an existing framework; the reformulation is entirely hidden from the user. For experimentation, we instantiate IPBT in QCheck2 for OCaml, and evaluate it on a lambda-calculus substitution example and the multicoretests suite for the OCaml 5 compiler.