Strictness analysis is critical to efficient implementation of languages with non-strict evaluation, mitigating much of the performance overhead of laziness. However, reasoning about strictness at the source level can be challenging and unintuitive. We propose a new definition of strictness that refines the traditional one by describing variable usage more precisely. We lay type-theoretic foundations for this definition in both call-by-name and call-by-push-value settings, drawing inspiration from the literature on type systems tracking effects and coeffects. We prove via a logical relation that the strictness attributes computed by our type systems accurately describe the use of variables at runtime, and we offer a strictness-annotation-preserving translation from the call-by-name system to the call-by-push-value one. All our results are mechanized in Rocq.

@article{paperbot3879,
  title = {Typing Strictness},
  author = {Daniel Sainati and Joseph W. Cutler and Benjamin C. Pierce and Stephanie Weirich},
  journal = {Proceedings of the ACM on Programming Languages},
  volume = {10},
  number = {POPL},
  year = {2026},
  doi = {10.1145/3776657}
}