Verified Calculations

Calculational proofs—proofs by stepwise formula manipulation—are praised for their rigor, readability, and elegance. It seems desirable to reuse this style, often employed on paper, in the context of mechanized reasoning, and in particular, program verification. This work leverages the power of SMT solvers to machine-check calculational proofs at the level of detail they are usually written by hand. It builds the support for calculations into the programming language and auto-active program verifier Dafny. The paper demonstrates that calculations integrate smoothly with other language constructs, producing concise and readable proofs in a wide range of problem domains: from mathematical theorems to correctness of imperative programs. The examples show that calculational proofs in Dafny compare favorably, in terms of readability and conciseness, with arguments written in other styles and proof languages.