Type inference is essential for statically-typed languages such as OCaml and Haskell. It can be decomposed into two (possibly interleaved) phases: a generator converts programs to constraints; a solver decides whether a constraint is satisfiable. Elaboration, the task of decorating a program with explicit type annotations, can also be structured in this way. Unfortunately, most machine-checked implementations of type inference do not follow this phase-separated, constraint-based approach. Those that do are rarely executable, lack effectful abstractions, and do not include elaboration.

To close the gap between common practice in real-world implementations and mechanizations inside proof assistants, we propose an approach that enables modular reasoning about monadic constraint generation in the presence of elaboration. Our approach includes a domain-specific base logic for reasoning about metavariables, and a program logic that allows us to reason abstractly about the meaning of constraints. To evaluate it, we report on a machine-checked implementation of our techniques inside the Coq proof assistant. As a case study, we verify both soundness and completeness for three type inferencers for the simply typed 𝜆-calculus with Booleans. Our results are the first demonstration that type inference algorithms can be verified in the same form as they are implemented in practice: in an imperative style, modularly decomposed into constraint generation and solving, and delivering elaborated terms to the remainder of the compiler chain.