Complete and Decidable Type Inference for GADTs

Tom Schrijvers ( K U Leuven, visiting Microsoft Research Cambridge )

GADTs have proven to be an invaluable language extension, for ensuring data invariants and program correctness among others. Unfortunately, they pose a tough problem for type inference: we lose the principal-type property, which is necessary for modular type inference. We present a novel and simplified type inference approach for local type assumptions from GADT pattern matches. Our approach is complete and decidable, while more liberal than previous such approaches. (Just work with S. Peyton Jones, M. Sulzmann, D. Vytiniotis.)