Coinduction inductively: mechanizing coinductive proofs in Liquid Haskell

Abstract

Liquid Haskell is an inductive verifier that cannot reason about codata. In this work we present two alternative approaches, namely indexed and constructive coinduction, to consistently encode coinductive proofs in Liquid Haskell. The intuition is that indices can be used to enforce the base case in the setting of classical logic and the guardedness check in the constructive proofs. We use our encodings to machine check 10 coinductive proofs, about unary and binary predicates on infinite streams and lists, showcasing how an inductive verifier can be used to check coinductive properties of Haskell code.