Typed Contracts for Functional Programming

A robust software component fulfills a contract: it expects data satisfying a certain property and promises to return data satisfying another property. The object-oriented community uses the design-by-contract approach extensively. Proposals for language extensions that add contracts to higher-order functional programming have appeared recently. In this paper we propose an embedded domain-specific language for typed, higher-order and first-class contracts, which is both more expressive than previous proposals, and allows for a more informative blame assignment. We take some first steps towards an algebra of contracts, and we show how to define a generic contract combinator for arbitrary algebraic data types. The contract language is implemented as a library in Haskell using the concept of generalised algebraic data types.