The proof-assistant Yarrow

Yarrow is an interactive proof assistant based on the theory of Pure Type Systems, a family of typed lambda calculi. Yarrow has been designed as a flexible environment for experimentation with various typed lambda calculi. It offers both graphical and textual interfaces. It has been coded entirely in Haskell, making use of the Fudget library for the graphical interface. In this paper we concentrate on the software architecture of Yarrow, in particular the use of monads, the coupling of user interface and proof engine, polymorphic output routines, and flexible representations of lambda terms. We also treat the presentation of proofs in the flag-style format.