# Fibrational Induction Meets Effects

## Publication Information

Robert Atkey, Neil Ghani, Bart Jacobs, and Patricia Johann.

*Fibrational Induction Meets Effects*. In Lars Birkedal, editor,

*Foundations of Software Science and Computational Structures*, volume 7213 of

*Lecture Notes in Computer Science*, pages 42-57. Springer, 2012.

DOI:

10.1007/978-3-642-28729-9_3## Abstract

This paper provides several induction rules that can be used to prove properties of effectful data types. Our results are semantic in nature and build upon Hermida and Jacobs' fibrational formulation of induction for polynomial data types and its extension to all inductive data types by Ghani, Johann, and Fumex. An effectful data type *μ (TF)* is built from a functor *F* that describes data, and a monad *T* that computes effects. Our main contribution is to derive induction rules that are generic over *all* functors *F* and monads *T* such that *μ (TF)* exists. Along the way, we also derive a principle of definition by structural recursion for effectful data types that is similarly generic. Our induction rule is also generic over the kinds of properties to be proved: like the work on which we build, we work in a general fibrational setting and so can accommodate very general notions of properties, rather than just those of particular syntactic forms. We give examples exploiting the generality of our results, and show how our results specialize to those in the literature, particularly those of Filinski and Støvring.

## Additional Information

I wrote a blog post explaining this paper in terms of stream processing in Haskell. A more accessible account of how to use the results of this paper for reasoning about Haskell programs is in our paper Interleaving data and effects.