This Is Driving Me Loopy: Efficient Loops in Arrowized Functional Reactive Programs

Arrowized Functional Reactive Programming (AFRP) is one approach to writing reactive programs declaratively, based on the arrows abstraction in Haskell. While AFRP elegantly expresses the relationships between inputs and outputs of a reactive system, na'ive implementations suffer from poor performance. In particular, the loop combinator depends on lazy semantics: this inflicts the overheads of lazy evaluation and simultaneously prevents existing optimisation techniques from being applied to it. We present a novel program transformation which utilises the Arrow and ArrowLoop laws to transform typical uses of loop into restricted forms that have an execution order that is known at compile-time and therefore can be executed strictly. We evaluate the performance gained from our transformations and prove that the transformations are correct.