Reflection without Remorse: Revealing a hidden sequence to speed up monadic reflection

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• Oleg Kiselyov
• Monday 09 February 2015, 15:00-16:00
• SS03.

If you have a question about this talk, please contact Raphael Proust.

A series of list appends or monadic binds for many monads performs algorithmically worse when left-associated. Continuation-passing style (CPS) is well-known to cure this severe dependence of performance on the association pattern. The advantage of CPS dwindles or disappears if we have to examine or modify the intermediate result of that series of appends or binds, before continuing the series. Such examination is frequently needed, for example, to control search in non-determinism monads.

We present an alternative approach that is just as general as CPS but more robust: it makes series of binds and other such operations efficient regardless of the association pattern—and also provides efficient access to intermediate results. The key is to represent such a conceptual sequence as an efficient sequence data structure. Efficient sequence data structures from the literature are homogeneous and cannot be applied as they are in a type-safe way to series of monadic binds. We generalize them to type aligned sequences and show how to construct their (assuredly order-preserving) implementations. We demonstrate that our solution solves previously undocumented, severe performance problems in iteratees, LogicT transformers, free monads and extensible effects.

This talk is part of the Computer Laboratory Programming Research Group Seminar series.

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