Factorising folds for faster functions

October 16th, 2009 by Graham

When I visited Galois a couple of years ago, I worked with Andy Gill on formalising the worker/wrapper transformation that is used within GHC to improve the performance of programs. The theory that we came up with was simple and general, but was based upon modelling recursive programs in terms of a fixed point operater, and hence wasn’t able to exploit additional structure that may be present in many programs.

In today’s talk (based upon joint work with Mauro Jaskelioff and Andy Gill), I showed how the worker/wrapper theory can be specialised to the structured pattern of recursion encapsulated by fold operators, and discussed the resulting benefits. I gave one simple example, deriving fast reverse, and one more complicated example, verifying the correctness of an efficient implementation of substitution from Voigtlander’s MPC 2008 paper. Further details are in our new paper.

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