Optimizing SYB is easy!

Abstract

The most widely used generic-programming system in the Haskell community, Scrap Your Boilerplate (SYB), also happens to be one of the slowest. Generic traversals in SYB are often an order of magnitude slower than equivalent handwritten, non-generic traversals. Thus while SYB allows the concise expression of many traversals, its use incurs a significant runtime cost. Existing techniques for optimizing other generic-programming systems are not able to eliminate this overhead.This paper presents an optimization that completely eliminates this cost. Essentially, it is a partial evaluation that takes advantage of domain-specific knowledge about the structure of SYB. It optimizes SYB-style traversals to be as fast as handwritten, non-generic code, and benchmarks show that this optimization improves the speed of SYB-style code by an order of magnitude or more.

Reasoning with the HERMIT: tool support for equational reasoning on GHC core programs

“…In the past [1,13] HERMIT has been used to successfully prototype GHC optimisations by encoding them as sequences of transformation steps. Now HERMIT can also be used to reason about any preconditions those transformation steps have, as well as to provide mechanical assurances about equational reasoning that is intended to prove properties of Haskell programs, including typeclass laws for instance declarations, and user optimisations stated as GHC rewrite rules.…”

“…The initial HERMIT implementation [12] only supported equational reasoning that was transformational in nature; that is, HER-MIT allowed the user to apply a sequence of correctness-preserving transformations to the Haskell program, resulting in an equivalent but (hopefully) more efficient program. This was sufficient to allow some specific instances of known program transformations to be mechanised [33], as well as for encoding prototypes of new optimisation techniques [1,13]. However, some of the transformation steps used were only valid in certain contexts, and HERMIT had no facility for checking the necessary preconditions.…”

Staged sums of products

“…From there, the features of MetaOCaml [Kiselyov 2014] are used for staging. Adams et al [2014] use the HERMIT plugin [Farmer 2015] as a different approach to optimising SYB. This works by interacting with GHC during the compilation process using a combination of supercompilation [Turchin 1979] and partial evaluation [Jones et al 1993].…”

Generic deriving of generic traversals

“…[Magalhães 2012] also considered the behaviour of the optimiser on similar generic programs. Adams et al [2014] analysed the poor performance of SYB and performed code optimizations expressed in HERMIT [Farmer 2015] to get better performance, in their extended work they show how this can be done with an adapted version of GHC [Adams et al 2015]. A large problem with their approach is that they rely on being able to symbolically evaluate type casts.…”