A short cut to deforestation

Abstract

Lists are often used as "glue" to connect separate parts of a program together.We propose an automatic teehnique for improving the efficiency of such programs, by removing many of these intermediate lists, based on a single, simple, local transformation.We have implemented the method in the Glasgow Haakell compiler.

Build, Augment and Destroy, Universally

“…The methodology of shortcut deforestation [10] proposes reprogramming summation, mapping and list generation in terms of foldr and a new combinator build as follows:…”

“…The method to encode inductive types via type quantification (the "impredicative encoding") was first described in [19,3]. Shortcut deforestation for lists was proposed by Gill et al [11,10]. For general inductive types, it was defined in [30,18].…”

“…Recently there has been a significant amount of work starting with Gill et al's [10] which proposes an alternative approach to programming with inductive types based on using the fold-combinator in interaction with a new constructor called build. This is related to deforestation [35] or semanticspreserving program transformation to eliminate intermediate datastructures.…”

A Supercompiler for Core Haskell

“…This technique has been extended in many ways to encompass higher order deforestation [10] and work on other data types [3]. Probably the most practically motivated work has come from those attempting to restrict deforestation, in particular shortcut deforestation [5], and newer approaches such as stream fusion [2]. In this work certain named functions are automatically fused together.…”

Datatype-Generic Programming

“…There exist slight generalizations of unfolds, such as monadic unfolds [109,110], apomorphisms [130] and futumorphisms [127], but these still all conform to the same structural scheme, and not all programs that generate data structures fit this scheme [46]. Gill et al [50] introduced an operator they called build for unstructured generation of data, in order to simplify the implementation and broaden the applicability of deforestation optimizations as discussed in the previous section. During the Spring School, Malcolm Wallace proposed the alternative term 'tectomorphism' for build , maintaining the Greek naming theme.…”