This paper presents an automatic deforestation system, stream fusion, based on equational transformations, that fuses a wider range of functions than existing short-cut fusion systems. In particular, stream fusion is able to fuse zips, left folds and functions over nested lists, including list comprehensions. A distinguishing feature of the framework is its simplicity: by transforming list functions to expose their structure, intermediate values are eliminated by general purpose compiler optimisations. We have reimplemented the Haskell standard List library on top of our framework, providing stream fusion for Haskell lists. By allowing a wider range of functions to fuse, we see an increase in the number of occurrences of fusion in typical Haskell programs. We present benchmarks documenting time and space improvements.

Abstract. Haskell is a functional language, with features such as higher order functions and lazy evaluation, which allow succinct programs. These high-level features present many challenges for optimising compilers. We report practical experiments using novel variants of supercompilation, with special attention to let bindings and the generalisation technique. 1

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We study various shortcut fusion rules for languages like Haskell. Following a careful semantic account of a recently proposed rule for circular program transformation, we propose a new rule that trades circularity for higher-orderedness, and thus attains better semantic properties. This also leads us to revisit the original foldr/build-rule, as well as its dual, and to develop variants that do not suffer from detrimental impacts of Haskell’s mixed strict/nonstrict semantics. Throughout, we offer pragmatic insights about our new rules to investigate also their relative effectiveness, rather than just their semantic correctness.

Supercompilation is a technique due to Turchin [1] which allows for the construction of program optimisers that are both simple and extremely powerful. Supercompilation is capable of achieving transformations such as deforestation [2], function specialisation and constructor specialisation [3]. Inspired by Mitchell’s promising results [4], we show how the call-by-need supercompilation algorithm can be recast to be based explicitly on an evaluator, and in the process extend it to deal with recursive let expressions.

We present a novel approach to regular, multi-dimensional arrays in Haskell. The main highlights of our approach are that it (1) is purely functional, (2) supports reuse through shape polymorphism, (3) avoids unnecessary intermediate structures rather than relying on subsequent loop fusion, and (4) supports transparent parallelisation. We show how to embed two forms of shape polymorphism into Haskell’s type system using type classes and type families. In particular, we discuss the generalisation of regular array transformations to arrays of higher rank, and introduce a type-safe specification of array slices. We discuss the runtime performance of our approach for three standard array algorithms. We achieve absolute performance comparable to handwritten C code. At the same time, our implementation scales well up to 8 processor cores. Categories and Subject Descriptors D.3.3 [Programming Languages]: Language Constructs and Features—Concurrent programming structures; Polymorphism; Abstract data types

Supercompilation is a powerful technique for program optimisation and theorem proving. In this paper we describe and evaluate three improvements to the Cambridge Haskell Supercompiler (CHSC). We reduce supercompiled program size by the use of a weak normaliser and aggressive rollback, and we improve the performance of supercompiled programs by heap speculation and generalisation. Our generalisation method is simpler than those in the literature, and is better at generalising computations involving primitive operations such as those on machine integers. We also provide the first comprehensive account of the tag-bag termination mechanism.

Abstract. Haskell is a functional language, with features such as higher order functions and lazy evaluation, which allow succinct programs. These high-level features are difficult for fast execution, but GHC is a mature and widely used optimising compiler. This paper presents a wholeprogram approach to optimisation, which produces speed improvements of between 10 % and 60 % when used with GHC, on eight benchmarks. 1

Abstract. Pure functional programming language offer many advantages over impure languages. Unfortunately, the absence of destructive update, imposes a complexity barrier. In imperative languages, there are algorithms and data structures with better complexity. We present our project for combining existing program transformation techniques to transform inefficient pure data structures into impure ones with better complexity. As a consequence, the programmer is not exposed to the impurity and retains the advantages of purity. 1

We describe a transformation which takes a higher-order program, and produces an equivalent first-order program. Unlike Reynoldsstyle defunctionalisation, it does not introduce any new data types, and the results are more amenable to subsequent analysis operations. We can use our method to improve the results of existing analysis operations, including strictness analysis, pattern-match safety and termination checking. Our transformation is implemented, and works on a Core language to which Haskell programs can be reduced. Our method cannot always succeed in removing all functional values, but in practice is remarkably successful. D.3 [Software]: Program-