Haskell 2024: Proceedings of the 17th ACM SIGPLAN International Haskell Symposium

Many Haskell textbooks explain the evaluation of pure functional programs as a process of stepwise rewriting using equations. However, usual implementation techniques perform program transformations that make producing the corresponding tracing evaluations difficult. This paper presents a tracing interpreter for a subset of Haskell based on the pattern matching calculus of Kahl. We start from a big-step semantics in the style of Launchbury and develop a small-step semantics in the style of Sestoft's machines. This machine is used in the implementation of a step-by-step educational interpreter. We also discuss some implementation decisions and present illustrative examples.

GHC’s rewrite rules enable programmers to write local program transformation rules for their own functions. The most notable use case are fusion optimizations, which merge multiple traversals of a data structure into one and avoids allocation of intermediate structures. However, GHC’s rewrite rules were too limited to express a rewrite rule that is necessary for proper fusion of the concatMap function in a variant of fusion called stream fusion. We introduce higher order patterns as a simple extension of GHC’s rewrite rules. Higher order patterns substantially broaden the expressiveness of rewrite rules that involve local variables. In particular, they enable the rewriting of concatMap such that it can be properly optimized. We implement a stream fusion framework to replace the existing fusion framework in GHC and evaluate it on GHC’s benchmark suite. Our stream fusion framework with higher order patterns shows an average speedup of 7% compared to our stream fusion framework without it. However, our stream fusion framework is not yet able to match the performance of GHC’s existing fusion framework. Additionally, we show that our higher order patterns can be used to simplify GHC’s existing mechanism for rolling back failed attempts at fusion and, at the same time, solve a problem that prevented proper optimization in one example program. However, evaluating it on GHC’s benchmark suite shows a small regression in performance overall.

Writing distributed applications is hard, as the programmer needs to describe the communication protocol between the different endpoints. If this is not done correctly, we can introduce bugs such as deadlocks and data races. Tierless and choreographic programming models aim to make this easier by describing the interactions of every endpoint in a single compilation unit. When such a program is compiled, ideally, a single endpoint is projected and the code for the other endpoints is removed. This leads to smaller binaries with fewer dependencies, and is called program partitioning. In this pearl, we show how we can use rewrite rules and specialisation to get GHC to partition our Haskell programs (almost) for free, if they are written using the Haste App or HasChor framework. As an example of why partitioning is useful, we show how an example application can be more easily built and deployed after being partitioned.

Most functional language runtime systems context switch between executing user code and a non-concurrent garbage collector (GC), exposing GC latency to overall wall-clock time. Recent concurrent software-based GCs reduce these latencies, but wall-clock times are instead increased due to their synchronisation and write barrier overheads, by as much as 21%. This GC overhead is exacerbated further for pure non-strict languages like Haskell, due to the abundance of allocations for storing immutable data structures and closures. This paper presents Cloaca, an FPGA-based hardware implementation of a concurrent, hybrid GC for a pure non-strict functional language. It combines mark-and-sweep tracing and one-bit reference counting. It traces live heap data using hardware-level synchronisation and write barriers, without damaging graph reduction performance. To ensure the correctness of Cloaca, three invariants of its Haskell-based implementation are verified with property-based testing. Despite GHC running on an Intel i7 CPU operating at a x25 higher clock frequency than Cloaca; Cloaca takes, on average, 4.1% of GHC's GC wall-clock time across 14 of 15 benchmarks.

At present, the most efficient implementations of Arrowized Functional Reactive Programming (AFRP), such as Scalable FRP (SFRP), do not support the loop combinator. This prevents us from expressing fundamental programs in such implementations, which limits AFRP’s use in domains where both performance and expressivity are required. We introduce Oxbow, which extends SFRP with support for the loop combinator by leveraging an improved variant of the rearrange technique. In benchmarks, Oxbow performs at least 2.2x better than Yampa, an AFRP implementation that supports the loop combinator.

Despite the intended use for prototyping or as a first step towards giving semantics to a new programming language, interpreters are often monolithic and difficult to extend. Higher-order effects and handlers promise modular composition of handlers and semantics; in practice, however, they have been mostly applied to toy examples. We present an approach that successfully combines algebraic, scoped, and latent effects into a handler pipeline for interpreting the functional logic programming language Curry. We show which effects make up Curry, discuss the infrastructure needed to combine effects of different classes and shed light on how these effects interact in order to lessen the barrier to entry for using effects in practical projects.

Improving Floating-Point Numbers (IFN) is a numerical computation library for Haskell. It allows the user to directly specify the accuracy of the result, making accurate computation easy. The library's computation process is based on adaptive control of accuracies, which propagates the demands for more accurate values from an expression to its appropriate subexpressions. However, despite its unique features, programs utilizing the IFN library often encounter efficiency issues regarding memory consumption and execution time due to its fine granularity of computations. This paper presents the computational granularity control mechanism through fusion transformation to resolve these problems and proposes two fusion strategies, the maximal fusion and chain fusion. We have successfully implemented a fusion system that automatically applies these strategies through program transformation. Its effectiveness was confirmed through numerical computation programs.

Formal reasoning about the time complexity of algorithms and data structures is usually done in interactive theorem provers like Isabelle/HOL. This includes reasoning about amortized time complexity which looks at the worst case performance over a series of operations. However, most programs are not written within a theorem prover and thus use the data structures of the production language. To verify the correctness it is necessary to translate the data structures from the production language into the language of the prover. Such a translation step could introduce errors, for example due to a mismatch in features between the two languages. We show how to prove amortized complexity of data structures directly in Haskell using LiquidHaskell. Besides skipping the translation step, our approach can also provide a didactic advantage. Learners do not have to learn an additional language for proofs and can focus on the new concepts only. For this paper, we do not assume prior knowledge of amortized complexity as we explain the concepts and apply them in our first case study, a simple stack with multipop. Moving to more complicated (and useful) data structures, we show that the same technique works for binomial heaps which can be used to implement a priority queue. We also prove amortized complexity bounds for Claessen’s version of the finger tree, a sequence-like data structure with constant-time cons/uncons on either end. Finally we discuss the current limitations of LiquidHaskell that made certain versions of the data structures not feasible.

Much work in the area of compiler calculation has focused on pure languages. While this simplifies the reasoning, it reduces the applicability. In this article, we show how an existing compiler calculation methodology can be naturally extended to languages with side effects. We achieve this by exploiting an algebraic approach to effects, which keeps the reasoning simple and provides flexibility in how effects are interpreted. To make the ideas accessible we only use elementary functional programming techniques.

MicroHs is a compiler for Haskell2010. It translates Haskell to SKI style combinators via λ-calculus. The runtime system is quite small with few dependencies.