Haskell Compiler Research Articles

Revisiting software transactional memory in Haskell

Software Transactional Memory (STM) has become very popular in Haskell. Currently, there are nearly 500 packages on Haskell’s package archive that directly use STM. Despite the widespread use in real world applications, Haskell’s STM implementation has seen very few updates since its introduction in 2005. In this work, we describe our efforts to redesign the STM implementation in the Glasgow Haskell Compiler (GHC), based on a TL2-like implementation that is able to support both <code>orElse</code> and <code>retry</code> without the use of traditional nested transactions. We argue that our implementation is simpler than the current GHC implementation while supporting opacity. We also demonstrate that our implementation performs better than the current GHC implementation on a number of benchmarks by multiple orders of magnitude for long-running transactions. In an effort to measure the performance of <code>orElse</code> and <code>retry</code>, we present an STM-based work stealing scheduler. With <code>orElse</code> and <code>retry</code>, we are able to elegantly implement the scheduler in just a few lines of code. We have modified the Par Monad, a real-world Haskell package that provides deterministic parallelism, to use our STM-based work stealing scheduler and show that it is not only simpler but is able to perform as well as the current scheduler.

Supermonads: one notion to bind them all

Several popular generalizations of monads have been implemented in Haskell. Unfortunately, because the shape of the associated type constructors do not match the standard Haskell monad interface, each such implementation provides its own type class and versions of associated library functions. Furthermore, simultaneous use of different monadic notions can be cumbersome as it in general is necessary to be explicit about which notion is used where. In this paper we introduce supermonads : an encoding of monadic notions that captures several different generalizations along with a version of the standard library of monadic functions that work uniformly with all of them. As standard Haskell type inference does not work for supermonads due to their generality, our supermonad implementation is accompanied with a language extension, in the form of a plugin for the Glasgow Haskell Compiler (GHC), that allows type inference for supermonads, obviating the need for manual annotations.

Non-recursive make considered harmful: build systems at scale

Most build systems start small and simple, but over time grow into hairy monsters that few dare to touch. As we demonstrate in this paper, there are a few issues that cause build systems major scalability challenges, and many pervasively used build systems (e.g. Make) do not scale well. This paper presents a solution to the challenges we identify. We use functional programming to design abstractions for build systems, and implement them on top of the Shake library, which allows us to describe build rules and dependencies. To substantiate our claims, we engineer a new build system for the Glasgow Haskell Compiler. The result is more scalable, faster, and spectacularly more maintainable than its Make-based predecessor.

Sequent calculus as a compiler intermediate language

The λ-calculus is popular as an intermediate language for practical compilers. But in the world of logic it has a lesser-known twin, born at the same time, called the sequent calculus . Perhaps that would make for a good intermediate language, too? To explore this question we designed Sequent Core, a practically-oriented core calculus based on the sequent calculus, and used it to re-implement a substantial chunk of the Glasgow Haskell Compiler.

From Push/Enter to Eval/Apply by Program Transformation

Push/enter and eval/apply are two calling conventions used in implementations of functional languages. In this paper, we explore the following observation: when considering functions with multiple arguments, the stack under the push/enter and eval/apply conventions behaves similarly to two particular implementations of the list datatype: the regular cons-list and a form of lists with lazy concatenation respectively. Along the lines of Danvy et al.'s functional correspondence between definitional interpreters and abstract machines, we use this observation to transform an abstract machine that implements push/enter into an abstract machine that implements eval/apply. We show that our method is flexible enough to transform the push/enter Spineless Tagless G-machine (which is the semantic core of the GHC Haskell compiler) into its eval/apply variant.

PAEAN: Portable and scalable runtime support for parallel Haskell dialects

AbstractOver time, several competing approaches to parallel Haskell programming have emerged. Different approaches support parallelism at various different scales, ranging from small multicores to massively parallel high-performance computing systems. They also provide varying degrees of control, ranging from completely implicit approaches to ones providing full programmer control. Most current designs assume a shared memory model at the programmer, implementation and hardware levels. This is, however, becoming increasingly divorced from the reality at the hardware level. It also imposes significant unwanted runtime overheads in the form of garbage collection synchronisation etc. What is needed is an easy way to abstract over the implementation and hardware levels, while presenting a simple parallelism model to the programmer. The PArallEl shAred Nothing runtime system design aims to provide a portable and high-level shared-nothing implementation platform for parallel Haskell dialects. It abstracts over major issues such as work distribution and data serialisation, consolidating existing, successful designs into a single framework. It also provides an optional virtual shared-memory programming abstraction for (possibly) shared-nothing parallel machines, such as modern multicore/manycore architectures or cluster/cloud computing systems. It builds on, unifies and extends, existing well-developed support for shared-memory parallelism that is provided by the widely used GHC Haskell compiler. This paper summarises the state-of-the-art in shared-nothing parallel Haskell implementations, introduces the PArallEl shAred Nothing abstractions, shows how they can be used to implement three distinct parallel Haskell dialects, and demonstrates that good scalability can be obtained on recent parallel machines.

Composable scheduler activations for Haskell

AbstractThe runtime for a modern, concurrent, garbage collected language like Java or Haskell is like an operating system: sophisticated, complex, performant, but alas very hard to change. If more of the runtime system were in the high-level language, it would be far more modular and malleable. In this paper, we describe a novel concurrency substrate design for the Glasgow Haskell Compiler that allows multicore schedulers for concurrent and parallel Haskell programs to be safely and modularly described as libraries in Haskell. The approach relies on abstracting the interface to the user-implemented schedulers through scheduler activations, together with the use of Software Transactional Memory to promote safety in a multicore context.

Formally proving a compiler transformation safe

We prove that the Call Arity analysis and transformation, as implemented in the Haskell compiler GHC, is safe, i.e. does not impede the performance of the program. We formalized syntax, semantics, the analysis and the transformation in the interactive theorem prover Isabelle to obtain a machine-checked proof and hence a level of rigor rarely obtained for compiler optimization safety theorems. The proof is modular and introduces trace trees as a suitable abstraction in abstract cardinality analyses. We discuss the breadth of the formalization gap.

A typechecker plugin for units of measure: domain-specific constraint solving in GHC Haskell

Typed functional programming and units of measure are a natural combination, as F# ably demonstrates. However, encoding statically-checked units in Haskell’s type system leads to inevitable disappointment with the usability of the resulting system. Extending the language itself would produce a much better result, but it would be a lot of work! In this paper, I demonstrate how typechecker plugins in the Glasgow Haskell Compiler allow users to define domain-specific constraint solving behaviour, making it possible to implement units of measure as a type system extension without rebuilding the compiler. This paves the way for a more modular treatment of constraint solving in GHC.

Injective type families for Haskell

Haskell, as implemented by the Glasgow Haskell Compiler (GHC), allows expressive type-level programming. The most popular type-level programming extension is TypeFamilies, which allows users to write functions on types. Yet, using type functions can cripple type inference in certain situations. In particular, lack of injectivity in type functions means that GHC can never infer an instantiation of a type variable appearing only under type functions. In this paper, we describe a small modification to GHC that allows type functions to be annotated as injective. GHC naturally must check validity of the injectivity annotations. The algorithm to do so is surprisingly subtle. We prove soundness for a simplification of our algorithm, and state and prove a completeness property, though the algorithm is not fully complete. As much of our reasoning surrounds functions defined by a simple pattern-matching structure, we believe our results extend beyond just Haskell. We have implemented our solution on a branch of GHC and plan to make it available to regular users with the next stable release of the compiler.

Adaptive lock-free maps: purely-functional to scalable

Purely functional data structures stored inside a mutable variable provide an excellent concurrent data structure—obviously correct, cheap to create, and supporting snapshots. They are not, however, scalable. We provide a way to retain the benefits of these pure-in-a-box data structures while dynamically converting to a more scalable lock-free data structure under contention. Our solution scales to any pair of pure and lock-free container types with key/value set semantics, while retaining lock-freedom. We demonstrate the principle in action on two very different platforms: first in the Glasgow Haskell Compiler and second in Java. To this end we extend GHC to support lock-free data structures and introduce a new approach for safe CAS in a lazy language.

Efficient communication and collection with compact normal forms

In distributed applications, the transmission of non-contiguous data structures is greatly slowed down by the need to serialize them into a buffer before sending. We describe Compact Normal Forms, an API that allows programmers to explicitly place immutable heap objects into regions, which can both be accessed like ordinary data as well as efficiently transmitted over the network. The process of placing objects into compact regions (essentially a copy) is faster than any serializer and can be amortized over a series of functional updates to the data structure in question. We implement this scheme in the Glasgow Haskell Compiler and show that even with the space expansion attendant with memory-oriented data structure representations, we achieve between x2 and x4 speedups on fast local networks with sufficiently large data structures.

Algorithmic debugging of real-world haskell programs: deriving dependencies from the cost centre stack

Existing algorithmic debuggers for Haskell require a transformation of all modules in a program, even libraries that the user does not want to debug and which may use language features not supported by the debugger. This is a pity, because a promising approach to debugging is therefore not applicable to many real-world programs. We use the cost centre stack from the Glasgow Haskell Compiler profiling environment together with runtime value observations as provided by the Haskell Object Observation Debugger (HOOD) to collect enough information for algorithmic debugging. Program annotations are in suspected modules only. With this technique algorithmic debugging is applicable to a much larger set of Haskell programs. This demonstrates that for functional languages in general a simple stack trace extension is useful to support tasks such as profiling and debugging.

Diagnosing type errors with class

Type inference engines often give terrible error messages, and the more sophisticated the type system the worse the problem. We show that even with the highly expressive type system implemented by the Glasgow Haskell Compiler (GHC)--including type classes, GADTs, and type families--it is possible to identify the most likely source of the type error, rather than the first source that the inference engine trips over. To determine which are the likely error sources, we apply a simple Bayesian model to a graph representation of the typing constraints; the satisfiability or unsatisfiability of paths within the graph provides evidence for or against possible explanations. While we build on prior work on error diagnosis for simpler type systems, inference in the richer type system of Haskell requires extending the graph with new nodes. The augmentation of the graph creates challenges both for Bayesian reasoning and for ensuring termination. Using a large corpus of Haskell programs, we show that this error localization technique is practical and significantly improves accuracy over the state of the art.

Glasgow Haskell Compiler上の遅延オブジェクト再利用手法の設計と実装

Glasgow Haskell Compiler上の遅延オブジェクト再利用手法の設計と実装

Systems demonstration

Most strongly typed, functional programming languages are not equipped with a reentrant garbage collector. Therefore such languages are not used for operating systems programming, where the virtues of types are most desired. We propose the use of Context-Local Heaps (CLHs) to achieve reentrancy, which also increasing the speed of garbage collection. We have implemented CLHs in Ajhc, a Haskell compiler derived from jhc, rewritten some NetBSD sound drivers using Ajhc, and benchmarked them. The reentrant, faster garbage collection that CLHs provide opens the path to type-assisted operating systems programming.

Experience report

Many of the bugs in scientific programs have their roots in mistreatment of physical dimensions, via erroneous expressions in the quantity calculus. Now that the type system in the Glasgow Haskell Compiler is rich enough to support type-level integers and other promoted datatypes, we can type-check the quantity calculus in Haskell. In addition to basic dimension-aware arithmetic and unit conversions, our units library features an extensible system of dimensions and units, a notion of dimensions apart from that of units, and unit polymorphism designed to describe the laws of physics. We demonstrate the utility of units by writing an astrophysics research paper. This work is free of unit concerns because every quantity expression in the paper is rigorously type-checked.

Promoting functions to type families in Haskell

Haskell, as implemented in the Glasgow Haskell Compiler (GHC), is enriched with many extensions that support type-level programming, such as promoted datatypes, kind polymorphism, and type families. Yet, the expressiveness of the type-level language remains limited. It is missing many features present at the term level, including case expressions, anonymous functions, partially-applied functions, and let expressions. In this paper, we present an algorithm - with a proof of correctness - to encode these term-level constructs at the type level. Our approach is automated and capable of promoting a wide array of functions to type families. We also highlight and discuss those term-level features that are not promotable. In so doing, we offer a critique on GHC's existing type system, showing what it is already capable of and where it may want improvement. We believe that delineating the mismatch between GHC's term level and its type level is a key step toward supporting dependently typed programming.

On Termination, Confluence and Consistent CHR-based Type Inference

AbstractWe consider the application of Constraint Handling Rules (CHR) for the specification of type inference systems, such as that used by Haskell. Confluence of CHR guarantees that the answer provided by type inference is correct and consistent. The standard method for establishing confluence relies on an assumption that the CHR program is terminating. However, many examples in practice give rise to non-terminating CHR programs, rendering this method inapplicable. Despite no guarantee of termination or confluence, the Glasgow Haskell Compiler (GHC) supports options that allow the user to proceed with type inference anyway, e.g. via the use of the UndecidableInstances flag. In this paper we formally identify and verify a set of relaxed criteria, namely range-restrictedness, local confluence, and ground termination, that ensure the consistency of CHR-based type inference that maps to potentially non-terminating CHR programs.

Automatic SIMD vectorization for Haskell

Expressing algorithms using immutable arrays greatly simplifies the challenges of automatic SIMD vectorization, since several important classes of dependency violations cannot occur. The Haskell programming language provides libraries for programming with immutable arrays, and compiler support for optimizing them to eliminate the overhead of intermediate temporary arrays. We describe an implementation of automatic SIMD vectorization in a Haskell compiler which gives substantial vector speedups for a range of programs written in a natural programming style. We compare performance with that of programs compiled by the Glasgow Haskell Compiler.