Simon Marlow

A practical subtyping system for Erlang

We present a type system for the programming language Erlang. The type system supports subtyping and declaration-free recursive types, using subtyping constraints. Our system is similar to one explored by Aiken and Wimmers, though it sacrifices expressive power in favour of simplicity. We cover our techniques for type inference, type simplification, and checking when an inferred type conforms to a user-supplied type signature, and report on early experience with our prototype.

Deforestation for Higher-Order Functions

Deforestation is an automatic transformation scheme for functional programs which attempts to remove unnecessary intermediate data structures. The algorithm presented here is a variant of the original, adapted for a higher order language. A detailed description of how this may be implemented in an optimising compiler is also given.

Avoiding Unnecessary Updates

Graph reduction underlies most implementations of lazy functional languages, allowing separate computations to share results when subterms are evaluated. Once a term is evaluated, the node of the graph representing the computation is updated with the value of the term. However, in many cases, no other computation requires this value, so the update is unnecessary. In this paper we take some steps towards an analysis for determining when these updates may be omitted.

Update Avoidance Analysis by Abstract Interpretation

A requirement of lazy evaluation is that the value of any subexpression in the program is calculated no more than once. This is achieved by updating an expression with its value, once computed. The problem is that updating is a costly operation, and experimentation has shown that it is only necessary in about 30% of cases (that is, 70% of expressions represent values that are only ever required once during execution). The aim of the analysis presented in this paper is to discover expressions that do not need to be updated, and thus reduce the execution time of the program. The analysis has been implemented in the Glasgow Haskell Compiler, and results are given.

There is no fork: an abstraction for efficient, concurrent, and concise data access

We describe a new programming idiom for concurrency, based on Applicative Functors, where concurrency is implicit in the Applicative <*> operator. The result is that concurrent programs can be written in a natural applicative style, and they retain a high degree of clarity and modularity while executing with maximal concurrency. This idiom is particularly useful for programming against external data sources, where the application code is written without the use of explicit concurrency constructs, while the implementation is able to batch together multiple requests for data from the same source, and fetch data from multiple sources concurrently. Our abstraction uses a cache to ensure that multiple requests for the same data return the same result, which frees the programmer from having to arrange to fetch data only once, which in turn leads to greater modularity. While it is generally applicable, our technique was designed with a particular application in mind: an internal service at Facebook that identifies particular types of content and takes actions based on it. Our application has a large body of business logic that fetches data from several different external sources. The framework described in this paper enables the business logic to execute efficiently by automatically fetching data concurrently; we present some preliminary results.

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.

Backpack: retrofitting Haskell with interfaces

Module systems like that of Haskell permit only a weak form of modularity in which module implementations depend directly on other implementations and must be processed in dependency order. Module systems like that of ML, on the other hand, permit a stronger form of modularity in which explicit interfaces express assumptions about dependencies, and each module can be typechecked and reasoned about independently. In this paper, we present Backpack, a new language for building separately-typecheckable *packages* on top of a weak module system like Haskells. The design of Backpack is inspired by the MixML module calculus of Rossberg and Dreyer, but differs significantly in detail. Like MixML, Backpack supports explicit interfaces and recursive linking. Unlike MixML, Backpack supports a more flexible applicative semantics of instantiation. Moreover, its design is motivated less by foundational concerns and more by the practical concern of integration into Haskell, which has led us to advocate simplicity---in both the syntax and semantics of Backpack---over raw expressive power. The semantics of Backpack packages is defined by elaboration to sets of Haskell modules and binary interface files, thus showing how Backpack maintains interoperability with Haskell while extending it with separate typechecking. Lastly, although Backpack is geared toward integration into Haskell, its design and semantics are largely agnostic with respect to the details of the underlying core language.

Desugaring Haskell's do-notation into applicative operations

Monads have taken the world by storm, and are supported by do-notation (at least in Haskell). Programmers are increasingly waking up to the usefulness and ubiquity of Applicatives, but they have so far been hampered by the absence of supporting notation. In this paper we show how to re-use the very same do-notation to work for Applicatives as well, providing efficiency benefits for some types that are both Monad and Applicative, and syntactic convenience for those that are merely Applicative. The result is fully implemented as an optional extension in GHC, and is in use at Facebook to make it easy to write highly-parallel queries in a distributed system.

Composable Scheduler Activations for Haskell

The 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 (GHC) 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 (STM) to promote safety in a multicore context.