What does focusing tell us about language design?

A blog post about Call-By-Push-Value by Rob Simmons: What does focusing tell us about language design?

I think that one of the key observations of focusing/CBPV is that programs are dealing with two different things - data and computation - and that we tend to get the most tripped up when we confuse the two.

  • Data is classified by data types (a.k.a. positive types). Data is defined by how it is constructed, and the way you use data is by pattern-matching against it.
  • Computation is classified by computation types (a.k.a. negative types). Computations are defined their eliminations - that is, by how they respond to signals/messages/pokes/arguments.

There are two things I want to talk about, and they're both recursive types: call-by-push-value has positive recursive types (which have the feel of inductive types and/or algebras and/or what we're used to as datatypes in functional languages) and negative recursive types (which have the feel of recursive, lazy records and/or "codata" whatever that is and/or coalgebras and/or what William Cook calls objects). Both positive and negative recursive types are treated by Paul Blain Levy in his thesis (section 5.3.2) and in the Call-By-Push Value book (section 4.2.2).

In particular, I want to claim that Call-By-Push-Value and focusing suggest two fundamental features that should be, and generally aren't (at least simultaneously) in modern programming languages:

  • Support for structured data with rich case analysis facilities (up to and beyond what are called views)
  • Support for recursive records and negative recursive types.

Previously on Rob's blog: Embracing and extending the Levy language; on LtU: Call by push-value, Levy: a Toy Call-by-Push-Value Language.

And let me also repeat CBPV's slogan, which is one of the finest in PL advocacy: Once the fine structure has been exposed, why ignore it?

Informed dissent: William Cook contra Bob Harper on OOP

Ongoing discussion that you can follow on William Cook's blog.

I am not going to take sides (or keep points). I know everyone here has an opinion on the issue, and many of the arguments were discussed here over the years. I still think LtU-ers will want to follow this.

Given the nature of the topic, I remind everyone to review our policies before posting here on the issue.

Specification and Verification: The Spec# Experience

Mike Barnett, Manuel Fahndrich, K. Rustan M. Leino, Peter Muller, Wolfram Schulte, and Herman Venter, Specification and Verification: The Spec# Experience" Preprint of an article appearing in the June 2011 CACM.

CACM tagline: Can a programming language really help programmers write better programs?

Spec# is a programming system that facilitates the development of correct software. The Spec# language extends C# with contracts that allow programmers to express their design intent in the code. The Spec# tool suite consists of a compiler that emits run-time checks for contracts, a static program verifier that attempts to mathematically prove the correctness of programs, and an integration into the Visual Studio development environment. Spec# shows how contracts and verifiers can be integrated seamlessly into the software development process. This paper reflects on the six-year history of the Spec# project, scientific contributions it has made, remaining challenges for tools that seek to establish program correctness, and prospects of incorporating verification into everyday software engineering.

Spec# is, in some ways, quite similar to JML+ESC/Java2. But Spec# is a language rather than a set of annotations, which allows it to incorporate features such as a non-null type system and a very tight integration with the IDE.

Spec# was previously mentioned on LtU back in 2005.

Parametric Prediction of Heap Memory Requirements

Parametric Prediction of Heap Memory Requirements, by Victor Braberman, Federico Fernandez, Diego Garbervetsky, Sergio Yovine:

This work presents a technique to compute symbolic polynomial approximations of the amount of dynamic memory required to safely execute a method without running out of memory, for Java-like imperative programs. We consider object allocations and deallocations made by the method and the methods it transitively calls. More precisely, given an initial configuration of the stack and the heap, the peak memory consumption is the maximum space occupied by newly created objects in all states along a run from it. We over-approximate the peak memory consumption using a scoped-memory management where objects are organized in regions associated with the lifetime of methods. We model the problem of computing the maximum memory occupied by any region configuration as a parametric polynomial optimization problem over a polyhedral domain and resort to Bernstein basis to solve it. We apply the developed tool to several benchmarks.

We've briefly discussed analyses to predict heap usage here on LtU, but I can't seem to find them. Anyone with a reference handy, please post in the comments!

First-class modules: hidden power and tantalizing promises

Oleg just posted a new page, First-class modules: hidden power and tantalizing promises, related to new features in OCaml 3.12 (on LtU).

First-class modules introduced in OCaml 3.12 make type constructors first-class, permitting type constructor abstraction and polymorphism. It becomes possible to manipulate and quantify over types of higher kind. We demonstrate that as a consequence, full-scale, efficient generalized algebraic data types (GADTs) become expressible in OCaml 3.12 as it is, without any further extensions. Value-independent generic programming along the lines of Haskell's popular ``Generics for the masses'' become possible in OCaml for the first time. We discuss extensions such as a better implementation of polymorphic equality on modules, which can give us intensional type analysis (aka, type-case), permitting generic programming frameworks like SYB.

It includes a nice intro to first-class modules by Frisch and Garrigue: First-class modules and composable signatures in Objective Caml 3.12.

OCaml definitely just got even more interesting.

Objects to Unify Type Classes and GADTs

Objects to Unify Type Classes and GADTs, by Bruno C. d. S. Oliveira and Martin Sulzmann:

We propose an Haskell-like language with the goal of unifying type classes and generalized algebraic datatypes (GADTs) into a single class construct. We treat classes as first-class types and we use objects (instead of type class instances and data constructors) to define the values of those classes. We recover the ability to define functions by pattern matching by using sealed classes. The resulting language is simple and intuitive and it can be used to define, with similar convenience, the same programs that we would define in Haskell. Furthermore, unlike Haskell, dictionaries (or
objects) can be explicitly (as well as implicitly) passed to functions and we can program in a simple object-oriented style directly.

A very interesting paper on generalizing and unifying type classes and GADTs. Classes are now first-class values, resulting in a language that resembles a traditional, albeit more elegant, object-oriented language, and which supports a form of first-class "lightweight modules".

The language supports the traditional use of implicit type class dispatch, but also supports explicit dispatch, unlike Haskell. The authors suggest this could eliminate much of the duplication in the Haskell standard library of functions that take a type class or an explicit function, eg. insert/insertBy and sort/sortBy, although some syntactic sugar is needed to make this more concise.

Classes are open to extension by default, although a class can also be explicitly specified as "sealed", in which case extension is forbidden and you can pattern match on the cases. Furthermore, GADTs can now also carry methods, thus introducing dispatch to algebraic types. This fusion does not itself solve the expression problem, though it does ease the burden through the first-class support of both types of extension. You can see the Scala influences here.

I found this paper via the Haskell sub-reddit, which also links to a set of slides. The authors acknowledge Scala as an influence, and as future work, suggest extensions like type families and to support more module-like features, such as nesting and opaque types.

Joe-E: A Security-Oriented Subset of Java

Joe-E: A Security-Oriented Subset of Java. Adrian Mettler, David Wagner, and Tyler Close. To appear at ISOC NDSS 2010.

We present Joe-E, a language designed to support the development of secure software systems. Joe-E is a subset of Java that makes it easier to architect and implement programs with strong security properties that can be checked during a security review. It enables programmers to apply the principle of least privilege to their programs; implement application-specific reference monitors that cannot be bypassed; introduce and use domain-specific security abstractions; safely execute and interact with untrusted code; and build secure, extensible systems. Joe-E demonstrates how it is possible to achieve the strong security properties of an object-capability language while retaining the features and feel of a mainstream object-oriented language...

Section 5.2 discuss how Joe-E leverages Java static typing. Joe-E is implemented as a source-code verifier not a bytecode verifier. Section 6 of the paper explains this design choice.

Joe-E was mentioned on LtU in the past and is available here.

Super and Inner — Together at Last!

Super and Inner — Together at Last! by David S. Goldberg, Robert Bruce Findler, and Matthew Flatt, 2004.
In an object-oriented language, a derived class may declare a method with the same signature as a method in the base class. The meaning of the re-declaration depends on the language. Most commonly, the new declaration overrides the base declaration, perhaps completely replacing it, or perhaps using super to invoke the old implementation. Another possibility is that the base class always controls the method implementation, and the new declaration merely augments the method in the case that the base method calls inner. Each possibility has advantages and disadvantages. In this paper, we explain why programmers need both kinds of method redeclaration, and we present a language that integrates them. We also present a formal semantics for the new language, and we describe an implementation for MzScheme.
To me, an interesting aspect was interleaving of overrides and augmentations of the same method.

Why Object-Oriented Languages Need Tail Calls

The Fortress blog has a recent post, Why Object-Oriented Languages Need Tail Calls, where Guy Steele argues for the necessity of proper tail call implementations without rehashing two of the classic arguments: state machines and the continuation passing style. It starts by mentioning William Cook's On Understanding Data Abstraction, Revisited:

In this blog post we extend one of his examples in order to make a completely different point: object-oriented programming languages need tail calls correctly implemented, not just as a "trivial and optional" optimization of the use of stack space that could be achieved by using iteration statements, but in order to preserve object-oriented abstractions.

The post also mentions other papers previously discussed on LtU: Automata as Macros, and A Tail-Recursive Machine with Stack Inspection.

Design Patterns 15 Years Later: An Interview with Erich Gamma, Richard Helm, and Ralph Johnson

Larry O'Brien recently interviewed three of the Gang of Four about their seminal work on patterns. Larry teased the interview's readers for awhile, but he eventually asked the pressing question that most language designers ask and debate about patterns ;) Here it is:

Larry: GoF came out relatively early in the ascent of OOP as the mainstream paradigm and, for better or worse, "patterns" seem to be associated with OO approaches. You even hear functional and dynamic advocates boasting that their languages "don't need" patterns. How do you respond to that?

Erich: Just as an aside, it is also easy to forget that we wrote design patterns before there was Java or C#.

Ralph: Some of those languages don't need some of the patterns in Design Patterns because their languages provide alternative ways of solving the problems. Our patterns are for languages between C++ and Smalltalk, which includes just about everything called "object-oriented," but they certainly are not for every programming language. I don't think anybody actually says that programmers in other languages don't need patterns; they just have a different set of patterns.

Erich: Design patterns eventually emerge for any language. Design déjà-vu is language neutral. While these experiences are not always captured as patterns, they do exist. The design principles for Erlang come to mind.

Larry: Where would a person go to learn about patterns for dynamic and functional languages? Who's making good contributions?

Ralph: If by "dynamic" you mean dynamic object-oriented languages like Smalltalk, Ruby or Python, then our patterns are applicable. Functional languages require different patterns, but I don't know who is working on them.

Note: At the end of the interview, Erich says that they tried refactoring the patterns into new categories in 2005. The draft breakdown he provides (accidentally???) takes out Memento, Chain of Responsibility, Bridge, Adapter, and Observer.

As I said above these are just notes in a draft state. Doing a refactoring without test cases is always dangerous...

UPDATE: The Gang of Four have an accompanying article for the interview that they wrote as a group. See A Look Back: Why We Wrote Design Patterns: Elements of Reusable Object-Oriented Software.

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