Keywords: assessment, code quality, empirical validation, feedback, programming education, rubrics

Keywords: Trusted code base; Source-code size; Auditability; Software implementation; Timing-attack protection; NaCl; Twitter

In this paper we present an elegant method for sequentializing arbitrary data using the generic language extension of the functional programming language Clean. We show how the proposed operations can be used to store values of any concrete data type in several kinds of IO containers (such as files or arrays of characters), and how to manipulate stored data efficiently. Moreover, by extending stored data with encoded type information, data manipulation will be type-safe. Defining these operations generically has the advantage that specific instances for user defined data types can be generated fully automatically. Compared to traditional sequentialization methods (or to common data manipulation, using relational data bases) our operations are an order of magnitude faster.

Keywords: Polytypic/generic programming, data storage/sequentialization, functional programming, type inference

In this case study we investigate the use of PVS for developing type theoretical concepts and verifying the correctness of a typing algorithm. PVS turns out to be very useful for efficient development of a sound basic theory about polymorphic typing. This research contributes to the PoplMark challenge on mechanizing metatheory.

The functional language Clean includes a hybrid type system: in addition to the traditional static type system it offers support for dynamically typed values, so called dynamics. An expression of arbitrary static type can be packed into a dynamic, and via pattern matching on types a dynamic can be converted back to a value of statically known type. The most important aspect of dynamics is that they can be serialized, written to file, and exchanged between independently compiled applications in a type-safe way. Since dynamics may contain functions and closures, they can be regarded as type-safe plugins. An application can combine plugins in a type safe way to new ones, without having to know what the actual contents and types of the original plugins are. In this paper we explain the underlying implementation and the architecture of the run-time system needed to make this all possible. To show the expressive power of Clean`s dynamics we present an interactive shell, called Esther. The command line language of this shell provides the basic functionality of a strongly typed lazy functional language. Esther behaves like an interpreter (direct response) yet its resulting performance is comparable to compiled code. This is achieved by using the dynamic facility of Clean. The shell actually combines compiled code; no help from a compiler is needed. Moreover, type checking is for free using the run-time type unification of dynamics. In this way we have obtained a novel architectural framework in which one can freely mix functionality created by the compiler, functionality stored in dynamics by compiled applications, and functionality interactively created by using the shell command line interpreter.

We present a new typing system for strictness analysis of functional programs. The system extends standard typing (including recursive data types) with strictness annotations and subtyping. Strictness typing is shown to be sound with respect to a natural operational semantics. We demonstrate that strictness types can be computed effectively.

Using PVS (Prototype Verification System), we prove that an industry designed scheduler for a smartcard personalization machine is safe and optimal. This scheduler has previously been the subject of research in model checked scheduling synthesis and verification. These verification and synthesis efforts have only been done for a limited number of personalization stations. We have created an executable model and prove the scheduling algorithm to be optimal and safe for any number of personalization stations. This result shows that theorem provers can be successfully used for industrial problems in cases where model checkers suffer from state explosion.

This paper is both a position paper on a particular approach in program correctness, and also a contribution to this area. The approach entails the generation of programs (code) from the executable content of logical theories. This capability already exists within the main theorem provers like Coq, Isabelle and ACL2 and PVS. Here we will focus on issues portraying the use of this methodology, rather than the underlying theory. We illustrate the power of the approach within PVS via two case studies (on unification and compression) that lead to actual running code. We also demonstrate its flexibility by extending the program generation capabilities. This paper fits in a line of ongoing integration of programming and proving.

Bracket abstraction is an algorithm that transforms lambda expressions into combinator terms. There are several versions of this algorithm depending on the actual set of combinators that is used. Most of them have been proven correct with respect to the operational semantics. In this paper we focus on typability. We present a fully machine verified proof of the property that bracket abstraction preserves types; the types assigned to an expression before and after performing bracket abstraction are identical. To our knowledge, this is the first time that (1) such proof has been given, and (2) the proof is verified by a theorem prover. The theorem prover used in the development of the proof is PVS.

Polytypic functional programming has the advantage that it can derive code for generic functions automatically. However, it is not clear whether it is useful for anything other than the textbook examples, and the generated polytypic code is usually too slow for real-life programs. As a real-life test, we derive a polytypic parser for the Haskell 98 syntax and look into other front-end compiler syntax tree operations. We present a types-as-grammar approach, which uses polytypic programming (in both Generic Haskell and Clean) to automatically derive the code for a parser based on the syntax tree type, without using external tools. Moreover, we show that using polytypic programming can even be useful for data-specific syntax tree operations in a (functional) compiler, such as scope checking and type inference. Simple speed tests show that the performance of polytypic parsers can be abominable for real-life inputs. However, we show that much performance can be recovered by applying (extended) fusion optimization on the generated code. We actually have a derived parser whose speed is close to one generated by a specialized Haskell parser generator.

Invertible programming occurs in the area of data conversion where it is required that the conversion in one direction is the inverse of the other. For that purpose, we introduce bidirectional arrows (bi-arrows). The bi-arrow class is an extension of Haskell's arrow class with an extra combinator that changes the direction of computation. The advantage of the use of bi-arrows for invertible programming is the preservation of invertibility properties using the bi-arrow combinators. Programming with bi-arrows in a polytypic or generic way exploits this the most. Besides bidirectional polytypic examples, including invertible serialization, we give the definition of a monadic bi-arrow transformer, which we use to construct a bidirectional parser/pretty printer.

Keywords: Haskell, Arrows, Invertible program construction, Polytypic programming

Generic/polytypic functional programming enables the compiler to generate most of the code. This style of programming works especially well for the generic textbook examples, but it generally performs poorly for bigger programs. This paper uses generic programming to derive a parser using a types-as-grammar approach, circumventing the need for an external parser generator and allowing the programmer to specify everything in the language of his choice. We show just how much we can recover from the loss of performance using an existing optimizer on several parser monads. The types-as-grammar approach yields a rich (non-abstract) syntax tree, and we explore the usability of generic programming for other syntax tree operations in a (functional) compiler, such as type inference, scope checking, and syntax transformations.

Generic programming is accepted by the functional programming community as a valuable tool for program development. Several functional languages have adopted the generic scheme of type-indexed values. This scheme works by specialization of a generic function to a concrete type. However, the generated code is extremely inefficient compared to its hand-written counterpart. The performance penalty is so big that the practical usefulness of generic programming is compromised. In this paper we present an optimization algorithm that is able to completely eliminate the overhead introduced by the specialization scheme for a large class of generic functions. The presented technique is based on consumer-producer elimination as exploited by fusion, a standard general purpose optimization method. We show that our algorithm is able to optimize many practical examples of generic functions. Keywords: program transformation, fusion, generic/polytypic programming.

Generic functions are defined by induction on the structural representation of types. As a consequence, by defining just a single generic operation, one acquires this operation over any particular type. An instance on a specific type is generated by interpretation of the type's structure. A direct translation leads to extremely inefficient code that involves many conversions between types and their structural representations. In this paper we present an optimization technique based on compile-time symbolic evaluation. We prove that the optimization removes the overhead of the generated code for a considerable class of generic functions. The proof uses typing to identify intermediate data structures that should be eliminated. In essence, the output after optimization is similar to hand-written code.

Deforestation was introduced to eliminate intermediate data structures used to connect separate parts of a functional program together. Fusion is a more sophisticated technique, based on a producer-consumer model, to eliminate intermediate data structures. It achieves better results. In this paper we extend this fusion algorithm by refining this model, and by adding new transformation rules. The extended fusion algorithm is able to deal with standard deforestation, but also with higher-order function removal and dictionary elimination. We have implemented this extended algorithm in the Clean 2.0 compiler.

We present two type systems for term graph rewriting: conventional typing and (polymorphic) uniqueness typing. The latter is introduced as a natural extension of simple algebraic and higher-order uniqueness typing. The systems are given in natural deduction style using an inductive syntax of graph denotations with familiar constructs such as let and case.

The conventional system resembles traditional Curry-style typing systems in functional programming languages. Uniqueness typing extends this with reference count information. In both type systems, typing is preserved during evaluation, and types can be determined effectively. Moreover, with respect to a graph rewriting semantics, both type systems turn out to be sound.