Jean-Philippe Bernardy

Testing polymorphic properties

This paper is concerned with testing properties of polymorphic functions. The problem is that testing can only be performed on specific monomorphic instances, whereas parametrically polymorphic functions are expected to work for any type. We present a schema for constructing a monomorphic instance for a polymorphic property, such that correctness of that single instance implies correctness for all other instances. We also give a formal definition of the class of polymorphic properties the schema can be used for. Compared to the standard method of testing such properties, our schema leads to a significant reduction of necessary test cases.

Realizability and parametricity in pure type systems

We describe a systematic method to build a logic from any programming language described as a Pure Type System (PTS). The formulas of this logic express properties about programs. We define a parametricity theory about programs and a realizability theory for the logic. The logic is expressive enough to internalize both theories. Thanks to the PTS setting, we abstract most idiosyncrasies specific to particular type theories. This confers generality to the results, and reveals parallels between parametricity and realizability.

A comparison of c++ concepts and haskell type classes

Earlier studies have introduced a list of high-level evaluation criteria to assess how well a language supports generic programming. Since each language that meets all criteria is considered generic, those criteria are not fine-grained enough to differentiate between languages for generic programming. We refine these criteria into a taxonomy that captures differences between type classes in Haskell and concepts in C++, and discuss which differences are incidental and which ones are due to other language features. The taxonomy allows for an improved understanding of language support for generic programming, and the comparison is useful for the ongoing discussions among language designers and users of both languages.

Type-theory in color

Dependent type-theory aims to become the standard way to formalize mathematics at the same time as displacing traditional platforms for high-assurance programming. However, current implementations of type theory are still lacking, in the sense that some obvious truths require explicit proofs, making type-theory awkward to use for many applications, both in formalization and programming. In particular, notions of erasure are poorly supported. In this paper we propose an extension of type-theory with colored terms, color erasure and interpretation of colored types as predicates. The result is a more powerful type-theory: some definitions and proofs may be omitted as they become trivial, it becomes easier to program with precise types, and some parametricity results can be internalized.

Lazy functional incremental parsing

Structured documents are commonly edited using a free-form editor. Even though every string is an acceptable input, it makes sense to maintain a structured representation of the edited document. The structured representation has a number of uses: structural navigation (and optional structural editing), structure highlighting, etc. The construction of the structure must be done incrementally to be efficient: the time to process an edit operation should be proportional to the size of the change, and (ideally) independent of the total size of the document. We show that combining lazy evaluation and caching of intermediate (partial) results enables incremental parsing. We build a complete incremental parsing library for interactive systems with support for error-correction.

Generic programming with c++ concepts and haskell type classes: A comparison

Earlier studies have introduced a list of high-level evaluation criteria to assess how well a language supports generic programming. Languages that meet all criteria include Haskell because of its type classes and C++ with the concept feature. We refine these criteria into a taxonomy that captures commonalities and differences between type classes in Haskell and concepts in C++ and discuss which differences are incidental and which ones are due to other language features. The taxonomy allows for an improved understanding of language support for generic programming, and the comparison is useful for the ongoing discussions among language designers and users of both languages.

A Presheaf Model of Parametric Type Theory

We extend Martin-Lofs Logical Framework with special constructions and typing rules providing internalized parametricity. Compared to previous similar proposals, this version comes with a denotational semantics which is a refinement of the standard presheaf semantics of dependent type theory. Further, this presheaf semantics is a refinement of the one used to interpret nominal sets with restrictions. The present calculus is a candidate for the core of a proof assistant with internalized parametricity.

Efficient divide-and-conquer parsing of practical context-free languages

We present a divide-and-conquer algorithm for parsing context-free languages efficiently. Our algorithm is an instance of Valiants (1975), who reduced the problem of parsing to matrix multiplications. We show that, while the conquer step of Valiants is O(n3) in the worst case, it improves to O(logn3), under certain conditions satisfied by many useful inputs. These conditions occur for example in program texts written by humans. The improvement happens because the multiplications involve an overwhelming majority of empty matrices. This result is relevant to modern computing: divide-and-conquer algorithms can be parallelized relatively easily.

Yi: an editor in haskell for haskell

Yi is a text editor written in Haskell and extensible in Haskell. We take advantage of Haskells expressive power to define embedded DSLs that form the foundation of the editor. In turn, these DSLs provide a flexible mechanism to create extended versions of the editor. Yi also provides some support for editing Haskell code.

Certified Context-Free Parsing: A formalisation of Valiant's Algorithm in Agda

Valiant (1975) has developed an algorithm for recognition of context free languages. As of today, it remains the algorithm with the best asymptotic complexity for this purpose. In this paper, we present an algebraic specification, implementation, and proof of correctness of a generalisation of Valiants algorithm. The generalisation can be used for recognition, parsing or generic calculation of the transitive closure of upper triangular matrices. The proof is certified by the Agda proof assistant. The certification is representative of state-of-the-art methods for specification and proofs in proof assistants based on type-theory. As such, this paper can be read as a tutorial for the Agda system.