Category theorists invented monads in the 1960's to concisely express certain aspects of universal algebra. Functional programmers invented list comprehensions in the 1970's to concisely express certain programs involving lists. This paper shows how list comprehensions may be generalised to an arbitrary monad, and how the resulting programming feature can concisely express in a pure functional language some programs that manipulate state, handle exceptions, parse text, or invoke continuations. A new solution to the old problem of destructive array update is also presented. No knowledge of category theory is assumed.

This is an introduction to the philosophy and use of OBJ, emphasizing its operational semantics, with aspects of its history and its logical semantics. Release 2 of OBJ3 is described in detail, with many examples. OBJ is a wide spectrum first-order functional language that is rigorously based on (order sorted) equational logic and parameterized programming, supporting a declarative style that facilitates verification and allows OBJ to be used as a theorem prover.

Generic control operators, such as fold, can be generated from algebraic type definitions. The class of types to which these techniques are applicable is generalized to all algebraic types definable in languages such as Miranda and ML, i.e. mutually recursive sums-of-products with tuples and function types. Several other useful generic operators, also applicable to every type in this class, also are described. A normalization algorithm which automatically calculates improvements to programs expressed in a language based upon folds is described. It reduces programs, expressed using fold as the exclusive control operator, to a canonical form. Based upon a generic promotion theorem, the algorithm is facilitated by the explicit structure of fold programs rather than using an analysis phase to search for implicit structure. Canonical programs are minimal in the sense that they contain the fewest number of fold operations. Because of this property, the normalization algorithm has important ...

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Contexts have been proposed as a means of performing strictness analysis on non-flat domains. Roughly speaking, a context describes how much a sub-expression will be evaluated by the surrounding program. This paper shows how contexts can be represented using the notion of projection from domain theory. This is clearer than the previous explanation of contexts in terms of continuations. In addition, this paper describes finite domains of contexts over the non-flat list domain. This means that recursive context equations can be solved using standard fixpoint techniques, instead of the algebraic manipulation previously used. Praises of lazy functional languages have been widely sung, and so have some curses. One reason for praise is that laziness supports programming styles that are inconvenient or impossible otherwise [Joh87, Hug84, Wad85a]. One reason for cursing is that laziness hinders efficient implementation. Still, acceptable efficiency for lazy languages is at last being achieved...

We present two complementary approaches to writing XML document-processing applications in a functional language. In the first approach, the generic tree structure of XML documents is used as the basis for the design of a library of combinators for generic processing: selection, generation, and transformation of XML trees. The second approach is to use a type-translation framework for treating XML document type definitions (DTDs) as declarations of algebraic data types, and a derivation of the corresponding functions for reading and writing documents as typed values in Haskell. 1 Introduction 1.1 Document markup languages XML (Extensible Markup Language) [1] is a recent simplification of the older SGML (Standardised Generalised Markup Language) standard that is widely used in the publishing industry. It is a markup language, meaning that it adds structural information around the text of a document. It is extensible, meaning that the vocabulary of the markup is not fixed -- each doc...

Several programming languages arising from widely diverse practical and theoretical considerations share a common high-level feature: their basic data type is an aggregate of other more primitive data types and their primitive functions operate on these aggregates. Examples of such languages (and the collections they support) are FORTRAN 90 (arrays), APL (arrays), Connection Machine LISP (xectors), PARALATION LISP (paralations), and SETL (sets). Acting on large collections of data with a single operation is the hallmark of data-parallel programming and massively parallel computers. These languages --- which we call collection-oriented --- are thus ideal for use with massively parallel machines, even though many of them were developed before parallelism and associated considerations became important. This paper examines collections and the operations that can be performed on them in a language-independent manner. It also critically reviews and compares a variety of collection-oriented languages...

In this paper we propose a new approach to implement web services based on the Common Gateway Interface (CGI). Since we use the multi-paradigm declarative language Curry as an implementation language, many of the drawbacks and pitfalls of traditional CGI programming can be avoided. For instance, the syntactical details of HTML and passing values with CGI are hidden by a wrapper that executes abstract HTML forms by translating them into concrete HTML code. This leads to a high-level approach to server side web service programming where notions like event handlers, state variables and control of interactions are available. Thanks to the use of a functional logic language, we can structure our approach as an embedded domain specic language where the functional and logic programming features of the host language are exploited to abstract from details and frequent errors in standard CGI programming.

We describe a facility for improving optimization of Haskell programs using rewrite rules. Library authors can use rules to express domain-specific optimizations that the compiler cannot discover for itself. The compiler can also generate rules internally to propagate information obtained from automated analyses. The rewrite mechanism is fully implemented in the released Glasgow Haskell Compiler. Our system is very simple, but can be effective in optimizing real programs. We describe two practical applications involving short-cut deforestation, for lists and for rose trees, and document substantial performance improvements on a range of programs. 1 Introduction Optimising compilers perform program transformations that improve the efficiency of the program. However, a compiler can only use relatively shallow reasoning to guarantee the correctness of its optimisations. In contrast, the programmer has much deeper information about the program and its intended behaviour. For example, a programmer may know that

Some modern superscalar microprocessors provide only imprecise exceptions. That is, they do not guarantee to report the same exception that would be encountered by a straightforward sequential execution of the program. In exchange, they offer increased performance or decreased chip area (which amount to much the same thing). This performance/precision tradeoff has not so far been much explored at the programming language level. In this paper we propose a design for imprecise exceptions in the lazy functional programming language Haskell. We discuss several designs, and conclude that imprecision is essential if the language is still to enjoy its current rich algebra of transformations. We sketch a precise semantics for the language extended with exceptions. The paper shows how to extend Haskell with exceptions without crippling the language or its compilers. We do not yet have enough experience of using the new mechanism to know whether it strikes an appropriate balance between expressiveness and pwrformance.