The use of monads to structure functional programs is described. Monads provide a convenient framework for simulating e ects found in other languages, such as global state, exception handling, output, or non-determinism. Three case studies are looked at in detail: how monads ease the modi cation of a simple evaluator; how monads act as the basis of a datatype of arrays subject to in-place update; and how monads can be used to build parsers.

The Standard ML of New Jersey compiler has been under development for five years now. We have developed a robust and complete environment for Standard ML that supports the implementation of large software systems and generates efficient code. The compiler has also served as a laboratory for developing novel implementation techniques for a sophisticated type and module system, continuation based code generation, efficient pattern matching, and concurrent programming features.

This paper presents a soft type systems that retains the expressiveness of dynamic typing, but offers the early error detection and improved optimization capabilities of static typing. The key idea underlying soft typing is that a type checker need not reject programs containing "ill-typed" phrases. Instead, the type checker can insert explicit run-time checks, transforming "ill-typed" programs into type-correct ones.

The type and effect discipline is a new framework for reconstructing the principal type and the minimal effect of expressions in implicitly typed polymorphic functional languages that support imperative constructs. The type and effect discipline outperforms other polymorphic type systems. Just as types abstract collections of concrete values, effects denote imperative operations on regions. Regions abstract sets of possibly aliased memory locations. Effects are used to control type generalization in the presence of imperative constructs while regions delimit observable side-effects. The observable effects of an expression range over the regions that are free in its type environment and its type; effects related to local data structures can be discarded during type reconstruction. The type of an expression can be generalized with respect to the variables that are not free in the type environment or in the observable effect. 1 Introduction Type inference [12] is the process that automa...

Abstract not found

. This paper describes a simple extension of the Hindley-Milner polymorphic type discipline to call-by-value languages that incorporate imperative features like references, exceptions, and continuations. This extension sacrifices the ability to type every purely functional expression that is typable in the Hindley-Milner system. In return, it assigns the same type to functional and imperative implementations of the same abstraction. Hence with a module system that separates specifications from implementations, imperative features can be freely used to implement polymorphic specifications. A study of a number of ML programs shows that the inability to type all Hindley-Milner typable expressions seldom impacts realistic programs. Furthermore, most programs that are rendered untypable by the new system can be easily repaired. Keywords: Continuations, functional programming, polymorphism, references, state 1. Polymorphism, Imperative Features, and Modules The Hindley-Milner polymorphic ty...

Standard ML is an excellent language for many kinds of programming. It is safe, efficient, suitably abstract, and concise. There are many aspects of the language that work well. However, nothing is perfect: Standard ML has a few shortcomings. In some cases there are obvious solutions, and in other cases further research is required.

We introduce the type theory ¯ v , a call-by-value variant of Parigot's ¯-calculus, as a Curry-Howard representation theory of classical propositional proofs. The associated rewrite system is Church-Rosser and strongly normalizing, and definitional equality of the type theory is consistent, compatible with cut, congruent and decidable. The attendant call-by-value programming language ¯pcf v is obtained from ¯ v by augmenting it by basic arithmetic, conditionals and fixpoints. We study the behavioural properties of ¯pcf v and show that, though simple, it is a very general language for functional computation with control: it can express all the main control constructs such as exceptions and first-class continuations. Proof-theoretically the dual ¯ v -constructs of naming and ¯-abstraction witness the introduction and elimination rules of absurdity respectively. Computationally they give succinct expression to a kind of generic (forward) "jump" operator, which may be regarded as a unif...

We study the typing properties of CPS conversion for an extension of F ! with control operators. Two classes of evaluation strategies are considered, each with call-by-name and call-by-value variants. Under the "standard" strategies, constructor abstractions are values, and constructor applications can lead to non-trivial control effects. In contrast, the "ML-like" strategies evaluate beneath constructor abstractions, reflecting the usual interpretation of programs in languages based on implicit polymorphism. Three continuation passing style sub-languages are considered, one on which the standard strategies coincide, one on which the ML-like strategies coincide, and one on which all the strategies coincide. Compositional, type-preserving CPS transformation algorithms are given for the standard strategies, resulting in terms on which all evaluation strategies coincide. This has as a corollary the soundness and termination of well-typed programs under the standard evaluation strategies. A similar result is obtained for the ML-like call-by-name strategy. In contrast, such results are obtained for the call-by value ML-like strategy only for a restricted sub-language in which constructor abstractions are limited to values.

We add functional continuations and prompts to a language with an ML-style type system. The operators significantly extend and simplify the control operators in SML/NJ, and can be themselves used to implement (simple) exceptions. We prove that well-typed terms never produce run-time type errors and give a module for implementing them in the latest version of SML/NJ.