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.

Functional nets combine key ideas of functional programming and Petri nets to yield a simple and general programming notation. They

## is an extension of the #-calculus with a binding construct for local names. The extension has properties analogous to classical #-calculus and preserves all observational equivalences of #. It is useful as a basis for modeling wide-spectrum languages that build on a functional core. 1 Introduction Recentyears have given us a good deal of theoretical research on the interaction of imperative programming #exempli#ed byvariable assignment# and functional programming #exempli#ed by higher order functions# #3,6,19,21, 24#. The common method of all these works is to propose a #-calculus extended with imperative features and to carry out an exploration of the operational semantics of the new calculus. Based on our own experience in devising such an extended #- calculus #13#, the presentwork singles out the name, whose only observational property is its identity, as an essential componentofany such extension. We present a simple extension of the pure #-calculus with names; we showby ex...

Type safety of imperative programs is an area fraught with difficulty and requiring great care. The SML solution to the problem, originally involving imperative type variables, has been recently simplified to the syntactic-value restriction. In Haskell, the problem is addressed in a rather different way using explicit monadic state. We present an operational semantics for state in Haskell and the first full proof of type safety. We demonstrate that the semantic notion of value provided by the explicit monadic types is able to avoid any problems with generalization. 1 Introduction When Launchbury and Peyton Jones introduced encapsulated monadic state [11, 12], it came equipped with a denotational semantics and a model-theoretic proof that different state threads did not interact with each other. The encapsulation operator runST had a type which statically guaranteed freedom of interaction, and the guarantee relied on a parametricity proof. What the paper failed to provide was any form...

Many object-oriented languages used in practice descend from Algol. With this motivation, we study the theoretical issues underlying such languages via the theory of Algollike languages. It is shown that the basic framework of this theory extends cleanly and elegantly to the concepts of objects and classes. An important idea that comes to light is that classes are abstract data types, whose theory corresponds to that of existential types. Equational and Hoare-like reasoning methods, and relational parametricity provide powerful formal tools for reasoning about Algol-like object-oriented programs. 1

. In an earlier paper, one of the present authors presented a preliminary account of an equational logic called PIM. PIM is intended to function as a "transformational toolkit" to be used by compilers and analysis tools for imperative languages, and has been applied to such problems as program slicing, symbolic evaluation, conditional constant propagation, and dependence analysis. PIM consists of the untyped lambda calculus extended with an algebraic rewriting system that characterizes the behavior of lazy stores and generalized conditionals. A major question left open in the earlier paper was whether there existed a complete equational axiomatization of PIM's semantics. In this paper, we answer this question in the affirmative for PIM's core algebraic component, PIM t , under the assumption of certain reasonable restrictions on term formation. We systematically derive the complete PIM logic as the culmination of a sequence of increasingly powerful equational systems starti...

The extension of Haskell with a built-in state monad combines mathematical elegance with operational efficiency: ffl Semantically, at the source language level, constructs that act on the state are viewed as functions that pass an explicit store data structure around. ffl Operationally, at the implementation level, constructs that act on the state are viewed as statements whose evaluation has the side-effect of updating the implicit global store in place. There are several unproven conjectures that the two views are consistent. Recently, we have noted that the consistency of the two views is far from obvious: all it takes for the implementation to become unsound is one judiciously-placed beta-step in the optimization phase of the compiler. This discovery motivates the current paper in which we formalize and show the correctness of the implementation of monadic state. For the proof, we first design a typed call-by-need language that models the intermediate language of the compiler, to...

It is well known that adding side effects to functional languages changes the operational equivalences of the language. We develop a new language construct, encap, that forces imperative pieces of code to behave purely functionally, i.e., without any visible side effects. The coercion operator encap provides a means of extending the simple reasoning principles for equivalences of code in a functional language to a language with side effects. In earlier work [36], similar coercion operators were developed, but their correctness required the underlying functional language to include parallel operations. The coercion operators developed here are simpler and are proven correct for purely sequential languages. The sequential setting requires the construction of fully abstract models for sequential call-by-value languages and the formulation of a weak form of "monad" suitable for expressing the semantics of call-by-value languages with side effects. 1 Introduction Two pieces of code are...

1 Introduction It's been said many times before: "Functional languages are great, but they can't deal with state! " to which functional programmers often reply: "But a compiler that's great, will eliminate state!" Although recent advances in compiler optimization techniques have eliminated many concerns over efficiency, optimizations have their own set of problems: (1) they are often expensive (in terms of compilation resources), (2) they aren't always good enough, (3) they are often hard to reason about, and (4) they are implementation dependent (and thus programs that depend on them are not portable). Perhaps more importantly, compiler optimizations aren't explicit, and in this sense are not "expressive " enough.

In a programming language with procedures and assignments, it is often important to isolate uses of state to particular program fragments. The frameworks of type, region, and effect inference, and monadic state are technologies that have been used to state and enforce the property that an expression has no visible side-effects. This property has been exploited to justify the deallocation of memory regions despite the presence of dangling pointers. Starting from an idea developed in the context of monadic state in Haskell, we develop an ML-like language with full assignments and an operator that enforces the encapsulation of effects. Using this language, we formalize and prove the folklore connection between effect masking and monadic encapsulation. Then, by employing a novel set of reductions to deal with dangling pointers, we establish the soundness of the type-based encapsulation with a proof based on a standard subject reduction argument. 1 Introduction Two of the recurring theme...