This document formally specifies the semantics of local modules and packages - dynamically typed modules that are first-class values - as an extension to the functional programming language Standard ML. The language thus defined is a substantial subset of a larger extension of Standard ML, a language known as Alice ML. Packages are the central feature of Alice ML that enables support for typed open programming.
we develop an approach for reconciling open programming -- the development of programs that support dynamic exchange of higher-order values with strong static typing. conventional functional language extended with a set of orthogonal features like higher-order modules, dynamic type checking, higher-order serialisation, and concurrency. On top of these a flexible system of dynamic components and a simple but expressive notion of distribution is realised. The central concept in this design is the packag}, a first-class value embedding a module along with its interface type. formal model for abstract types that is not invalidated by the presence of primitives for dynamic type inspection, as is the case for the standard model based on existential quantification. For that we present an idealised language in form of an extended lambda-calculus, which can express dynamic generation of types.
We introduce a new lambda calculus with futures, Lambda(fut), that models the operational semantics of concurrent statically typed functional programming languages with mixed eager and lazy threads such as Alice ML, a concurrent extension of Standard ML. Lambda(fut) is a minimalist extension of the call-by-value lambda-calculus that is sufficiently expressive to define and combine a variety of standard concurrency abstractions, such as channels, semaphores, and ports. Despite its minimality, the basic machinery of Lambda(fut) is sufficiently powerful to support explicit recursion and call-by-need evaluation. We present a static type system for Lambda(fut) and distinguish a fragment of Lambda(fut) that we prove to be uniformly confluent. This result confirms our intuition that reference cells are the sole source of indeterminism.
Despite its powerful module system, ML has not yet evolved for the modern world of dynamic and open modular programming, to which more primitive languages have adapted better so far. We present the design and semantics of a simple yet expressive first-class component system for ML. It provides dynamic linking in a type-safe and type-flexible manner, and allows selective execution in sandboxes. The system is defined solely by reduction to higher-order modules plus an extension with simple module-level dynamics, which we call packages. To represent components outside processes we employ generic pickling. We give a module calculus formalising the semantics of packages and pickling.