The practice of computing has reached a stage where computers are seen as parts of a global computing platform. The possibility of exploiting resources on a global scale has given rise to a new paradigm -- the mobile computation paradigm -- for computation in large-scale distributed networks. Languages which enable the mobility of code over the network are becoming widely used for building distributed applications. This thesis explores distributed computation with languages which adopt functions as the main programming abstraction and support code mobility through the mobility of functions between remote sites. It aims to highlight the benefits of using languages of this family in dealing with the challenges of mobile computation. The possibility of exploiting existing static analysis techniques suggests that having functions at the core of a mobile code language is a particularly apt choice. A range of problems which have impact on the safety, security and performance of systems are discussed here. It is shown that types extended with effects and other annotations can capture a significant amount of information about the dynamic behaviour of mobile functions and offer solutions to the problems under investigation. The thesis presents a survey of the languages Concurrent ML, Facile and PLAN which remain loyal to the principles of the functional language ML and hence inherit its strengths in the context of concurrent and distributed computation. The languages which are defined in the subsequent chapters have their roots in these languages. Two chapters focus on using types to statically predict whether functions are used locally or may become mobile at runtime. Types are exploited for distributed calltracking to estimate which functions are invoked at which sites in the system. Compilers for mobile code languages would benefit from such estimates in dealing with the heterogeneity of the network nodes, in providing static profiling tools and in estimating the resource-consumption of programs. Two chapters are devoted to the use of types in controlling the flow of values in a system where users have different trust levels. The confinement of values within a specified mobility region is the subject of one of these. The other focuses on systems where values are classified with respect to their confidentiality level. The sources of undesirable flows of information are identified and a solution based on noninterference is proposed.

This thesis describes a computational model for interpreting natural language expressions in an interactive multimodal query system integrating both natural language text and graphic displays. The primary concern of the model is to interpret expressions that might involve graphical attributes and expressions whose referents could be objects on the screen. Graphical objects on the screen are used to visualise entities in the application domain and their attributes (in short, domain entities and domain attributes). This is why graphical objects are treated as descriptions of those domain entities/attributes in the literature. However, graphical objects and their attributes are visible during the interaction, and are thus known by the participants of the interaction. Therefore, they themselves should be part of the mutual knowledge of the interaction. This poses some interesting problems in language processing. As part of the mutual knowledge graphical attributes could be used in expressions, and graphical objects could be referred to by expressions. In consequences, there could be ambiguities about whether an attribute in an expression belongs to a graphical object or to a domain entity. There could also be ambiguities about the referent of an expression is a graphical object or a domain entity. The main contributions of this thesis consist of analysing the above ambiguities, designing, implementing and testing a computational model and a demonstrational system for resolving these ambiguities. Firstly, a structure and corresponding terminology are set up, so these ambiguities can be clarified as ambiguities derived from referring to different databases, the screen or the application domain (in short, source ambiguities). Secondly, a meaning representation language is designed which explicitly represents the information about which database an attribute/entity comes from. Several linguistic regularities inside and among referring expressions are described so that they can be used as heuristics in the ambiguity resolution. Thirdly, a computational model based on constraint satisfaction is constructed to resolve simultaneously some reference ambiguities and source ambiguities.

The primary goal of this research is to produce stochastic models which can be used to generate fundamental frequency contours for synthetic utterances. The models produced are binary decision trees which are used to predict a parameterized description of fundamental frequency for an utterance. These models are trained using the sort of information which is typically available to a speech synthesizer during intonation generation. For example, the speech database is annotated with information about the location of word, phrase, segment, and syllable boundaries. The decision trees ask questions about such information. One obvious problem facing the stochastic modelling approach to intonation synthesis models is obtaining data with the appropriate intonation annotation. This thesis presents a method by which such an annotation can be automatically derived for an utterance. The method uses Hidden Markov Models to label speech with intonation event boundaries given fundamental frequency, energy, and Mel frequency cepstral coefficients. Intonation events are fundamental frequency movements which relate to constituents larger than the syllable nucleus. Even if there is an abundance of fully labelled speech data, and the intonation synthesis models appear robust, it is important to produce an evaluation of the resulting intonation contours which allows comparison with other intonation synthesis methods. Such an evaluation could be used to compare versions of the same basic methodology or completely different methodologies. The question of intonation evaluation is addressed in this thesis in terms of system development. Objective methods of evaluating intonation contours are investigated and reviewed with regard to their ability to regularly provide feedback which can be used to improve the systems being evaluated. The fourth area investigated in this thesis is the interaction between segmental (phone) and suprasegmental (intonation) levels of speech. This investigation is not undertaken separately from the other investigations. Questions about phone-intonation interaction form a part of the research in both intonation synthesis and intonation analysis.

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This thesis proposes a safety-oriented variant of Statecharts, called Safecharts, . devoted exclusively for safety-critical systems design. Safecharts maintains two separate representations for functional and safety requirements, bringing the distinctions and dependencies between such :tequirements into sharper focus. A fundamental concept of Safecharts, on which many of its features are based, is an explicit ordering of system states according to the risks posed by them. Mathematically, this takes the form of a risk ordering relation. Recognising the possibility of gaps and inaccuracies in the definition of such a relation (for example, due to human error or the lack of knowledge) , Safecharts imposes an additional clustering of states into what is referred to as risk bands and constructs a risk graph of these states. Based on the above, Safecharts incorporates ways to represent various safety requirements of the system, equipment failures and'failure handli.n. g mechanisms. Safecharts classifies transitions according to their risk natur&'into safe, unsafe and neutraL It also extends their labelling to call for additional safeguards against unsafe transitions and prompt enforcement of safe ones. Relying on the concept of risk distance of transitions, Safecharts provides a safety-oriented scheme for resolving any unpredictable nondeterministic patterns of behaviour. The thesis clarifies the above concepts using illustrations and precise mathematical definitions.

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