A number of authors have exported domain-theoretic techniques from denotational semantics to the operational study of contextual equivalence and order. We further develop this, and, moreover, we additionally export topological techniques. In particular, we work with an operational notion of compact set and show that total programs with values on certain types are uniformly continuous on compact sets of total elements. We apply this and other conclusions to prove the correctness of non-trivial programs that manipulate infinite data. What is interesting is that the development applies to sequential programming languages, in addition to languages with parallel features. 1

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in his 87th year whose influence on me and help cannot be overstated. model for PCF of hereditarily sequential functionals is not ω-complete and therefore not continuous in the traditional terminology (in contrast to the old fully abstract continuous dcpo model of Milner). This is also applicable to a wider class of models such as the recently constructed by the author fully abstract (universal) model for PCF + = PCF + parallel if. Here we will present an outline of a general approach to this kind of “natural ” domains which, although being non-dcpos, allow considering “naturally ” continuous functions (with respect to existing directed “pointwise”, or “natural ” least upper bounds) and also have appropriate version of “naturally ” algebraic and “naturally ” bounded complete “natural ” domains. This is the non-dcpo analogue of the wellknown concept of Scott domains, or equivalently, the complete f-spaces of Ershov. In fact, the latter version of natural domains, if considered under “natural ” Scott topology, exactly corresponds to the class of f-spaces, not necessarily complete. 1

hereditarily-sequential functionals is not ω-complete (in contrast to the old fully abstract continuous dcpo model of Milner). This is also applicable to a potentially (universal) model for PCF + = PCF + pif (parallel if). Here we will present an outline of a general approach to this kind of ‘natural ’ domains which, although being non-dcpos, allow considering ‘naturally ’ continuous functions (with respect to existing directed ‘pointwise’, or ‘natural ’ least upper bounds). There is also an appropriate version of ‘naturally ’ algebraic and ‘naturally ’ bounded complete ‘natural’ domains which serves as the non-dcpo analogue of the well-known concept of Scott domains, or equivalently, the complete f-spaces of Ershov. It is shown that this special version of ‘natural ’ domains, if considered under ‘natural ’ Scott topology, exactly corresponds to the class of f-spaces, not necessarily complete. Key words: domain theory, dcpo and non-dcpo domains, Scott topology,

The results reported in Part III consist of joint work with Martín Escardó [14]. All the other results reported in this thesis are due to the author, except for background results, which are clearly stated as such. Some of the results in Part IV have already appeared as [28]. Note This version of the thesis, produced on October 31, 2006, is the result of completing all the minor modifications as suggested by both the examiners in the viva report (Ref: CLM/AC/497773). We develop an operational domain theory to reason about programs in sequential functional languages. The central idea is to export domaintheoretic techniques of the Scott denotational semantics directly to the study of contextual pre-order and equivalence. We investigate to what extent this can be done for two deterministic functional programming languages: PCF (Programming-language for Computable Functionals) and FPC (Fixed Point Calculus).