A totally constraint-based computational architecture is proposed and applied to natural language processing. Here constraint means a design orthogonal to information flow, so as to implement diverse patterns of computation requisite in intelligent systems. In this architecture, declarative semantics is given by measuring the degree of violation of a constraint in terms of real valued potential energy, and context-sensitive control of computation emerges from the dynamics defined thereof. Joint utterance with role switching between a speaker and a hearer in the midst of one sentence is accounted for as a corollary of constraint-based design plus this dynamical control, without recourse to any procedures specific to sentence parsing, generation, role switching, and so on. Since constraint does not refer to information flow, input and output are tightly coupled within a constraint module which is embodied as feedback loops coupling the system and its environment. Joint utterance emerges when the dynamical effect towards that constraint changes and flips this constraint between input and output, while some other constraints are continuously satisfied.
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