A type system for object models is described that supports subtyping, unions, and overloading of relation names. No special features need be added to the modelling language; in particular, there are no casts, and the meaning of an object model can be understood without mentioning types. A type error is associated with an expression that can be proved to be _irrelevant_, in the sense that it can be replaced by an empty set or relation without affecting the value of its enclosing constraint. Relevance is computed by a simple abstract interpretation.
Schematic tables are a new representation for conditionals. Roughly a cross between decision tables and data flowgraphs, they represent computation and decision-making orthogonally. They unify the full range of conditional constructs, from if statements through pattern matching to polymorphic predicate dispatch. Program logic is maintained in a declarative canonical form that enforces completeness and disjointness among choices. Schematic tables can beused either as a code specification/generation tool, or as a self-contained diagrammatic programming language. They give program logic the clarity of truth tables, and support high-level direct manipulation of that logic, avoiding much of the mental computation demanded by conventional conditionals.
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