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% Schemas constrain by membership: primitive domains, refinements,
% and foreign keys are one mechanism. A program whose facts violate
% them gets no world at all; this one is admitted.
Relation named: Department schema: #(
[id domain: Integer]
[title domain: String]
)
[Department id: 1 title: "Engineering"]
[Department id: 2 title: "Design"]
Relation named: Employee schema: #(
[name domain: String]
% A foreign key is a membership criterion, nothing more special.
% Only the subject D is free; other attributes stay existential.
[dept domain: ([Department id: D])]
% A refinement: a natural at most 100.
[score domain: ([Natural value: S] :and [LessThanEqual left: S right: 100])]
)
[Employee name: "Marcos" dept: 1 score: 90]
[Employee name: "Ana" dept: 2 score: 75]
Query expression: [Employee name: N dept: D score: S]
% Join through the foreign key.
Query expression:
[Employee name: N dept: D score: S] :and
[Department id: D title: T]
% Try it yourself: append one of these lines and watch admission
% reject the whole program.
% [Employee name: "Ghost" dept: 9 score: 50]
% [Employee name: "Over" dept: 1 score: 101]
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