73 lines
2.8 KiB
Markdown
73 lines
2.8 KiB
Markdown
# InMemoryDB.hs Summary
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This module implements an in-memory Datalog database with support for facts, rules, and queries.
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## Data Types
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### Value
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A simple algebraic data type representing values in the database.
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- `ValueInt Int` - Integer value
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- `ValueSymbol String` - Symbolic value
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### InMemoryDB
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The main database structure containing:
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- `relations :: Map RelationId Relation` - Map of relation names to relations
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- `constants :: Set Constant` - The Herbrand universe (all constants in the database)
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### RuleElement
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Represents an entry occurring in a head or body relation.
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- `RuleElementConstant Constant` - A constant term
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- `RuleElementVariable Text` - A variable name
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### RelationRule
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A rule definition containing:
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- `headVariables :: [Text]` - List of variable names in the rule head
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- `body :: [(Relation, [RuleElement])]` - List of body relations with their elements
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### Relation
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A database relation with:
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- `_name :: Text` - Relation name
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- `_arity :: Int` - Number of columns
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- `_tuples :: Set [Constant]` - Set of tuples (facts)
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- `_rules :: [RelationRule]` - Associated rules for deriving new tuples
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### ConstraintElement
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Represents an entry in a rule body constraint, using indices for variables.
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- `ConstraintElementConstant Constant` - A constant term
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- `ConstraintElementIndex Int` - Index into the variable list
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### BodyConstraint
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A body constraint used during rule processing:
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- `_relation :: Relation` - The relation being constrained
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- `_elements :: [ConstraintElement]` - List of constraint elements
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### RuleContext
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Context maintained while processing rules:
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- `__relation :: Relation` - The relation being defined
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- `_headVariables :: [RuleElement]` - Variables collected from head and body
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- `_bodyConstraints :: [BodyConstraint]` - Constraints from rule body
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- `_db :: InMemoryDB` - Current database state
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### InMemoryDBException
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Exception types for error handling:
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- `CannotParseStatementException` - Failed to parse Datalog text
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- `NonFactException` - Expected a fact but got another statement type
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- `NonRuleException` - Expected a rule but got another statement type
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- `NonQueryException` - Expected a query but got another statement type
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- `BadArityException` - Mismatched arity for a relation
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- `VariableLookupException` - Variable not found in context
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- `UnexpectedConstantException` - Found constant where variable expected
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## Type Aliases
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- `Constant = Term` - Constants are Datalog terms (ints, variables, symbols)
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- `RelationId = Text` - Relation identifiers are text strings
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## Key Functions
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- `emptyDB` - Creates an empty database
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- `withFacts` - Creates a database from a list of fact texts
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- `withFactsAndRules` - Creates a database from facts and rules
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- `lookupRelation` - Finds or creates a relation by name
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- `query` - Executes a query (currently not yet implemented)
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