pyrites/haskell-experiments/notes/InMemoryDB-summary.md

InMemoryDB.hs Summary

This module implements an in-memory Datalog database with support for facts, rules, and queries.

Data Types

Value

A simple algebraic data type representing values in the database.

• ValueInt Int - Integer value
• ValueSymbol String - Symbolic value

InMemoryDB

The main database structure containing:

• relations :: Map RelationId Relation - Map of relation names to relations
• constants :: Set Constant - The Herbrand universe (all constants in the database)

RuleElement

Represents an entry occurring in a head or body relation.

• RuleElementConstant Constant - A constant term
• RuleElementVariable Text - A variable name

RelationRule

A rule definition containing:

• headVariables :: [Text] - List of variable names in the rule head
• body :: [(Relation, [RuleElement])] - List of body relations with their elements

Relation

A database relation with:

• _name :: Text - Relation name
• _arity :: Int - Number of columns
• _tuples :: Set [Constant] - Set of tuples (facts)
• _rules :: [RelationRule] - Associated rules for deriving new tuples

ConstraintElement

Represents an entry in a rule body constraint, using indices for variables.

• ConstraintElementConstant Constant - A constant term
• ConstraintElementIndex Int - Index into the variable list

BodyConstraint

A body constraint used during rule processing:

• _relation :: Relation - The relation being constrained
• _elements :: [ConstraintElement] - List of constraint elements

RuleContext

Context maintained while processing rules:

• __relation :: Relation - The relation being defined
• _headVariables :: [RuleElement] - Variables collected from head and body
• _bodyConstraints :: [BodyConstraint] - Constraints from rule body
• _db :: InMemoryDB - Current database state

InMemoryDBException

Exception types for error handling:

• CannotParseStatementException - Failed to parse Datalog text
• NonFactException - Expected a fact but got another statement type
• NonRuleException - Expected a rule but got another statement type
• NonQueryException - Expected a query but got another statement type
• BadArityException - Mismatched arity for a relation
• VariableLookupException - Variable not found in context
• UnexpectedConstantException - Found constant where variable expected

Type Aliases

• Constant = Term - Constants are Datalog terms (ints, variables, symbols)
• RelationId = Text - Relation identifiers are text strings

Key Functions

• emptyDB - Creates an empty database
• withFacts - Creates a database from a list of fact texts
• withFactsAndRules - Creates a database from facts and rules
• lookupRelation - Finds or creates a relation by name
• query - Executes a query (currently not yet implemented)