# 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)