Expression Engine

The expression engine evaluates the predicates and functions that appear inside Datalog :where clauses. It is a row-at-a-time evaluator over a tree of expressions built from variables, literals and supported predicates and functions.

There are two ways to use an expression in a clause:

• Predicates — a boolean expression that filters rows.

  [(< ?age 30)]

• Function binding — an expression whose result is bound to a new variable.

  [(+ ?x 1) ?y]
  [(upper ?name) ?upper-name]

Predicates

Predicates return a boolean and are used as filters.

Predicate	Syntax	Argument types
<	(< a b)	any comparable, same kind
<=	(<= a b)	any comparable, same kind
>	(> a b)	any comparable, same kind
>=	(>= a b)	any comparable, same kind
=	(= a b)	any
not=, !=	(not= a b)	any
not	(not x)	boolean

Functions

Functions return a value and are used in a function binding to introduce a new variable.

Arithmetic

Function	Syntax	Argument types	Result
+	(+ a b)	long/double	long/double
-	(- a b)	long/double	long/double
*	(* a b)	long/double	long/double
/, quot	(/ a b)	long/double	long/double
mod	(mod a b)	long/double	long/double
abs	(abs x)	long/double	long/double

Notes:

• Integer arithmetic uses checked operations — overflow makes the row drop out.
• Division or modulo by zero is treated as no result rather than an error.
• Mixed long/double arithmetic promotes both sides to double.

String

Function	Syntax	Argument types	Result
concat	(concat a b)	string, string	string
upper	(upper s)	string	string
lower	(lower s)	string	string
strlen	(strlen s)	string	long
str	(str x)	long, bigint, double, float, boolean, string, uuid	string

Notes:

• str falls back to a debug-style representation for any argument type not listed above.

Date/Time

Function	Syntax	Argument type	Result
year	(year t)	instant	long
month	(month t)	instant	long

Evaluation semantics

• Expressions evaluate against a row’s variable bindings. An unbound variable, a type mismatch, or an arithmetic failure (overflow, divide-by-zero) all produce “no result” — the row is excluded from a predicate and produces no binding from a function.
• Predicate evaluation returns true only when the expression yields boolean true; anything else is treated as false.
• Expressions can be nested freely, for example:

  [(not (< ?x 10))]
  [(+ (* ?x 2) 10) ?y]