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
regexp_like	(regexp_like subject "pattern")	string, string literal pattern

regexp_like uses Rust regular expression syntax. Its pattern must be a string literal and is compiled when the query is planned. It is only available as a predicate, not as a function binding.

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

Conditional

Function	Syntax	Argument types	Result
if	(if condition then else)	boolean, any, any	selected branch

if evaluates to the then expression only when its condition produces boolean true; otherwise it evaluates to the else expression.

Evaluation semantics

• Expressions evaluate against a tuple’s variable bindings. A type mismatch, or an arithmetic failure (overflow, divide-by-zero) or generally an argument that is nonsensical for a predicate or function 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]

Note

The evaluation of an expression just “silently failing” might be a bit confusing to users and is different to most EDN Datalog flavoured engines. We are aware of this and will likely consider different error semantics for the expression engine at some point.