RDR Syntax

rule --> if conjunction then conclusion.
rule --> if conjunction then conclusion except rule.
rule --> if conjunction then conclusion else rule.

conjunction --> condition.
conjunction --> condition and conjunction.

condition --> any Prolog predicate

conclusion --> term
conclusion --> term because term

term --> any Prolog term

RDR Semantics

RDR's are essentially if-then-else statements that return values, eg.

X is if 1 == 2 then 2+3 else 4*5?

X = 20

That is, if is a function. Note that the result in the then part of the statement is evaluated.

The except part of the rule is evaluate if the conjunction of conditions succeeds, eg.

X is if 1 == 1 then 2+3 except if 2 < 3 then 4*5?

X = 20

In this case, the result of the except part over-rides the original conclusion.

Operator Precedence Table