backward_rule.n3

@prefix : <urn:example:> .
@prefix log: <http://www.w3.org/2000/10/swap/log#> .
@prefix list: <http://www.w3.org/2000/10/swap/list#> .

:Alice a :Person .

{
    :Alice :built-in-1 :Ok .

    :Alice :built-in-3 :Ok .
    ?Who :built-in-3 :Ok .
    ?Who log:equalTo :Alice .

    :Alice :built-in-4 ?X .
    :Alice :built-in-4 (:Alice) .
}
=>
{
    :built-in-4 :has ?X .
    :test :is true .
} .

# A backward rule is like a user defined built-in. In the body you specify
# specify what must be true, in the head you define the built-in template.
# It works a bit like stating in an imperative language:
#
#   function (:Alice,:built-in-1,:Ok) {
#       :Alice a :Person .
#   }
{
    :Alice :built-in-1 :Ok .
}
<= # Notice the backward arrow here
{
    :Alice a :Person .
} .

# The head can only contain one triple. The example below will not work!
#{
#    :Alice :built-in-2 :NotOk;
#           :test 123 .
#}
#<=
#{
#    :Alice a :Person .
#} .

# More interesting backward rules can be created by adding variab;es
{
    ?Who :built-in-3 :Ok .
}
<=
{
    ?Who a :Person .
} . 

# Or, adding other built-ins
{
    ?Who :built-in-4 ?Result .
}
<=
{
    ?Who a :Person .
    ( () (?Who)) list:append ?Result .
    # The result will be bound to the result in the head
} .