# Forward Chaining

Forward chaining rules are processed automatically as each rule base is activated.

When a rule base is activated, all of its forward-chaining rules are run in the order that they appear in the .krb file for that rule base.

## Overview of Forward-Chaining

To do forward-chaining, Pyke finds rules whose if clause matches Pyke's list of already known facts (the if clause may match, or succeed, multiple time; see backtracking). Each time a rule succeeds, it fires this rule, which adds the facts in the then clause of that rule to the list of already known facts.

These new facts may fire other forward-chaining rules by matching their if clause. This can go on to any depth. So Pyke ends up linking (or chaining) the then clause of the first rule to the if clause of the next rule.

Note

Forward-chaining continues until no more rules can be fired.

### Reviewing

1. Pyke starts with the if clause of the first rule and checks to see if it matches the known facts.
2. If so, it proceeds to the then clause of that rule (firing the rule).
3. Which may link (or chain) to the if clause of another rule.

Since Pyke processes these rules from if to then to if to then in the manner that we normally think of using rules, it's called forward chaining.

## "Foreach", "Assert" Rather than "If", "Then"

Finally, since the statements within the if clause of the rule contain patterns; they may each match several facts. In this case, the rule will succeed and be fired multiple times.

The statements in the then clause of the rule also contain patterns. Each time the rule is fired, the pattern variables within the then statements are bound to different values so that different facts are asserted.

To avoid confusion, Pyke uses the words foreach and assert rather than if and then for forward-chaining rules. This is to suggest that "for each" combination of facts matching the first list of statements, the rule is fired to "assert" the facts in the second list of statements.

Note

The use of foreach and assert identifies the rule as a forward-chaining rule.

## Example

This example will figure out the paternal ancestry of individuals given a list of starting statements about who the sons of each father are. (Daughters and mothers are omitted to keep the example brief). These facts are stored in a fact base called `family1` as `son_of(son, father)`:

    1  son_of(david, bruce)
    2  son_of(bruce, thomas)
    3  son_of(thomas, frederik)
    4  son_of(frederik, hiram)

We want to derive `father_son` relationships of the following form:

    father_son($father, $son, $prefix)

where

> $son: | is the name of the son (e.g., david)
> $father: | is the name of the father (e.g., bruce)
> $prefix: | is a tuple of prefixes before the 'father' and 'son' titles to indicate the number of generations (e.g., `()` for direct father_son relationships, `(grand)`, `(great, grand)`, etc).

This is done using three forward-chaining rules. Each rule is presented as a separate step:

- Step 1: Direct_father_son
  - Step 1 demonstrates the use of pattern matching to transfer values from one statement within the rule to another statement.
- Step 2: Grand_father_son
  - Step 2 demonstrates backtracking within the premises of a forward-chaining rule. Understanding this will help you to understand backward-chaining rules.
- Step 3: Great_grand_father_son
  - Step 3 demonstrates a recursive forward-chaining rule.

Finally, you will be shown how to Run the Example yourself.

## Step 1: Direct_father_son

First we need to establish the direct father_son relationships (those whose `$prefix` is `()`):

    1  direct_father_son
    2      foreach
    3          family1.son_of($son, $father)
    4      assert
    5          family1.father_son($father, $son, ())

### The Use of Pattern Variables

This demonstrates how pattern variables are used to transfer values from one statement within a rule into another statement within the rule.

This rule has a single statement in its `foreach` clause (on line 3). This statement matches all four `son_of` facts, so the rule succeeds four times; but with different bindings for the `$son` and `$father` pattern variables. This causes different facts to be asserted when the same `assert` clause (on line 5) is run four times; because each time line 5 is run, the values for `$son` and `$father` are transferred from the statement on line 3 to the statement on line 5.

When the rule fires matching line 3 to:

    1  son_of(david, bruce)

It runs line 5 to assert:

    5  father_son(bruce, david, ())

And when the rule fires a second time matching line 3 to:

    2  son_of(bruce, thomas)

It runs line 5 a second time to assert:

    6  father_son(thomas, bruce, ())

The rule fires twice more for the remaining `son_of` facts, asserting two more `father_son` relationships. So this rule adds a total of four new facts:

    5  father_son(bruce, david, ())
    6  father_son(thomas, bruce, ())
    7  father_son(frederik, thomas, ())
    8  father_son(hiram, frederik, ())

## Step 2: Grand_father_son

Now we want to add grand son-father relationships. We have a new rule for this:

     6  grand_father_son
     7      foreach
     8          family1.father_son($father, $grand_son, ())
     9          family1.father_son($grand_father, $father, ())
    10      assert
    11          family1.father_son($grand_father, $grand_son, (grand))

### The Use of Backtracking

The `grand_father_son` rule is run for all combinations of `father_son` facts that satisfy the two `foreach` statements (on lines 8 and 9) and asserts a `(grand)` `father_son` statement (on line 11) for each combination.

This rule is a good example for backtracking and will help later in your understanding of backtracking with backward-chaining. So let's follow the backtracking in the execution of this rule.

The `foreach` clause has two statements (on lines 8 and 9) in it that are both looking for `father_son` facts with a prefix of `()`:

    8  family1.father_son($father, $grand_son, ())
    9  family1.father_son($grand_father, $father, ())

These will be matched to the following `family1` facts (facts 5 through 8):

    5  father_son(bruce, david, ())
    6  father_son(thomas, bruce, ())
    7  father_son(frederik, thomas, ())
    8  father_son(hiram, frederik, ())

Pyke starts at the top of the list of premises and looks for a match for the first premise (on line 8). This matches fact 5, so the first premise succeeds, binding `$father` to `bruce`:

    8  family1.father_son($father, $grand_son, ())    => fact 5, SUCCESS
    9  family1.father_son($grand_father, $father, ())

Success means go down, so Pyke goes to the next premise on line 9. This succeeds with fact 6 (because `$father` is bound to `bruce`):

    8  family1.father_son($father, $grand_son, ())    => fact 5
    9  family1.father_son($grand_father, $father, ()) => fact 6, SUCCESS

Success means go down, but Pyke is at the end of the list of premises, so the rule succeeds and Pyke fires the rule to assert:

    9  father_son(thomas, david, (grand))

Since this is a forward-chaining rule, Pyke wants to get all of the answers from it that it can, so it continues as if it had a failure (i.e., as if it's not happy with this answer).

Note

You'll see later that Pyke doesn't do this automatically with backward-chaining rules.

So Pyke fails back up to the second premise and looks for another `father_son` after fact 6 with `bruce` as the first argument. This fails:

    8  family1.father_son($father, $grand_son, ())    => fact 5
    9  family1.father_son($grand_father, $father, ()) => FAILS

Fail means go up, so Pyke goes up to the first premise and looks for another `father_son` after fact 5, which succeeds for fact 6, binding `$father` to `thomas`:

    8  family1.father_son($father, $grand_son, ())    => fact 6, SUCCESS
    9  family1.father_son($grand_father, $father, ())

Success means go down, so Pyke goes down to the second premise which succeeds for fact 7:

    8  family1.father_son($father, $grand_son, ())    => fact 6
    9  family1.father_son($grand_father, $father, ()) => fact 7, SUCCESS

Success means go down, but Pyke is at the end of the list of premises, so the rule succeeds and Pyke fires the rule to assert:

    10 father_son(frederik, bruce, (grand))

Then Pyke fails back up to the second premise, and continues looking for another match after fact 7. This fails:

    8  family1.father_son($father, $grand_son, ())    => fact 6
    9  family1.father_son($grand_father, $father, ()) => FAILS

Fail means go up, so Pyke goes back to the first premise and continues looking for another match after fact 6. (Since fact 7 is just like the last case, we'll skip matching fact 7 and go straight to the last fact, fact 8). The match to fact 8 succeeds, binding `$father` to `hiram`:

    8  family1.father_son($father, $grand_son, ())    => fact 8, SUCCESS
    9  family1.father_son($grand_father, $father, ())

Success means go down, so Pyke goes to the second premise and looks for a `father_son` for `hiram`. This fails:

    8  family1.father_son($father, $grand_son, ())    => fact 8
    9  family1.father_son($grand_father, $father, ()) => FAILS

Fail means go up, so Pyke goes back up to the first premise and looks for another match after fact 8. There are no more facts, so this fails:

    8  family1.father_son($father, $grand_son, ())    => FAILS
    9  family1.father_son($grand_father, $father, ())

Fail means go up, but Pyke is at the top of the list of premises, so the rule fails and Pyke is done processing it.

Important

Note that the last statement in the `foreach` clause may succeed multiple times (which fires the `assert` clause multiple times).

But the first statement in the `foreach` clause may only fail once. When that happens, the whole rule fails and the show's over for this rule!

So running the `grand_father_son` rule results in addition of these three facts:

    9  father_son(thomas, david, (grand))
    10 father_son(frederik, bruce, (grand))
    11 father_son(hiram, thomas, (grand))    (this is the one we skipped)

## Step 3: Great_grand_father_son

Finally, we want to add great(...) grand son-father relationships. We have a final rule for this:

    12  great_grand_father_son
    13      foreach
    14          family1.father_son($father, $gg_son, ())
    15          family1.father_son($gg_father, $father, ($prefix1, *$rest_prefixes))
    16      assert
    17          family1.father_son($gg_father, $gg_son,
                                    (great, $prefix1, *$rest_prefixes))

Note

Note how the $prefixes for the statement on line 15 are specified as `($prefix1, *$rest_prefixes)`, rather than just `$prefix`. This is done so that it does not match `()`. (But it will still match `(grand)` by binding `$rest_prefixes` to `()`).

This is the only rule that can be recursive. As this rule asserts new facts, those facts may be used by the same rule (by matching the statement on line 15) to produce even more great, great, ... `father_son` relationships.

### Recursive Rules

Running this rule normally will assert the following two facts:

    12 father_son(frederik, david, (great, grand))
    13 father_son(hiram, bruce, (great, grand))

But, since these facts may also be used by the same rule (on line 15), Pyke checks each one by trying to run the rule again just for that new fact.

Trying this for the first new fact: `father_son(frederik, david, (great, grand))` fails to find anything because `david` is not a father.

Trying this for the second new fact: `father_son(hiram, bruce, (great, grand))` results in one more new fact:

    14 father_son(hiram, david, (great, great, grand))

Now this last new fact is tried again with this rule, which fails again because `david` is not a father.

So at this point Pyke is finished with this rule. The rule ended up firing three times, asserting:

    12 father_son(frederik, david, (great, grand))
    13 father_son(hiram, bruce, (great, grand))
    14 father_son(hiram, david, (great, great, grand))

## Running the Example

These rules could be run as follows:

>     >>> from pyke import knowledge_engine
>     >>> engine = knowledge_engine.engine(__file__)
>     >>> engine.activate('fc_related')     # This is where the rules are run!
>     >>> engine.get_kb('family1').dump_specific_facts()
>     father_son('bruce', 'david', ())
>     father_son('thomas', 'bruce', ())
>     father_son('frederik', 'thomas', ())
>     father_son('hiram', 'frederik', ())
>     father_son('thomas', 'david', ('grand',))
>     father_son('frederik', 'bruce', ('grand',))
>     father_son('hiram', 'thomas', ('grand',))
>     father_son('frederik', 'david', ('great', 'grand'))
>     father_son('hiram', 'bruce', ('great', 'grand'))
>     father_son('hiram', 'david', ('great', 'great', 'grand'))

Copyright © 2007-2009 Bruce Frederiksen

Page last modified Mon, Mar 08 2010.