Pyke logical programming (8): Pattern Matching: "answers to pathological questions"

Answers to pathology questions

This is the answer to the following question:

A pathological problem

After the two tuples match in one mode, what is the value of the pattern variable $ y constraint:

• Tuples Mode:
  • (Ho, $ _, ($ A, $ A), ($ A, $ A, $ B), ($ A, * $ B ))
• Tuples mode B:
  • ($ X, $ X, $ Y)

Answer

Let's match and combine the members of the two tuples one by one.

• (Ho, $ _, ($ A, $ A) matches with $ X.

  Matched successfully:

  $ X constraints to (Ho, $ _, ($ A, $ ))

• ($ A, $ A, $ B) matches with $ X.

  Because $ X has bound the value above, this matching actually becomes:

  ($ A, $ A, $ B) matches with (Ho, $ _, ($ A, $)

  The following constraints are imposed:

  $ A to Ho

  $ B to ($ A, $)

  $ _ Is an anonymous variable with no value. It cannot constrain other variables or be subject to other variables.

• ($ A, * $ B) matches with $ Y.

  Because $ A and $ B already have constraints, this match actually becomes:

  (Ho, Ho, Ho) matches with $ y

  The following constraints are imposed:

  $ Y to (Ho, Ho, Ho)

In short, the matching process completes the constraints of the following values:

$ X to (Ho, $ _, ($ A, $ ))

$ A to Ho

$ B to ($ A, $)

$ Y to (Ho, Ho, Ho)

The conclusion is: $ Y equals (Ho, Ho, Ho ).

Note: If the answer is correct, you should use Pyke.