Non-failure Analysis

• Based on the intuitively simple notion of a set of tests ``covering'' the type of the input variables.

• Clause: set of primitive tests followed by various unifications and body goals.

• The tests at the beginning determine whether the clause should be executed or not (may involve pattern matching, arithmetic tests, type tests, etc.)

• For any given clause, we refer to the conjunction of its primitive tests as ``the test of the clause''.

• The disjunction of all the tests of the clauses that define a particular predicate is referred to as ``the test of that predicate''.

• Example:

  p(X, Y):- X  > 0, Y is X + 1.
  p(X, Y):- X =< 0, Y is -X.

• Test of p/2: .

• For any call to p/2 with X bound to a integer and Y free, at least one of the tests will succeed.

• ``The test of the predicate p/2 covers the type of X''.

• Since the rest of the body literals of p/2 are guaranteed not to fail, at least one of the clauses will not fail, and thus the call will also not fail.