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Calling Patterns

For any given predicate with arity greater than 0, each argument may be intended to have one of three calling patterns:

Input ---indicated with a +
Output ---indicated with a -
Indeterminate ---indicated with a ? (+ or -)

For example, successor/2 above requires a calling pattern of

1st argument must be + 2nd argument can be + or - and is therefore ?

We write this as

 
 mode successor(+,?).

The notation used here is consistent with the mode declarations found in many Prolog libraries. For a further example, the mode declaration of is/2 is mode is(?,+).

Because of the discrepancy between the declarative and the procedural aspects of Prolog we often need to think carefully about the intended usage of a predicate. It is good practice to comment your code to indicate a predicate's intended usage.

Paul Brna
Mon May 24 20:14:48 BST 1999