Declarative and Procedural Semantics.

The term declarative semantics refers to interpreting Prolog statements as predicate logic. Other statements that logically follow from the program are considered to be true. Consider for example,

mother(anne,    bridget).
mother(abigail, bridget).
mother(bridget, carol).

grandmother(C, GM) <= mother(C, M) and mother(M, GM).

? grandmother(anne, carol).

{ Grand Mother }

The query logically follows from the other statements because it matches the general rule if C is anne, GM is carol, and M is bridget.

[*Exercise: Now change the query above to ask "of whom is carol a grandmother?"]

The term procedural semantics refers to interpreting Prolog statements as procedure definitions to be called and run. The three Prolog statements above can be thought of as definitions of procedures `mother' and `grandmother'. In order to answer the previous query, call the procedure `grandmother' which calls `mother' twice. Procedural semantics require notions of the order of execution, a search strategy and so on.

Declarative semantics are easier to think about and to program in terms of and procedural semantics are needed for implementations. Unfortunately there is a gap between the two semantics. In some problems it is necessary to consider the procedural semantics to be able to write a correct Prolog program. Much research is aimed at reducing this gap.

[Previous Page] [Next Page] [Index] © L. Allison, Dept. Computer Science, Monash University