Guide to Prolog Programming

© Roman Barták, 1998

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Meta-Programming

Meta-programming is a programming technique that enables manipulation with program structures. Because Prolog uses the same data structures to represent programs as well as data, Prolog is suitable for writing meta-programs.

In the section "Prolog Data Structures", we discussed terms, basic Prolog data structure, and some predicates (arg,functor,=..) which work with terms. Now we extend this set of predicates by predicate "call" that is used to call arbitrary Prolog goal. In fact, one can build a Prolog goal using =.. and then use call to execute this goal.

In the following code we use the features of =.. and call to design a procedure which can apply (map) given function to each element of list. The name of this function is an input parameter of the procedure.

map(FunctionName,[H|T],[NH|NT]):-
   Function=..[FunctionName,H,NH],
   call(Function),
   map(FunctionName,T,NT).
map(_,[],[]).

Now, one can call

?-map(neg,[1,2,3],L).          % result is L=[-1,-2,-3]

where neg is defined in a following way:

neg(A,B):-B is -A.

to get the procedure that negates elements of list.

The same procedure map can be used to get other list processors by adding code of functions, e.g.,

inc(A,B):-B is A+1.
dec(A,B):-B is A-1.

and goal:

?-map(inc,[1,2,3],X),map(dec,X,Y).      % result is X=[2,3,4], Y=[1,2,3]

Compare the above approach with generalized list processor.

Prolog is ready for meta-programing.

In the following sections, we will discuss two particular usage of meta-programming techniques, namely:

Designed and maintained by Roman Barták

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