Prolog Macros



Introduction

In the Lisp community, macros are always a big deal. A Lisp macro rewrites Lisp code.

Very often, the phrase "code is data" is used in this context. But this also holds for example for C and Java code: C code is also data, and you can store and access C code as a plain text file. Therefore, this does not capture the main reason why we should care about Lisp macros especially. In a more limited sense, a C preprocessor also supports macros.

What, then, is the core difference between Lisp and C macros? Clearly, Lisp macros are much more powerful because they can easily analyze Lisp code. Why is this? Because Lisp code has a natural, structure-preserving representation as a Lisp data structure! This is not the case for C or Java. For example, there is no built-in data structure in C that faithfully represents C source code in a structure-preserving way. We can represent C source code within C as an array of characters, but that does not preserve the abstract structure of the source code, and it makes basic operations extremely hard. For example: These are questions that are hard to answer in the case of C because it is hard to reason about the structure of C programs in an abstract way, at least much harder than it is in the case of Lisp.

The point of this page is to show you that Prolog has a mechanism that works much like macros in Lisp. In fact, macros are a lot easier to write in Prolog for several reasons. This simplicity is the main reason why Prolog programmers are not as hysteric about this feature as several other language communities. In Prolog, this facility is simply available, and easily used when needed.

Code is more than data

Of course, code is data: We write it down, we read it. Prolog code is more than simply "data" though: Every Prolog clause is a valid Prolog term. This is structured data, which we can analyze and process systematically using built-in features of Prolog. For example, consider a simple Prolog rule:
f(X, Y) :- g(X), h(Y).
    
This is the following Prolog term: If you are ever unsure about what a term "actually" is, use write_canonical/1 to obtain a canonical representation:
?- write_canonical( (f(X, Y) :- g(X), h(Y)) ).
:-(f(A,B),','(g(A),h(B)))
    
Note that we have used parentheses around the term to properly indicate the single argument of the predicate.

Rewriting Prolog code

All widely used Prolog systems provide mechanisms that let you rewrite Prolog code at compilation time.

The advantage is clear: You can perform complex transformations of Prolog code once at compilation time, resulting in often faster code at run time.

In Prolog systems, this facility is typically provided by extensible predicates called term_expansion/2 and goal_expansion/2. These predicates take 2 arguments: If a clause of this extensible predicate succeeds, then the replacement term is used instead of the original term during compilation.

If your Prolog system does not provide this facility, you can easily implement it yourself, using the built-in predicate read/1 to read Prolog terms from a file.

Examples

See the implementation of DCGs for an example of term expansion that is used in almost all widely available Prolog systems. In this case, the expansion mechanism is used to automatically equip DCG rules and all DCG nonterminals with 2 additional arguments in a systematic way.

In SWI-Prolog, integer arithmetic serves as an example of goal expansion. For instance, consider the following Prolog rule whose body consists of a single goal:
integer_successor(I0, I) :- I #= I0 + 1.
    
To see how this is automatically expanded, use listing/2:
?- listing(integer_successor/2).
integer_successor(B, A) :-
        (   integer(A)
        ->  (   integer(B)
            ->  A=:=B+1
            ;   C=A,
                clpfd:clpfd_equal(C, B+1)
            )
        ;   integer(B)
        ->  (   var(A)
            ->  A is B+1
            ;   C is B+1,
                clpfd:clpfd_equal(A, C)
            )
        ;   clpfd:clpfd_equal(A, B+1)
        ).
    
This example illustrates that low-level integer arithmetic is automatically used whenever possible, even if you use higher-level constructs which also work in other directions.


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