A.25.1 Defining and using macros

Macros are defined for the current module using one of the three constructs below.

#define(Macro, Replacement).
#define(Macro, Replacement) :- Code.
#import(ModuleFile).

Macro is a callable term, not being define(_,_), or import(_). Replacement is an arbitrary Prolog term. Code is a Prolog body term that must succeed and can be used to dynamically generate (parts of) Replacement.

The #import(ModuleFile) definition makes all macros from the given module available for expansion in the module it appears. Normally this shall be appear after local macro definitions.

A macro is called using the term #(Macro). # is defined as a low-priority (10) prefix operator to allow for #Macro. Macros can appear at the following places:

• An entire sentence (clause)
• Any argument of a compound. This implies also the head and body of a clause.
• Anywhere in a list, including as the tail of a list
• As a value for a dict key or as a dict key name.

Macros can not appear as name of a compound or tag of a dict. A term #Macro appearing in one of the allowed places must have a matching macro defined, i.e., #Macro is always expanded. An error is emitted if the expansion fails. Macro expansion is applied recursively and thus, macros may be passed to macro arguments and macro expansion may use other macros.

Macros are matched to terms using Single Sided Unification (SSU), implemented using Head => Body rules. This implies that the matching never instantiates variables in the term that is being expanded.

Below are some examples. The first line defines the macro and the indented line after show example usage of the macro.

#define(max_width, 100).
    W < #max_width

#define(calc(Expr), Value) :- Value is Expr.
    fact(#calc(#max_width*2)).

#define(pt(X,Y), point{x:X, y:Y}).
    reply_json(json{type:polygon,
                    points:[#pt(0,0), #pt(0,5), #pt(5,0)]}).

Macro expansion expands terms #(Callable). If the argument to the #-term is not a callable, the #-term is not modified. This notably allows for #(Var) as used by library(clpfd) to indicate that a variable is constraint to be an (clp(fd)) integer.