This library defines a macro expansion mechanism that operates on
arbitrary terms. Unlike term_expansion/2
and goal_expansion/2,
a term is explicitly designed for expansion using the term #(Macro).
Macros are first of all intended to deal with compile time constants.
They can also be used to construct terms at compile time.
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:
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.
A macro #define(Macro, Expanded) :- Body. is, after some
basic sanity checks, translated into a rule
'$macro'(Macro, Var), Body => Var = Expanded.
The #import(File) is translated into :- use_module(File, [])
and a
link clause that links the macro expansion from the module
defined in
File to the current module.
Macro expansion is realised by creating a clause for term_expansion/2
in the current module. This clause results from expanding the first
#define or #import definition. Thus, if macros
are defined before any other local definition for term_expansion/2
it is executed as the first step. The macro expansion fails if no macros
were encounted in the term, allowing other term_expansion rules local to
the module to take effect. In other words, a term holding macros is not
subject to any other term expansion local to the module. It is subject
to term expansion defined in module user and system
that is performed after the local expansion is completed.
#import(File). It calls’$macro’/2 in M,
but fails silently in case Macro is not defined in M
as it may be defined in another imported macro file or further down in
the current file.This predicate is not intended for direct usage.
File:Line:LinePos. If File is unknown it is
unified with -. If Line and/or LinePos are unknown they are
unified with 0. This predicate can be used in the body of a macro
definition to provide the source location. The example below defines #pp(Var)
to print a variable together with the variable name and source location.
#define(pp(Var), print_message(debug, dump_var(Pos, Name, Var))) :-
( var_property(Var, name(Name))
-> true
; Name = 'Var'
),
macro_position(Pos).
:- multifile prolog:message//1.
prolog:message(dump_var(Pos,Name,Var)) -->
[ url(Pos), ': ',
ansi([fg(magenta),bold], '~w', [Name]), ' = ',
ansi(code, '~p', [Var])
].