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macros.pl -- Macro expansion |
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]) ].