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12.2  Using the macros

The following declarations and built-ins control macro expansion:
local macro(+TermClass, +TransPred, +Options)
define a macro for the given TermClass. The transformation will be performed by the predicate TransPred.
export macro(+TermClass, +TransPred, +Options)
as above, but available to other modules.
erase_macro(+TermClass, +Options)
erase a currently defined macro for TermClass. This can only be done in the module where the definition was made.
current_macro(?TermClass, ?TransPred, ?Options, ?Module)
retrieve information about currently defined visible macros.
Macros are selectively applied only to terms of the specified class. TermClass can take two forms:
transform all terms with the specified functor
transform all terms of the specified type, where Type is one of compound, string, integer, rational, float, breal, atom, goal1.
The +TransPred argument specifies the predicate that will perform the transformation. It has to be of arity 2 or 3 and should have the form:
trans_function(OldTerm, NewTerm [, Module]) :- ... .
At transformation time, the system will call TransPred in the module where macro/3 was invoked. The term to transform is passed as the first argument, the second is a free variable which the transformation predicate should bind to the transformed term, and the optional third argument is the module where the term is read or written.

Options is a list which may be empty (in this case the macro defaults to a local read term macro) or contain specifications from the following categories: The following shorthands exist:
local/export portray(+TermClass, +TransPred, +Options)
portray/3 is like macro/3, but the write-option is implied.
inline(+PredSpec, +TransPred)
inline/2 is the same as a goal-read-macro. The visibility is inherited from the transformed predicate.
Here is an example of a conditional read macro:
[eclipse 1]: [user].
 trans_a(a(X,Y), b(Y)) :-    % transform a/2 into b/1,
        number(X),           % but only under these
        X > 0.               % conditions

:- local macro(a/2, trans_a/2, []).
  user       compiled traceable 204 bytes in 0.00 seconds

[eclipse 2]: read(X).
        a(1, hello).

X = b(hello)                 % transformed
[eclipse 3]: read(X).
        a(-1, bye).

X = a(-1, bye)               % not transformed
If the transformation function fails, the term is not transformed. Thus, a(1, zzz) is transformed into b(zzz) but a(-1, zzz) is not transformed. The arguments are transformed bottom-up. It is possible to protect the subterms of a transformed term by specifying the flag protect_arg.

A term can be protected against transformation by quoting it with the “protecting functor” (by default it is no_macro_expansion/1):
[eclipse 4]: read(X).
        a(1, no_macro_expansion(a(1, zzz))).
X = b(a(1, zzz)).
Note that the protecting functor is itself defined as a macro:
trprotect(no_macro_expansion(X), X).
:- export macro(no_macro_expansion/1, trprotect/2, [protect_arg]).
A local macro is only visible in the module where it has been defined. When it is defined as exported, then it is copied to all other modules that contain a use_module/1 or import/1 for this module. The transformation function should also be exported in this case. There are a few global macros predefined by the system, e.g. for -->/2 (grammar rules, see below) or with/2 and of/2 (structure syntax, see section 5.1). These predefined macros can be hidden by local macro definitions.

The global flag macro_expansion can be used to disable macro expansion globally, e.g. for debugging purposes. Use set_flag(macro_expansion, off) to do so.

The next example shows the use of a type macro. Suppose we want to represent integers as s/1 terms:
[eclipse 1]: [user].
 tr_int(0, 0).
 tr_int(N, s(S)) :- N > 0, N1 is N-1, tr_int(N1, S).
 :- local macro(type(integer), tr_int/2, []).

[eclipse 2]: read(X).

X = s(s(s(0)))
When we want to convert the s/1 terms back to normal integers so that they are printed in the familiar form, we can use a write macro. Note that we first erase the read macro for integers, otherwise we would get unexpected effects since all integers occurring in the definition of tr_s/2 would turn into s/1 structures:
[eclipse 3]: erase_macro(type(integer)).

[eclipse 4]: [user].
 tr_s(0, 0).
 tr_s(s(S), N) :- tr_s(S, N1), N is N1+1.
 :- local macro(s/1, tr_s/2, [write]).

[eclipse 2]: write(s(s(s(0)))).

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