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As discussed in the User's Guide, TRALE's logical-variable macros
(unlike ALE macros) assume structure-sharing of their variables if
they are used more than once in the definition of the macro. What
TRALE does during compilation of logical-variable macros is simply
translate into ALE macros such that structure-sharing between
variables is preserved. For example, the following logical-variable
macro
foo(X,Y) := f:X, g:X, h:Y.is translated into
foo(X,Y) macro f:(X,_A), g:(X,_A), h:(Y,_B).The added variables
_A and _B are there to enforce
structure sharing among multiple occurrences of the logical-variable
macros X and Y. If a guard declaration is given in the
definition of the logical-variable macro to restrict the type of the
variables, then that is translated as a description as in the
following example:
foo(X-a,Y-b) := f:X, g:X, h:Y foo(X,Y) macro f:(X,a,_A), g:(X,a,_A), h:(Y,b,_B)To reiterate, the added variables
_A and _B make ALE
treat the variables in the right-hand side not as logical variables
but as normal Prolog variables.
Let us now see how TRALE performs this translation. First, TRALE
gathers all :=/2 predicates (logical-variable macros) in a list
and hands them to compute_macros/2 which in turn goes through
each macro one by one generating left-hand side arguments for a
corresponding macro/2 definition with the same predicate name
and arity as the left-hand side of the original :=/2. The
descriptions used as arguments in the new left-hand side, however, are
provided by add_lv_macro_descs/4.