:- inline(mypred/1, mytranspred/2).
arranges for mytranspred/2 to be invoked at compile time for each
call to the predicate mypred/1 before it is being compiled.
The transformation predicate receives the original call to mypred/1 as its first argument, and is expected to return a replacement goal in its second argument. This replacement goal replaces the original call in the compiled code. Usually, the replacement goal would be semantically equivalent, but more efficient than the original goal. When the transformation predicate fails, the original goal is not replaced.
A transformation predicate can have an optional third argument which supplies the module in which the call is being compiled.
The inline/2 directive must be issued from the definition module of Pred, and TransPred must be visible from this module.
The transformation predicate for a predicate can be obtained by calling get_flag(Pred, inline, TransPred).
Setting TransPred to =/2 will erase any previously attached transformation predicate.
Transformation can be disabled for debugging purposes by adding
:- pragma(noexpand).
to the compiled file, or by setting the global flag
:- set_flag(goal_expansion, off).
The global flag also controls whether transformations are applied
to goals entered at the interactive toplevel prompt.
:- inline(double/2, trans_double/2).
double(X, Y) :-
Y is 2*X.
trans_double(double(X, Y), Y=Result) :-
not nonground(X), % if X already known at compile time:
Result is 2*X. % do calculation at compile time!
% If we now compile the following predicate involving double/2:
sample :-
double(12,Y), % will be transformed into: Y=24
...
double(Y,Z). % will be compiled as is