:- 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 substitution takes place:
trans_pred(OldGoal, NewGoal [, Module]) :- ...
Moreover, a transformation predicate can be source annotation aware; it is then of arity 4 or 5, and should look as follows:
trans_pred(OldGoal, NewGoal, OldAnnGoal, NewAnnGoal [, Module]) :- ...
In this setting, the annotated version of the original goal term
(see read_annotated/2,3) is passed in the third argument, and the
transformation should bind the fourth argument to the annotated
transformed goal term. If no source annotation information was
available for the original goal, the third argument is passed in as
a free variable, and the transformation should not bind the fourth
argument. The optional last argument is again the module in which
the substitution takes place, i.e. where the goal was called.
If inlining is applied to an exported predicate, one must be aware that the replacement goal will be textually substituted for the original goal in an unknown module context. That means that the replacement goal should only contain calls to builtins or explicitly qualified calls to other exported predicates, since the visibility of predicates generally cannot be guaranteed in the module where the substitution takes place.
The inline/2 directive must be issued from the definition module of Pred, and TransPred must be visible from (and is usually defined in) this same module.
The transformation predicate for a predicate can be queried 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) :-
ground(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