I want to implement a tracing predicate that outputs which predicate is being “currently resolved” from within any predicates that get called during its resolution. To do this I am trying to find a way to set global state to a value only: during execution of a predicate or during any of the children called to “prove” it. Here’s an example that uses an unimplemented setGlobalDuringPredicate/2 predicate as an example: :- dynamic(globalValue/1). globalValue(none). writeGlobal(Tag) :- globalValue(X), writeln(value(Tag, X)). parentPredicate :- % Unclear how to implement setGlobalDuringPredicate % so that I get the output below setGlobalDuringPredicate(value1, childPredicate), writeGlobal(parentPredicate). childPredicate :- writeGlobal(childPredicate). ?- parentPredicate. value(childPredicate, value1) value(parentPredicate, none) I tried implementing setGlobalDuringPredicate using setup_call_cleanup. It almost worked. The problem was that cleanup is only called after all backtracking is done in call. So if childPredicate and parentPredicate were written so that execution backtracks once it would execute like this: :- dynamic(globalValue/1). globalValue(none). setGlobalDuringPredicate(Value, Goal) :- setup_call_cleanup( ( globalValue(PreviousValue), retractall(globalValue(_)), assert(globalValue(Value)) ), ( Goal ), ( retractall(globalValue(_)), assert(globalValue(PreviousValue)) ) ). writeGlobal(Tag) :- globalValue(X), writeln(value(Tag, X)). parentPredicate :- % Unclear how to implement setGlobalDuringPredicate % so that I get the output below setGlobalDuringPredicate(value1, childPredicate), writeGlobal(parentPredicate), fail. backtrack(a). backtrack(b). childPredicate :- backtrack(X), writeln(X), writeGlobal(childPredicate). ?- parentPredicate. a value(childPredicate,value1) value(parentPredicate,value1) % not what I want. I want `none` b value(childPredicate,value1) value(parentPredicate,none) false. Is there a way to get the behavior I want (outside of making every predicate take an argument where I pass along the “current predicate”)?

Typically, by far the most efficient way is to misuse the stack inspection. This is the first step. The no_lco/1 call ensures call_with_context/2 remains on the stack and passing the argument ensures Prolog will not garbage collect the argument. call_with_context(Goal, Context) :- call(Goal), no_lco(Context). no_lco(_). The next step is to retrieve the context. You do that using get_context(Context) :- prolog_current_frame(Me), prolog_frame_attribute(Me, parent_goal, call_with_context(_, Context)). Not tested. Should be pretty close though. This is fairly efficient. It slows down if you call get_context/1 inside deeply nested goals that could not be optimized using LCO (Last Call Optimization). In most scenarios this will be a lot faster than trying to maintain a global variable though.

I see, thanks! It’s a bit complicated but I think I get it :slight_smile:

How to set a global value during execution of a predicate

Help!

stassa.p February 26, 2021, 5:37pm 23

I see, thanks! It’s a bit complicated but I think I get it

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How to set a global value during execution of a predicate

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