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spy_term(?Term, ++Cond)

Create a SPYTERM-port in the debugger and prepare for tracing modifications to Term as MODIFY-ports.
A variable or any term containing variables.
A trigger condition (as in suspend/3).


This predicate allows to trace modifications to variables in Term.

When the debugger is on, this predicate causes a SPYTERM-port to be displayed. In the subsequent execution, any variable modification in Term which satisfies the trigger condition Cond will be shown as a MODIFY-port. The SPYTERM and MODIFY-port have the same unique invocation number, therefore the invocation-skip command (i) can be used to follow the chain of modifications.

The trigger conditions are specified in the same way as in the suspend/3 built-in. This feature is implemented using high-priority (1) delayed goals which create the MODIFY-ports. These goals are visible to the user as monitor_term/4 goals among the delayed goals.

CAUTION: Term is interpreted by the debugger as a goal. If Term looks like a call of a visible predicate, this predicate's settings (spy, traceable, etc) are taken into account for the SPYTERM and MODIFY ports as well. In particular, don't use a list for Term, since that looks like the compile-built-in ./2 which is untraceable.

Modes and Determinism


This predicate is sensitive to its module context (tool predicate, see @/2).


(6) out of range
Not a valid trigger condition.


[eclipse 1]: lib(fd).

[eclipse 1]: trace.
Debugger switched on - creep mode

[eclipse 3]: [X, Y] :: 1..9,
	     spy_term(v(X,Y), v(X,Y)->any),
	     X #> Y, Y #> X.
  (1) 1 CALL  [X, Y] :: 1..9   %> creep
  (1) 1 EXIT  [X{[... .. ...]}, Y{[...]}] :: 1..9   %> creep
  (3) 2 SPYTERM  v(X{[1..9]}, Y{[1..9]})   %> jump to invoc: [3]? 
  (3) 3 MODIFY  v(X{[2..9]}, Y{[1..8]})   %> jump to invoc: [3]? 
  (3) 3 MODIFY  v(X{[2..7]}, Y{[3..8]})   %> jump to invoc: [3]? 
  (3) 4 MODIFY  v(X{[4..7]}, Y{[3..6]})   %> jump to invoc: [3]? 
  (3) 4 MODIFY  v(X{[4, 5]}, Y{[5, 6]})   %> jump to invoc: [3]? 

no (more) solution.

See Also

suspend / 3, spy_var / 1, trace_point_port / 3, trace / 1