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spy_var(?Var)

Create a SPYTERM-port in the debugger and prepare for tracing modifications to Var as MODIFY-ports.
Var
A variable, possibly attributed.

Description

This predicate allows to trace modifications to the variable Var.

When the debugger is on, this predicate causes a SPYTERM-port to be displayed. In the subsequent execution, any variable modification in Var 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.

This is equivalent to 

    spy_term(Var, Var->(constrained of suspend))
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.

Modes and Determinism

Modules

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

Examples

[eclipse 1]: lib(fd).
yes.

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

[eclipse 3]: [X, Y] :: 1..9, spy_var(X), X #> Y, Y #> X.
  (1) 1 CALL  [X, Y] :: 1..9   %> creep
  (1) 1 EXIT  [X{[1..9]}, Y{[1..9]}] :: 1..9   %> creep
  (3) 2 SPYTERM  X{[1..9]}   %> jump to invoc: [3]? 
  (3) 3 MODIFY  X{[2..9]}   %> jump to invoc: [3]? 
  (3) 4 MODIFY  X{[4..7]}   %> jump to invoc: [3]? 

no (more) solution.

See Also

suspend / 3, spy_term / 2, trace_point_port / 3, trace / 1