SWI-Prolog -- freeze/2

Documentation
- Reference manual
  - Constraint Logic Programming
    - Coroutining
- Packages

Availability:built-in

freeze(+Var, :Goal)

Delay the execution of Goal until Var is bound (i.e. is not a variable or attributed variable). If Var is bound on entry freeze/2 is equivalent to call/1. The freeze/2 predicate is realised using an attributed variable associated with the module freeze. Use frozen(Var, Goal) to find out whether and which goals are delayed on Var.