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Constraint Logic Programming: Procedures

• Procedure abstraction (not unlike ``call by reference''): 0.430.43

  foo(Z,X) :- Y=2, X=Y+Z.

  main :- foo(K,W), K = 3, write(W).

  • Y local variable; Z,X formal parameters.
  • Calling foo(3,W) adds W=5 to calling context.
• But different ``calling modes'' allowed:
  • Calling foo(K,5) adds K=3 to calling context.
  • Calling foo(K,W) adds W=2+K to calling context (e.g., main above prints ``5'').

• Can be seen as ``reversible'' versions of their imperative syntactic counterparts.
• Procedures define a relation (rather than a function) among arguments.