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Evaluation Strategy

Crucial issue of parameters

• Call-by-value
  • Evaluate actual parameter immediately at point of invocation (eager evaluation).
• Call-by-name
  • Delay evaluation of actual parameter until the point of its use in the body of the abstraction (lazy evaluation).
• Example:
  • var A: newint
    proc P(M: intexp) = A:=M; A:=M
    in call P(@A+1)
  • Eager evaluation yields A=2.
  • Lazy evaluation yields A=3!