求值策略

求值策略（英文：evaluation strategy）係指一隻程式語言用咩方法嚟同啲表達式做求值。

主要策略

舉例說明，想像家陣要行以下噉嘅表達式^[1]

  def double x = (plus x x)
  def average x y = (divide (plus x y) 2)

用正常次序求值（normal-order evaluation）嘅話，會做嘅嘢係「吓吓都郁手拆咗最左嗰個子程序先」→

(double (average 2 4)) =>
(plus (average 2 4) (average 2 4)) =>
(plus (divide (plus 2 4) 2) (average 2 4)) => ...

而用應用次序求值（applicative-order evaluation）嘅話，會做嘅嘢係「吓吓都郁手拆咗最入嗰個子程序先」→

double (average 2 4) =>
double (divide (plus 2 4) 2) =>
double (divide 6 2) => ...

唔同嘅求值策略都有各自嘅利弊，同一條表達式，有可能用一種求值策略會攞到正確答案，但用另一種求值策略就會搞到輕機^[2]。

↑ Abelson, Harold; Sussman, Gerald Jay (1996). Structure and Interpretation of Computer Programs (Second ed.). Cambridge, Massachusetts: The MIT Press. p. 21-22.
↑ Normal, Applicative and Lazy Evaluation. Kevin Sookocheff.