1.1-The-Elements-of-Programming

1.1.5 The Substitution Model for Procedure Application

• Normal-order evaluation:
  • Evaluate the leftmost functions first
  • Evaluate every functions before evaluating any arguments

(sum-of-squares (+ 5 1) (* 5 2))
(+    (square (+ 5 1)).       (square (* 5 2))   )
(+    (* (+ 5 1) (+ 5 1))     (* (* 5 2) (* 5 2)))
(+    (* 6 6))                (* 10 10))
(+       36                      100)
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• Applicative-order evaluation:
  • arguments are evaluated before the function is applied
  • recursive functions may not terminated

(sum-of-squares (+ 5 1) (* 5 2))
(+ (square 6) (square 10))
(+ (* 6 6) (* 10 10))
(+ 36 100)
136