Exercise 1.5: Ben Bitdiddle has invented a test to determine whether the in- terpreter he is faced with is using applicative-order evaluation or normal-order evaluation. He defines the following two procedures:
(define (p) (p))
(define (test x y) (if (= x 0) 0 y))
Then he evaluates the expression
(test 0 (p))
What behavior will Ben observe with an interpreter that uses applicative-order evaluation? What behavior will he observe with an interpreter that uses normal- order evaluation? Explain your answer. (Assume that the evaluation rule for the special form if is the same whether the interpreter is using normal or applicative order: The predicate expression is evaluated first, and the result determines whether to evaluate the consequent or the alternative expression.)
(test 0 (p)) gives
0 if normal-order is used; the program hangs if applicative-order is used.
(p) is pointed to itself which introduces an infinite interpretation loop. And in normal-order the error will have no chance to expose since the
0 circuit is triggered first.
Notice: This Gitbook uses
klipse plugin to run the inlined scheme code, and under the hood it uses
biwascheme to interpret the code, which is actually using applicative-order, so I disabled the running for
(test 0 (p))
For more information, see this issue: https://github.com/biwascheme/biwascheme/issues/214