Exercise 4.28 · 习题

Exercise 4.28: Eval uses

actual-value rather than eval to evaluate the operator before

passing it to apply, in order to force the value of the operator. Give

an example that demonstrates the need for this forcing.

练习 4.28：eval 在将运算符传给 apply 之前，使用 actual-value 而非 eval 对其求值，以强迫求出运算符的值。请给出一个例子，说明为何必须进行这种强迫求值。