The Way 2 Inner Peace.

尾递归改写为递归
let,let*和letrec

十诫

Qians posted @ 2012年6月08日 23:11 in scheme with tags scheme , 1077 阅读

来自<the little schemer>

 

The First Commandment
When recurring on a list of atoms, lat, ask two questions about it: (null? lat) and else.
When recurring on a number, n, ask two questions about it: (zero? n) and else.
When recurring on a list of S-expressions, l,ask three question about it: (null? l), (atom?(car l)), and else.

 

The Second Commandment
Use cons to build lists.

 

The Third Commandment
When building a list, describe the first typical element, and then cons it onto the natu­ral recursion.

 

The Fourth Commandment
Always change at least one argument while recurring.
When recurring on a list of atoms,lat,use (cdr lat).
When recurring on a num­ .ber, n, use (sub1 n).
And when recurring on a list of S-expressions, l, use (car l) and (cdr l) if neither (null? l) nor (atom? (car l)) are true.
It must be changed to be closer to termina­tion. The changing argument must be tested in the termination condition:
when using test termination with cdr, test termination with null?
and when using sub1, test termination with zero?.

 

The Fifth Commandment
When building a value with + ,always use 0 for the value of the terminating line, for adding 0 does not change the value of an addition.
When building a value with x, always use 1 for the value of the terminating line, for multiplying by 1 does not change the value of a multiplication.
When building a value with cons, always consider '()  for the value of the terminating line.

 

The Sixth Commandment
Simplify only after the function is correct.

 

The Seventh Commandment
Recur on the subparts that are of the same nature:
 • On the sublists of a list.
 • On the subexpressions of an arithmetic expression.

 

The Eighth Commandment
Use help functions to abstract from represen­tations.

 

The Ninth Commandment
Abstract common patterns with a new func­tion.


The Tenth Commandment
Build functions to collect more than one value at a time.
 


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