Exercise 4.8. "Named let" are a variant of let that have the form
<var><bindings><body>)
The and is just as in ordinary let, except that is bound within to a procedure whose body is and whose parameters are the variables in the . Thus, one can repeatedly execute the by invoking the procedure named . For example, the iterative Fibonacci procedure (section 1.2.2) can be rewritten using named let as follows:
(Define (fib N) ( LetFib-iter((a 1)(b 0)(count N)) (if (= count 0) b (fib-iter (+ a B) a (- count 1)))))Modify Let->combination of Exercise 4.6 to also support named Let.
AnalysisI hope we still have something to look at the original text, I have seen many times the modify original is the meaning of the change.
About named let's some comparison of what, you can see here: "Scheme induction" 3 compare do, allow, loop.
From the code of the topic, we can also see that Named-let's name can be taken out by Cadr, which is the Fib-iter in the book. The body section can also be seen with 3 Cdr and a car from the code below.
<var><bindings><body>)
The parameter problem has been said to be binding in the variable, remove the binding with CADDR, and take out the topic example of a, B and count, etc. are used map and car. Remove 1, 0, and N from the topic example using Map and Cadr.
Then we also need to convert named-let into functions, with list to construct these, first of all, of course, ' Define, and then use cons to name and parameter together, and finally the body.
Of course, in the let->combination we need to judge is not Named-let?, then how to judge, first judge whether it is let, and then judge the name of expr is not the symbol (symbol?).
Finally, you can write let-combination. First use the written named-let predicate to judge expr, and then invoke the No. 257 page of the SEQUENCE->EXP function for real, otherwise use cons to continue the construction.
Code(define (named-let-name expr)(cadr expr)) (define (named-let-body expr)(cadddr expr)) (define (named-let-parameters expr)(map car (caddr expr )))(define (named-let-exp expr)(map cadr (caddr expr)) ) (define (named-let. expr)( and ( let) expr) (symbol? ) (cadr expr )))) (define (named-let->func expr)(list ' define (cons (named-let-name EPXR) (named-let-parameters expr)) (named-let-body expr )))(Define (let->combination expr) (if (named-let. expr) (sequence->exp(list (named-let->func expr)(cons (named-let-name expr) (named-let-exp expr )))) (cons (make-lambda (let-vars expr)(list ( Let-body expr )))(let-exp expr ))))"SICP Exercise" 152 Exercise 4.8
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