Re: [Chicken-users] scheme student perplexed by failures ...

[Thomas Chust]

On Sat, 11 Aug 2012, john saylor wrote:
> [...]
> and here is my propsed solution:
> define js-reverse
>    (lambda (ls)
>        ((reverse-rec ls '()))))
> [...]

Hello John,

your code above lacks an opening paranthesis before the define and it has 
an extraneous pair of parentheses around the call to reverse-rec.

> [...]
> (define js-reverse-rec
>    (lambda (asc dsc)
>        ((if (null? asc)
>            dsc
>            ;(js-reverse-rec (cdr asc) (list dsc (car asc)))))))
>            (js-reverse-rec (cdr asc) (cons (car asc) dsc))))))
> [...]

Again, there is an extraneous pair of parentheses around the use of the if 
syntax.

In Scheme, parentheses are relevant syntax that indicate uses of syntax or 
calls to procedures. The expression

  ((reverse-rec ls '()))

means

  "Call reverse-rec with the arguments ls and '(), then call the result
   with no arguments."

Since the results of reverse-rec and of the conditional in js-reverse-rec 
are not procedures but lists, these extra calls you programmed cause the 
type errors you see.

Ciao,
Thomas


--
When C++ is your hammer, every problem looks like your thumb.

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