[plt-scheme] (no subject)

According to your contracts, you are attempting to make 'hour->wages' 
(a function that consumes and produces lists) use 'wage' (a function 
that consumes and produces numbers.)  However, the body of 'wage' uses 
'empty?' ... a function that requires a list.  I would take a few 
minutes to use a design recipe to define and simplify 'wage', a 
function intended to provide the arithmetic that empowers 
'hours->wages' to convert lists.

The second problem needs a contract-purpose-header so that others can 
determine what it does.  Also, an unwritten law of programming is: 
don't use the letter l, it looks like the number 1.  'list' shouldn't 
be used either, because it has too many uses.  On the rare occasion 
that a more descriptive name cannot be determined, I enjoy using liszt 
even though I rarely listen to his music.

rac


On Jan 21, 2006, at 3:07 AM, igor denisov wrote:

> Hello there
>
> Can somebody help me to figure out what is wrong with these codes. 
> DrScheme V208
> Language: Beginning Student.
>
> ;; hours->wages : list-of-numbers  ->  list-of-numbers
> ;; to create a list of weekly wages from a list of weekly hours (alon)
> (define (hours-wages alon)
>   (cond
>     ((empty? alon) empty)
>     (else (cons (wage (first alon)) (hours-wages (rest alon))))))
>
> ;; wage : number  ->  number
> ;; to compute the total wage (at $12 per hour)
> ;; of someone who worked for h hours
> (define (wage h)
>   (cond
>     ((empty? h) 0)
>   (else
>    (* 12 (first h)
>      (wage (rest h))
>      );;multiply
>       );;else
>    );;cond
>   );;define
> Produces a bug: first: expects argument of type <non-empty list>; 
> given 0
>
> (define (dollar-store l)
>   (cond
>     ((empty? l) 0)
>             ((and
>               (< (first l) 1)
>              (< (dollar-store (rest l)) 1))
>              'l)))
> (define l
>   (cons .98 (cons .87 (cons .76 (cons .34 empty)))))
> Produces a bug: <: expects argument of type <real number>; given 'l
>
> Thanks in advance.
> Igor.
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