[racket] Fwd: Re: Is this a good design

From: Roelof Wobben (r.wobben at home.nl)
Date: Fri Mar 2 04:51:41 EST 2012

Op 2-3-2012 10:37, Pierpaolo Bernardi schreef:
> On Fri, Mar 2, 2012 at 09:41, Roelof Wobben<r.wobben at home.nl>  wrote:
>> I did some study on that chapter but I cannnot make it work,
>> This is what I have so  far :
>> ;;String ->  String
>> ;; Get the first character of a string (s) if there is one. error otherwise.
>> ;; given "aaaa" expected "a"
>> ;; given "" expected "
>> (define (first-string s)
>>   (cond
>>     [(eq? s "") (error "first-string" "Empty string found")]    ;; check for
>> a empty string.if so put a error message
>>     [(and (string? s) (not(eq? s "")))  (string-ith s 0)]              ;; if
>> it's a non-empty string then do the trick.
>>     [(number? s) (error "first-string" "No string found")]      ;; if it's a
>> number then put a error message
>>     [(boolean? s) (error "first-string" "No string found")]     ;; if it's a
>> boolean then put a error message
>>     [(image? s) (error "first string" "No string found")]         ;; if it's
>> a image put a error message
>>     ))
>> But when I do (first-string ") then still DrRacket gives his own error
>> message instead of the one I made.
>> Any tips are welcome here.
> The immediate problem is that eq? is not the right predicate for
> checking for empty strings.  Use string=? or string-length.
> Once fixed this, what happens if you call (first-string '(a b c)) ?
> P.

Thanks ,
This works in Windows well.
When I do (first-string ' ( a b c)) I see this error message : quote: 
expected the name of the symbol after the quote, but found a part

I see I made a error. With String -> String I only have to check for 
empty strings so i can delete the (string?s) (boolean?s)  (image?s) 
('number?s) parts.


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