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

Op 2-3-2012 10:51, Roelof Wobben schreef:
> 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.
>
> Roelof
>
>
>

Hoi,

I have studied the docs of error handeling but I don't see how I can 
implement this.
Maybe someone can give me a example or I have to wait till I have 
studied the chapter about structs.

Roelof