On Fri, Mar 2, 2012 at 09:41, Roelof Wobben <r.wob...@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. ____________________ Racket Users list: http://lists.racket-lang.org/users
