[plt-scheme] a couple of questions

On Jan 20, 2007, at 2:20 PM, Danny Yoo wrote:
> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
>   (let loop ([input-val (read-prompt)])
>     (with-handlers
>         ((exn:fail:user?
>           (lambda (exn) (printf "Error!~n") (loop (read-prompt)))))
>       (if (not (eof-object? input-val))
>           (begin
>             (printf "~a~n~n" (eval-strict input-val env))
>             (loop (read-prompt)))
>           (printf "~a~n" exit-message))))
> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

A minor point, but perhaps worth mentioning:  I wrote code like this  
piece above about fifty times, and every time it bothered me that I  
had to write 'read-prompt' in three places.  Finally I realized that  
there's no need:

(let loop ()
   (let ([input-val (read-prompt)])
     (with-handlers
         ((exn:fail:user?
           (lambda (exn) (printf "Error!~n") (loop))))
       (if (not (eof-object? input-val))
           (begin
             (printf "~a~n~n" (eval-strict input-val env))
             (loop))
           (printf "~a~n" exit-message)))))

I'm glad to see that I'm not the only one who somehow assumed that  
since this binding was changing on every pass through the loop, that  
it would have to be a loop variable.

All the best,

John Clements

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