[plt-scheme] evaluating contents of a string

On Tue, 25 Jul 2006, David Richards wrote:

> "(lambda () (current-seconds))"
>
> What is the 'best practice', or most elegant way to obtain a scheme 
> procedure from the contents of a string?

Hi David,

Is a requirement that we allow arbitrary lambdas, or can something more 
restrictive do?  We can create a mapping between symbols and function 
values:

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
(require (lib "etc.ss"))
   ;; etc.ss contains the hash-table helper function
(define functions
   (hash-table ('square (lambda (x) (* x x)))
               ('add1 add1)
               ('double (lambda (x) (* x 2)))))
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

which is a bit more controlled.  (This code could be modified to work on 
strings instead of symbols.)


Use of any function in this table can still be driven by the user:

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
> (let loop ()
     (let ([cmd (read)])
       (printf "(~a 42) = ~a~n" cmd ((hash-table-get functions cmd) 42))
       (loop)))
square
(square 42) = 1764
double
(double 42) = 84
add1
(add1 42) = 43
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;


Best of wishes!