On Tue, Feb 11, 2020 at 3:28 PM Alain De Vos <devosalai...@gmail.com> wrote:

> But first i need a list of characters. :)
> Does the language has a conversion operator ?

Yes, `string->list`:
But read on …

On Tue, Feb 11, 2020 at 3:28 PM Alain De Vos <devosalai...@gmail.com> wrote:

> 0) Given a string,
> 1) convert to  a list of characters,
> 2) allow me to iterate,
> 3) convert a character to an int ,
> 4) subtract corresponding value of zero,
> 5) allow me to some positional stuff and addition.

If you really want to implement the algorithm you described, here's one way
to do it:
#lang typed/racket
(: string->integer (-> String Integer))
(define (string->integer str)
  (define base (char->integer #\0))
  (for/fold ([acc : Integer 0])
            ([char (in-string str)])
    (+ (* 10 acc) (- (char->integer char) base))))

Again, I actually would do this with `string->number`, as I illustrated
earlier, unless this arithmetic with Unicode scalars is really what you
want to compute. For example, what about "-"?

On Tue, Feb 11, 2020 at 2:15 PM Alain De Vos <devosalai...@gmail.com> wrote:

> But at each step I should raise , this is not ok.
> Otherwise the GUI just keeps eating memory ...

I don't understand what you mean here. Raising an exception should not
cause a memory leak. If you mean that it makes the GUI non-responsive, then
you should catch the exception and show a message to the user (or do the
equivalent using a designated "bad value" instead of an exception).


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