Sure:
#lang racket
(define/contract [unsafe-factorial n]
(-> (and/c integer? (>=/c 0)) (and/c integer? (>=/c 0)))
(if (zero? n)
1
(* n (factorial (- n 10)))))
(define (factorial n) (unsafe-factorial n)) ;; cross the boundary and go right
back in
(unsafe-factorial 5) ; Does not terminate
I suspect you can implement this with a macro now.
> On May 5, 2017, at 5:48 PM, Daniel Prager <daniel.a.pra...@gmail.com> wrote:
>
> Hi Matthias
>
> Thanks for the pointer to Robby's 2014 keynote.
>
> Here's the link for those interested — it's really good:
> https://www.youtube.com/watch?v=gXTbMPVFP1M
> <https://www.youtube.com/watch?v=gXTbMPVFP1M>
>
> * * *
>
> In regard to Racket's use of boundaries, am I right in thinking that it's
> difficult to pull the boundaries in really *tight*?
>
> For instance, the following example does not terminate and cannot help with
> the necessary debugging because the contract is only checked on the call
> through the module boundary, with recursive calls incorrectly and implicitly
> trusted:
>
>
> #lang racket
>
> (module server racket
> (provide (contract-out
> [unsafe-factorial (-> (and/c integer? (>=/c 0))
> (and/c integer? (>=/c 0)))]))
>
> (define (unsafe-factorial n)
> (if (zero? n) ; Naïvely relying on the
> contract to rule out -ve n's
> 1
> (* n (unsafe-factorial (- n 10)))))) ; Bug
>
>
> (require 'server)
>
> (unsafe-factorial 5) ; Does not terminate
>
>
> My best idea at the moment is to add some extra checking, e.g. (just
> duplicating the pre-conditions) ...
>
> (define (safe-factorial n)
> (unless (integer? n)
> (raise-argument-error 'safe-factorial "n must be an integer" n))
> (unless (>= n 0)
> (raise-argument-error 'safe-factorial "n must be >= 0" n))
> (if (zero? n)
> 1
> (* n (safe-factorial (- n 10))))))
>
> ... but this is verbose, no good for higher-oder conditions, and redundant:
> once it has been debugged the contract at the module-boundary suffices.
>
> What do others do / suggest?
>
>
> Dan
>
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