~~

  #lang typed-scheme
  (define-struct: Nothing ())
  (define-struct: (a) Just ([v : a]))  
 
  (define-type-alias (Maybe a) (U Nothing (Just a)))  
 
  (: find (Number (Listof Number) -> (Maybe Number)))
  (define (find v l)
  (cond [(null? l) (make-Nothing)]
        [(= v (car l)) (make-Just v)]
        [else (find v (cdr l))]))

2 Responses to “PLT Scheme 4.0 is out!”

1. Sam TH Says:
   June 17th, 2008 at 2331

   Here’s an example of how to deconstruct such a value:

   (let* ([l (list 1 2 3 4)]
           [v (find 3 l)])
       (if (Just? v)
           (Just-v v)
           (error 'not-found)))

   Thanks for trying Typed Scheme!

2. jartur Says:
   June 18th, 2008 at 1238

   Oh, I’ll try that.
   Thanks.

Leave a Reply

Tips [+]

• $$\alpha$$ for inline latex.
• $$!\alpha$$ for block latex. Leave a blank line after it.
• `code` surrounded by blank lines to preserve whitespace.