[racket] Contract for methods with optional arguments

From: Matthew Flatt (mflatt at cs.utah.edu)
Date: Sat Aug 31 11:53:41 EDT 2013

I think want `case->`:

 (define/contract get-whatever (case->
                                (-> (vectorof integer?))
                                (-> exact-nonnegative-integer? integer?))
   (lambda ([pos #f])
     (if (not pos)
         (vector-ref inner-vector-of-int pos))))

You may even want to use `case-lambda` for the implementation:

 (define/contract get-whatever (case->
                                (-> (vectorof integer?))
                                (-> exact-nonnegative-integer? integer?))
    [() inner-vector-of-int]
    [(pos) (vector-ref inner-vector-of-int pos)]))

At Sat, 31 Aug 2013 13:39:30 +0200, Diego Sevilla Ruiz wrote:
> Hi all:
> 	I have encountered a situation in which I don't know how to specify the 
> contract for a method that:
> 1. Has a final optional argument
> 2. The value of the optional argument makes the method change the return 
> type of the method.
> It is similar to this:
> (define/public (get-whatever (pos #f))
>    (if (not pos)
>        inner-vector-of-int
>        (vector-ref inner-vector-of-int pos)))
> (that is, if you don't specify a position, return the whole vector, but if 
> you specify a position, return that position).
> 	For what I've seen in the documentation, there exists the ->i function 
> that allos the specification of named parameters, such as (not working but 
> you get the idea):
> (->i () ;; mandatory
>       ([pos (or/c number? boolean?)])  ;; optional
>       [result (pos) (or/c (and/c (=/c #f pos) (vectorof number?))
>                           (and/c (number? pos) number?))])
> (I don't think the result expression is correct, but it expresses what I want).
> 	The real problem comes with methods, because I have ->m and ->dm, but 
> not 
> ->im (->dm does not allow specifying dependent previous values in the 
> result, for example, as far as I know).
> 	So the question is, how to write a method contract for this method?
> 	Thanks in advance,
> 	diego.
> -- 
> Diego Sevilla Ruiz -- http://ditec.um.es/~dsevilla/ -- dsevilla at um.es _.___
> Dep. Ingeniería y Tecnología de Computadores, Facultad de Informática D|TEC
> Univ.de Murcia,Campus Espinardo,30080 Murcia (SPAIN),Tel.+34868887571
> ____________________
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