[racket] keyword arguments for type constructors?

Hi Alexander,
I asked this question some time ago:
http://article.gmane.org/gmane.comp.lang.racket.user/3420/match=structure+question

I got a very good answer from Matthias Felleisen who showed how this
could be achieved:
http://article.gmane.org/gmane.comp.lang.racket.user/3437/match=structure+question

Hope this helps in some way.

-- 
Manfred



On Fri, 6 Jun 2014 16:59:41 -0400
"Alexander D. Knauth"
<alexander at knauth.org> wrote:

> Is there a way to define a type constructor that accepts keyword
> arguments?  
> 
> I know that things like the Class type constructor do this already,
> so is there a way to define something like this myself?  
> 
> The reason is I have a struct that represents a dimension (as in
> dimensional analysis and units) and I want a type constructor that
> lets me make types for specific instances of it without having to
> remember what order the fields go in:
> 
> (struct: (Length-Expt Mass-Expt Time-Expt Charge-Expt Temp-Expt)
>   dimension
>   ([length-expt : Length-Expt]
>    [mass-expt : Mass-Expt]
>    [time-expt : Time-Expt]
>    [charge-expt : Charge-Expt]
>    [temperature-expt : Temp-Expt])
>   )
> 
> (define (dimension--kws #:length^ [length-expt 0]
>                         #:mass^ [mass-expt 0]
>                         #:time^ [time-expt 0]
>                         #:charge^ [charge-expt 0]
>                         #:temperature^ [temp-expt 0])
>   (dimension length-expt mass-expt time-expt charge-expt temp-expt))
> 
> ;; I want to be able to do something that would do the same thing as
> this: (it doesn't have to be this easy) (define-type (Dimmension--kws
> #:length^ Length-Expt #:mass^ Mass-Expt
>                               #:time^ Time-Expt
>                               #:charge^ Charge-Expt
>                               #:temperature^ Temp-Expt)
>   (dimension Length-Expt Mass-Expt Time-Expt Charge-Expt Temp-Expt))
> 
> 
>