Re: [Caml-list] Subclasses and pattern matching

[Michal Moskal <malekith@pld-linux.org>]

On Fri, Sep 19, 2003 at 10:21:56AM -0400, katre wrote:
> Hello again!
> 
> I'm working on a project where I want to create a list of data, which is
> all similar but not identical.  In other languages with which I am
> familiar, I would do this with a base class and a few subclasses, as
> such:
> 
> class virtual abstractClass = 
>     object
> 
>     method virtual to_string : unit -> string
> end
> 
> class subclass1 = 
>     object
>     inherit abstractClass
> 
>     method thing () = "subclass1 thing"
>     method to_string () = "subclass 1"
> end
> 
> class subclass2 = 
>     object
>     inherit abstractClass
> 
>     method thing2 () = "subclass2 thing"
>     method to_string () = "subclass 2"
> end
> 
> And I would create the list as
> 
> let l = [ (new subclass1 :> abstractClass); (new subclass :> abstractClass); (new subclass1 :> abstractClass); (new subclass2 :> abstractClass) ];
> 
> This seems to work fine.

The best way that would allow _ -> ... thing is something like this:

type specific = Subclass1 of subclass1 | Subclass2 of subclass2 | ... |
SubclassN of subclassN

type data = abstractClass * specific

match l with
| (_, Subclass1 x) :: xs -> ... 
| (_, Subclass2 x) :: xs -> ...
| (x, _) :: xs -> ...

In general there are no runtime types in OCaml, you have to organize
them yourself.

-- 
: Michal Moskal :: http://www.kernel.pl/~malekith : GCS {C,UL}++++$ a? !tv
: When in doubt, use brute force. -- Ken Thompson : {E-,w}-- {b++,e}>+++ h

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