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[Caml-list] extensible records again
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Date: -- (:)
From: Michael Vanier <mvanier@c...>
Subject: Re: [Caml-list] extensible records again

> Date: Sun, 21 Mar 2004 03:08:23 -0500
> From: Oleg Trott <oleg_trott@columbia.edu>
> 
> Michael Vanier wrote:
> 
> >[...] I want users to be able to add
> >their own types to the language without hacking the core type definition.
> >  
> >
> 
> Note that in a language like Scheme, users can not add new types other 
> than by combining the primitives. So, I suppose you aren't really 
> talking about language users, but those who extend the language.

Exactly.  The analogy I give is with a language like python, where you can
add new types at the C level.  Of course, this can be done in e.g. PLT
scheme or guile or perl or ruby as well.

> 
> The obvious thing to do is to use polymorphic variants. E.g.
> 
> (* "core language" *)
> 
> let plus a b =
>    match (a, b) with
>    |  (`Int x), (`Int y) -> `Int (x + y)
>     (*  ... *)
>    |  _ -> failwith "runtime type error: argument is not a number"
> 
> >I can do this as follows:
> > 
> >    (* types.mli *)
> >    type data =
> >        Int of int
> >      | Float of float
> >      | ...
> >      | Extension of exn
> > 
> >    (* files.ml *)
> >    exception File of out_channel
> >                                                                                        
> >    let close (d : data) =
> >      match data with
> >        Extension ext ->
> >          (match ext with
> >              File f -> (* close the file *)
> >            | _ -> raise (Failure "invalid type")))
> >        | _ -> raise (Failure "invalid type")
> >                                                                                        
> >This is OK, but it abuses the exception system.  I asked for ways to get the
> >same result with polymorphic variants.  A suggested approach (thanks to
> >Aleksey Nogin) was this:
> >                                                                                        
> ># type 'a data = Int of int | Ext of 'a;;
> >type 'a data = Int of int | Ext of 'a
> ># let open_file f =
> >      match f with
> >         Ext `File f -> print_string ("Opening file" ^ f)
> >       | _ -> raise (Invalid_argument "open_file")
> >   ;;
> >val open_file : [> `File of string] data -> unit = <fun>
> ># let run ( f :  [> ] data -> unit) = f (Ext (`Foo 1));;
> >val run : ([> `Foo of int] data -> unit) -> unit = <fun>
> ># run open_file;;
> >Exception: Invalid_argument "open_file" 
> >  
> >
> 
> 
> (* language exension example, adding "files" *)
> 
> let open = function `File f -> open_file f | _ -> failwith "runtime type 
> error: argument is not a file"
> 
> 
> HTH
> Oleg
> 

I don't know if I can do this.  I have a top-level "data" type which all
data objects must be instances of.  So in the case above, "open" would be a
function which took an argument of type "data".  The question is then: how
do I specify the "data" type without explicitly making it polymorphic?

Mike

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