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polymorphic lists, existential types and asorted other hattery
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Date: -- (:)
From: Peng Zang <peng.zang@g...>
Subject: Re: [Caml-list] polymorphic lists, existential_types_and_asorted_other hattery
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Ahh, right!  Sorry, I forgot to mention I'm looking for a possible solution 
without classes.

I ask because most of my code base is modules and functor based and it would 
be a pain to convert over.  Also because performance is typically better with 
just functions and data types.

I feel like a solution without the OO side is possible through perhaps an 
analog of existential types?

Peng

On Tuesday 13 November 2007 04:14:06 pm Dmitri Boulytchev wrote:
>     Try using classes for this purpose:
>
> let show l = List.map (fun x -> x#show) l
>
> class integer x =
>   object
>     method show = string_of_int x
>   end
>
> class floating x =
>   object
>     method show = string_of_float x
>   end
>
> class boolean x =
>   object
>     method show = string_of_bool x
>   end
>
>
> let _ =
>   List.iter
>     (Printf.printf "%s\n")
>     (show
>        [
>      new integer 10;
>          new floating 3.14;
>          new boolean true;
>        ]
>     )
>
>     Best regards,
>     Dmitri Boulytchev,
>     St.Petersburg State University.
>
> > Hi,
> >
> > Is there a way to create lists in which the elements may be of
> > differing types but which all have some set of operations defined
> > (eg. tostr) in common?  One can then imagine mapping over such lists
> > with "generic" versions of those common operations.  Here's a concrete
> > example of what I mean:
> >
> >   module Int = struct
> >     type t = int
> >     let show x = string_of_int x
> >   end
> >   module Float = struct
> >     type t = float
> >     let show x = string_of_float x
> >   end
> >   module Bool = struct
> >     type t = bool
> >     let show x = string_of_bool x
> >   end
> >
> >   let xs = [`Int 1; `Float 2.0; `Bool false]
> >   let showany x = match x with
> >
> >     | `Int x -> Int.show x
> >     | `Float x -> Float.show x
> >     | `Bool x -> Bool.show x
> >
> >   ;;
> >   List.map showany xs;;
> >
> > Essentially we have ints, floats and bools.  All these types can be
> > shown.  It would be nice to be able to create a list of them [1; 2.0;
> > false] that you can then map a generalized show over.  In the above
> > example, I used polymorphic variants in order to get them into the
> > same list and then had to define my own generalized show function,
> > "showany".  This is fine as there is only one shared operation but if
> > there is a large set of these common operations, it becomes
> > impractical to define a generalized version for each of them.
> >
> > I've come across a way to do this in haskell using what they call
> > "existential types".
> >
> >   http://www.haskell.org/haskellwiki/Existential_type
> >
> > I don't really understand existential types however and don't know if
> > OCaml has them nor how to use them.
> >
> > So.  How can one do this in OCaml?  Is there perhaps a camlp4
> > extension that can do this?  Is there a possible functor trick that
> > can take N modules as arguments and spit out a new module with a
> > generalized type that can take on any of the types in the arguments
> > and also make generalized versions of operations common to the N
> > modules?  Are there existential types or equivalents in OCaml?  If so
> > how does one go about using them?
> >
> > Thanks in advance to anyone who forays into this bundle of questions.
> >
> > Peng
>
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