Re: [Caml-list] Polymorphic Variants

[Tom <tom.primozic@gmail.com>]

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On 17/01/07, Jacques GARRIGUE <garrigue@math.nagoya-u.ac.jp> wrote:
>
> From: Tom <tom.primozic@gmail.com>
> > So... why actually are polymorphic variants useful? Why can't they
> simply be
> > implemented as normal, concrete (or how would you call them? ...)
> variants?
>
> The original motivation was for the LablTk library, where some types
> (index for instance) have lots of small variations. At that point
> there where several options
> * overloading (but ocaml loathes overloading, you could say that the
>   total absence of overloading is essential to the language)
>

Is there a reason for that? Is it only hard to implement or are there any
conceptual / strategical / theoretical reasons?

> OCaml does not, as far as I know, have any structural typing for
> records..

Hm... Actually, what I had in mind is nominal subtyping... similar to
objects, in fact, objects in C++-like languages, just that they have no
class methods.

Now... close your eyes (but try to continue reading this ;) ) and imagine
you're in a dreamworld. You are programming in a language that has
  * function overloading that allows you to have
       length "abcd" + length [1; 2; 3]
  * Constructor overloading, eliminating the need of
       type parse_expression =
           Pexp_name of string
         | Pexp_constant of constant
         | Pexp_let of (pattern * parse_expression) * parse_expression
         | Pexp_apply of parse_expression * parse_expression list
         | Pexp_try of parse_expression * (pattern * parse_expression) list

       type typed_expression =
           Texp_ident of ident
         | Texp_constant of constant
         | Texp_let of (pattern * typed_expression) * typed_expression
         | Texp_apply of typed_expression * typed_expression list
         | Texp_try of typed_expression * (pattern * typed_expression) list
    as it can be coded as
       type parse_expression =
           Name of string
         | Constant of constant
         | ...

       type typed_expression =
           Ident of ident
         | Constant of constant
         | ...

  * nominal subtyping of records, with overloaded field names:
       type widget = {x : float; y : float; width: float; height: float} (*
top-level type *)
       type button = {widget | text : string }
       type checkbox = {button | checked : bool}
       type image = {widget | url : string}

       type vector = {x : float; y : float}
       type document {url : url}

    so subtypes could be applied to a function
       fun move : widget -> (float * float) -> unit

       let chk = {x = 0.0; y = 0.0; width = 10.0; height = 12.0; text =
"Check me!"; checked = false}
       move chk (3.0, 3.0)
    and types could be "discovered" at runtime:
       let draw widget =
         typematch widget with
             w : widget -> draw_box (w.x, w.y, w.height, w.width)
           | b : button -> draw_box (b.x, b.y, b.height, b.width); draw_text
b.text
           | i : image -> draw_image i.url (i.x, i.y)
           | ...

Do you think you would be "satisfied" even without polymorphic variants?

I am not saying this just for fun... I want to create a language with
overloading, but I kinda don't really like polymorphic variants... thou if
they turn out to be really useful, I would probably start to like them.

Any comments?

- Tom

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