Re: [Caml-list] accessing the type of a polymorphic parameter

["\"Mark\"" <mark@proof-technologies.com>]

Superb, thanks David, Lilian, Gabriel.  This may be just about sufficient
for my purposes.  I'll have a play.

As well as predefined OCaml primitive datatypes, what would perfect for me
would be the ability to also do newly created abstract datatypes, and
composite datatypes like lists of strings.  Are either of these possible
with your solutions?

Mark.

on 26/10/12 7:29 PM, David Allsopp <dra-news@metastack.com> wrote:

> ...
> For this explicit example, you could write:
>
> let foo x =
> match Obj.tag (Obj.repr x) with
> x when x = Obj.int_tag -> ...
> | x when x = Obj.string_tag -> ...
>
> but this probably isn't what you're after. The type of foo will also be 'a
> -> '_b so you won't get any help from the type system in the calling of
foo.
> ...


on 26/10/12 7:50 PM, Lilian Jean BESSON <lilian.besson@ens-cachan.fr> wrote:

> ...
> In my code "surcharge.ml", the main functions are "typage" : 'a ->
> string, and "dump" : a' -> string.
> Examples:
> #9> Surcharge.typage;;
> - : 'a -> string = <fun>
> #10> Surcharge.typage 4  (* ok *);;
> - : string = "int"
> #11> Surcharge.typage 8.7  (* ok *);;
> - : string = "float"
> #12> Surcharge.typage "ok";; (* ok *)
> - : string = "string"
> #13> Surcharge.typage 'k'  (* not ok *);;
> - : string = "int"
> #14> Surcharge.typage [4]  (* not ok *);;
> - : string = "array"
> #15> Surcharge.typage [|4|];;
> - : string = "array"
> #16> Surcharge.typage cos  (* not ok *);;
> - : string = "unknown"
> But this function "typage" is very limited : no difference between arrays
> and strings etc...
> ...


on 26/10/12 8:24 PM, Gabriel Scherer <gabriel.scherer@gmail.com> wrote:

> ...
> Long answer: I presume that your implementation works for any type
> providing some fixed set of operations, but that you wish those
> operations to be implemented differently for each type. You can define
> a type ('a ops) describing these operations on the type 'a (as a
> record of functions for example), and use the polymorphic type ('a ->
> 'a ops -> foo) instead of ('a -> foo) to write your function.
> Depending on your specific needs there may be slightly different
> approaches (using a algebraic representation of runtime types rather
> than a typeclass-like dictionary), a very general way being described
> in this blog post by Alain Frisch :
> http://www.lexifi.com/blog/dynamic-types
> It could help to have more details on your needs to suggest a better
> solution.
>
> Wrong answer: yes, using Obj.magic you can access the runtime
> representation of values. You can't distinguish between boolean and
> integers and the empty list, but worrying about that is for the
> coward, let's shoot yourself in the foot -- starting by reading the
> documentation on the representation of OCaml values, chapter
> "interacting with C" in the manual.
> ...