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

[Anil Madhavapeddy <anil@recoil.org>]

You could use the Dyntype extension for this, if you don't mind a spot
of Camlp4 being involved: https://github.com/mirage/dyntype

Toplevel:

$ #require "dyntype.syntax" ;;
$ type t = { foo: int; bar: string }  with type_of;;
type t = { foo : int; bar : string; }
val type_of_t : Dyntype.Type.t = Dyntype.Type.Ext ("t", 
   Dyntype.Type.Dict (`R, [("foo", `RO, Dyntype.Type.Int (Some 63)); 
   ("bar", `RO, Dyntype.Type.String)]))

...and you can collect and inspect the dynamic types in other code.

This won't work with polymorphic types, but you can probably rephrase
your problem to it work with this approach.

Note that this suffers from the usual drawbacks of camlp4-based
interpretation of type declarations, but it works fairly well "in the
wild" and with an existing, unmodified OCaml toolchain.

-anil

On 26 Oct 2012, at 11:01, "Mark" <mark@proof-technologies.com> wrote:

> Hello.
> 
> Is it possible in OCaml for a polymorphic function to somehow have
> access to the concrete type of an actual parameter used in a call of
> the function?
> 
> For example, is there some way of writing a polymorphic function 'foo'
> list this:
> 
>     let foo x =
>        match (type_of x) with
>          Int    -> ....
>        | String -> ....
> 
> where 'type_of' is some function that returns a representation of the
> concrete type of its argument.
> 
> I want this because I want to implement an ad-hoc polymorphic
> function.  I know this is outside the bounds of "normal" OCaml, but is
> there some nasty part of the language like Obj.magic that I could use
> to achieve this?
> 
> Mark.
> 
> -- 
> Caml-list mailing list.  Subscription management and archives:
> https://sympa.inria.fr/sympa/arc/caml-list
> Beginner's list: http://groups.yahoo.com/group/ocaml_beginners
> Bug reports: http://caml.inria.fr/bin/caml-bugs