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

[Gabriel Scherer <gabriel.scherer@gmail.com>]

Short answer: no, it is not possible to do this.

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.

On Fri, Oct 26, 2012 at 8:01 PM, "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.
>
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