Re: [Caml-list] Generic printer patch

["François Bobot" <bobot@lri.fr>]

Le 08/12/2011 03:33, Edgar Friendly a écrit :
> I'm interested in having compile-time reflection like this, but maybe
> with a bit more generality. Maybe an identifier can be built out of the
> type in a generic way...
>
> external show : 'a -> string = "%type_to_string"
> external print : out_channel -> 'a -> unit
> external cmp : 'a -> 'a -> int = "%type_compare"

When does this function return something different than 0?

Jérémie Dimino a écrit :
>  If we want to really use it we can do something more generic
> than just generating printers. For example we could generate type
> representation values from which we would generate printers,
> marshallers, ...

I'm not sure that we can use a function like :
external type_expr : 'a -> 'a type_expr (with GADT)
(or just external type_expr : 'a -> type_expr)

in order to write a generic printer, since during the definition the 
value is polymorphic. We only know the type at the call site. Of course 
the user can call the function himself :

val gen_to_string : 'a type_expr -> 'a -> string

let v = ... in
(** debug *)
print_string (gen_to_string (type_expr v) v)


Perhaps we can ask that the first argument is implicitly given by the 
OCaml compiler (extending the "type a" notation) :

let gen_to_string type (a:repr) (x:a) =
	...  (repr : a type_expr) ...

....

let v = ... in
(** debug *)
print_string (gen_to_string v)


In fact the LexiFi extension to OCaml called implicit argument, can do 
something similar :
  http://www.lexifi.com/blog/implicit-values
(You can look for  "Combined with GADTs, one could do something like 
(not tested!):")

But with all these solutions we will never be able to write a generic 
printer which can print values that come from a dynamic plugin (um.. 
wait and with first order modules?...)


-- 
François Bobot