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Date: Wed, 17 Feb 2010 17:34:24 +0100
From: =?utf-8?B?U3TDqXBoYW5l?= Gimenez <stephane.gimenez@pps.jussieu.fr>
To: The Caml Mailing List <caml-list@inria.fr>
Subject: type generalization of recursive calls
Message-ID: <20100217163424.GA38458@tenebreuse>
Mail-Followup-To: =?utf-8?B?U3TDqXBoYW5l?= Gimenez <stephane.gimenez@pps.jussieu.fr>,
	The Caml Mailing List <caml-list@inria.fr>
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X-Spam: no; 0.00; gimenez:01 gimenez:01 recursive:01 ocaml:01 recursive:01 syntactical:01 checker:01 pointers:01 cheers:01 phane:98 phane:98 rec:01 recursively:01 pps:01 jussieu:01 

Hi,

I just realized that ocaml generalizes the type of a recursively
defined function *only* for calls which are outside it's own
definition. Recursive calls cannot use a generalized type.

In fact, I'm tring to work with such a data type:

type 'a t =
  | E
  | D of 'a t * 'a t
  | O of 'a
  | I of 'a t t

And, I'm forced to use some dark magic to define simple operations on
it:

let rec map (f : 'a -> 'b) : 'a t -> 'b t =
  begin function
  | E -> E
  | D (t1, t2) -> D (map f t1, map f t2)
  | O a -> O (f a)
  | I tt ->
      I ((Obj.magic map : ('a t -> 'b t) -> 'a t t -> 'b t t) (map f) tt)
  end

Questions:
  - Is it theoreticaly safe to generalize recursive calls ?
  - Is there a syntactical trick to use generalized recursive calls ?
  - Could such a generalization be added to the type checker ?
      - Performance issues ?
      - More obfusctated type checking errors ?

In a related disscution I found, one asked about generalization
between mutualy recursive definitions (same problem). No answers, but
maybe I just lack pointers.

Cheers,
Stéphane