Polymorphic method question

brogoff
 Richard Jones
 Jacques Garrigue
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Date:   (:) 
From:  Jacques Garrigue <garrigue@m...> 
Subject:  Re: [Camllist] Polymorphic method question 
From: brogoff <brogoff@speakeasy.net> Subject: [Camllist] Polymorphic method question Date: Mon, 10 Jul 2006 12:21:26 0700 (PDT) > Hi, > I'm sure I'll slap my forehead in disgust when someone lifts the > scales from my eyes, but I'm once again perplexed by a type error > from OCaml when using objects. Can someone tell me why I get the > error from the second case (with the classes connected by "and") when > the first case is OK? > # class virtual ['a] bar = > object > method virtual get : 'a > end > and foobar = > object > method virtual f : 'a . 'a bar > 'a > end;; > Characters 115132: > method virtual f : 'a . 'a bar > 'a > ^^^^^^^^^^^^^^^^^ > This type scheme cannot quantify 'a : > it escapes this scope. Because bar is not yet completely defined when you use it in foobar. This is all related with parameter constraint inference: we don't know yet wether bar's parameter is really polymorphic or is constraint (e.g. 'a = 'b * 'c). Note that the problem does not occur with direct type definitions, as constraint inference is not done in that case (this is one of the reasons you cannot combine type and class definitions.) # type 'a bar = < get: 'a > and foobar = < f : 'a. 'a bar > 'a >;; type 'a bar = < get : 'a > and foobar = < f : 'a. 'a bar > 'a > The only way to solve this problem would be to remove parameter constraint inference from classes. It would certainly break some programs, so this seems difficult. By the way, if you're ready to define your types first, then you can do this properly. # class virtual ['a] bar = object method virtual get : 'a end and virtual foobar = object method virtual f : 'a . 'a bar_t > 'a end;; class virtual ['a] bar : object method virtual get : 'a end and virtual foobar : object method virtual f : 'a bar_t > 'a end > BTW, This example was distilled from one in which I could not cleanly > remove the class recursion, one involving an "extensible visitor" in > which there is a recursion between the visited and visitor classes. > Assuming there's an obvious answer to my first question, is there a > nice workaround in this case? Extensible visitor is more complex. You can already find the polymorphic variant solution at http://www.math.nagoyau.ac.jp/~garrigue/papers/ There is a new solution using private rows and recursive modules inside "Private rows: abstracting the unnamed". With Keiko Nakata, we found another solution using objects in place of polymorhic variants. This is rather verbose, but here is the code. This is close to the Odersky&Zenger approach in Scala, but here our visitor is purely functional. Note how we are not able to use eval's self directly for recursion, as we need an abstract row. Jacques Garrigue (* OO decomposition in Objective Caml *) class type ['a,'exp] visit_add = object method visitNum : int > 'a method visitAdd : 'exp > 'exp > 'a end module type AddTE = sig type ('a,'exp) t type exp = < accept : 'a. ('a, exp) t > 'a > val num : int > exp val add : exp > exp > exp end module type AddT = sig include AddTE val eval : (int, exp) t Lazy.t end module AddF(X : AddT with type ('a,'exp) t = private ('a,'exp) #visit_add) = struct type ('a, 'exp) t = ('a, 'exp) X.t class type exp = object ('e) method accept : 'a. ('a, 'e) t > 'a end class num x = object (_ : #exp) method accept v = v#visitNum x end let num x = new num x class add a b = object (_ : #exp) method accept v = v#visitAdd a b end let add x = new add x class eval = object method visitNum (i : int) = i method visitAdd (r : exp) (l : exp) = r#accept (Lazy.force X.eval) + l#accept (Lazy.force X.eval) end let eval = lazy (new eval) end module rec Add : AddT with type('a,'exp) t = ('a,'exp) visit_add = AddF(Add) class type ['a,'exp] visit_add_neg = object inherit ['a,'exp] visit_add method visitNeg : 'exp > 'a end module type AddNegT = sig include AddT val neg : exp > exp end module AddNegF(X : AddNegT with type ('a,'exp) t = private ('a,'exp) #visit_add_neg) = struct module Add = AddF(X) include (Add : AddTE with type ('a,'e) t = ('a,'e) X.t) class neg x = object (_ : #Add.exp) method accept v = v#visitNeg x end let neg x = new neg x class eval = object inherit Add.eval method visitNeg (e : exp) =  e#accept (Lazy.force X.eval) end let eval = lazy (new eval) end module rec AddNeg : AddNegT with type ('a,'exp) t = ('a,'exp) visit_add_neg = AddNegF(AddNeg) # open Add;; # let e1 = (add (num 2) (num 3));; val e1 : Add.exp = <obj> # e1#accept (Lazy.force eval);;  : int = 5 # open AddNeg;; # let e2 = neg (e1 : Add.exp :> exp);; val e2 : AddNeg.exp = <obj> # e2#accept (Lazy.force eval);;  : int = 5