]> On Wed, Jan 14, 2009 at 04:31:24PM +0100, Julien SIGNOLES wrote: > In the article "Many Holes in Hindley-Milner" [1], Sam Lindley claims > that the type of x is ('a * 'a s, int) NList.t in the following ocaml > program because of Garrigue's relaxed value restriction [2]. > ========== > type 'a s > > module NList : sig > type (+'length, +'elem_type) t > val nil : ('m*'m, 'a) t > val cons: 'a * ('m*'n, 'a) t -> ('m*'n s,'a) t > end = struct > type ('i,'a) t = 'a list > let nil = [] > let cons (x, l) = x :: l > end > > let x = NList.cons (1, NList.nil) > ========== > > But, both with ocaml v3.10.2 and ocaml v3.12.0+dev1 (2008-12-03) (that > is the current cvs version), the infered type of [x] only contains a > weak type variable: ('_a * '_a s, int) NList.t. > > I quickly look at the typing rules introduced by Jacques Garrigue in [2] > and it seems to me that Sam Lindley is right: [x] is generalisable in > the above program. > > So, what's wrong here? You need to say "type +'a s", otherwise the compiler (since "s" is an abstract type) cannot know the variance of that type parameter inside a value of type 'a s. That information is needed to determine how to generalize the type of x. Mark