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Date: | 2006-12-28 (08:07) |
From: | Jason Hickey <jyh@c...> |
Subject: | Re: [Caml-list] Pure visitor patterns |
> On 12/27/06, Jacques Garrigue <garrigue@math.nagoya-u.ac.jp> wrote: >> From: Jason Hickey <jyh@cs.caltech.edu> >> >> > Here are my questions: >> > >> > - Why does 'a escape its scope in the recursive definition? >> >> Because during recursive definitions parameters of these definitions >> are handled as monomorphic. So you cannot generalize the 'a locally. Ah, that makes perfect sense. If I understand correctly, the quantifiers in a mutual recursive class definition are hoisted, like this: The definition class type ['a] c1 = ... and ['b] c2 = ... is really more like the following (pardon my notation): ['a, 'b] (class type c1 = ... and c2 = ...) The mistake is to think of it like simple recursive type definitions, like the following (rather useless) definition. type 'a visitor = { visit_foo : 'a -> foo -> 'a; visit_bar : 'a -> bar -> 'a } and foo = { accept : 'a. 'a -> 'a visitor -> 'a; examine : int } and bar = { accept : 'a. 'a -> 'a visitor -> 'a; examine : bool };; I'm not complaining--the fact that you can write any of these types is very cool. >> Another trick to make this pattern more scalable is to use >> constraints >> for parameters. Very good suggestion. This makes it _much_ easier to deal with the multiplicity of types, since the constraints are linear, not quadratic, in the number of cases. Many thanks for your explanation! Jason -- Jason Hickey http://www.cs.caltech.edu/~jyh Caltech Computer Science Tel: 626-395-6568 FAX: 626-792-4257