Hmm ... How do you solve this in Haskell ?
-------- Original Message --------
Subject: Re: type class VS struct/functor
Date: Sat, 19 Jan 2002 01:34:32 GMT
From: neelk(a)alum.mit.edu (Neelakantan Krishnaswami)
Reply-To: neelk(a)alum.mit.edu
Organization: ATT Broadband
Newsgroups: comp.lang.functional
References: <3C48C86E.9282BE00(a)bawi.org>
On Sat, 19 Jan 2002 10:14:22 +0900, =?EUC-KR?B?vsix4r+1?=
<kyagrd(a)bawi.org>
wrote:
>
> What does ML struct/functor have anything better than type classes ?
> For the user type classes feels like implicit functor istantiations
> to specific sturucture, and struct/functor seems just bugglling the
> user to do more typing which can be automated by using type classes.
The advantage of functors shows up when you need to have multiple
implementations of a module for a type. For instance, suppose you
want to implement a set, and you write the functors (OCaml below):
module type EQ =
sig
type t
val eq : t -> t -> bool
end
module type SET =
sig
type elt
type set
val empty : set
val add : elt -> set -> set
val mem : elt -> set -> bool
end
module Set(Eq : EQ) : SET with type elt = Eq.t =
struct
type elt = Eq.t
type set = elt list
let empty = []
let add elt set = elt :: set
let rec mem elt set =
match set with
| [] -> false
| x :: xs -> if Eq.eq x elt then true else mem elt xs
end
Now, suppose you want two kinds of sets of string, one of which is
case-sensitive and one of which is not. You can easily do this with
functors like so
module SensitiveCase =
struct
type t = string
let eq s s' = (s = s')
end
module InsensitiveCase =
struct
type t = string
let eq s s' = (String.lowercase s) = (String.lowercase s')
end
module SensitiveSet = Set(SensitiveCase)
module InsensitiveSet = Set(InsensitiveCase)
Each of these Set types takes a string, but the membership test is
different. This is an annoying case in Haskell, because you can make a
String a member of the Eq typeclass in only one way.
However, I think it's true that typeclasses are less verbose in the
common case. Personally, I want a language with both typeclasses and
functors, so that I can use either as the problem requires.
Neel
