From: Jeff Meister <nanaki@gmail.com>
To: Caml List <caml-list@inria.fr>
Subject: [Caml-list] Writing the function Set.map using first-class modules and 4.00 inference
Date: Thu, 1 Nov 2012 14:00:00 -0700 [thread overview]
Message-ID: <CAHaHOqSD=dYrBxjNTnCT0+uT2j06w6pC-_QEGVDUUFZx9-QkQw@mail.gmail.com> (raw)
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I found an interesting (to me, anyway) use of OCaml's first-class modules,
and particularly the new 4.00 type inference features, which I thought was
worth sharing with the list. This has probably been observed by someone
else already, but I haven't seen it discussed.
In the OCaml standard library, the polymorphic set data structure is
implemented as a functor, which takes a module containing a type t and
total ordering function over t, and returns a module representing sets
whose elements have type t. Like so:
module StringSet = Set.Make(String)
module CharSet = Set.Make(Char)
One disadvantage of this method is that once the functor has been called,
the type of the set elements is fixed. As a consequence, OCaml's set
interface has no map function. If we had a polymorphic type like 'a set,
this function would have type 'a set -> ('a -> 'b) -> 'b set. But
StringSet.t and CharSet.t are not polymorphic; the corresponding type elt
in each module cannot be changed.
However, using first-class modules, we can write a function map for sets,
which takes as an extra argument the packaged module representing the set
we're mapping from. Maybe this function is better called map_from. Check it
out:
# module Set = struct
module type OrderedType = Set.OrderedType
module type S = Set.S
module Make(Ord : OrderedType) = struct
include Set.Make(Ord)
let map (type e') (type t') (module OtherSet : S with type elt = e'
and type t = t') os f =
OtherSet.fold (fun x accu -> add (f x) accu) os empty
end
end;;
[... bunch of output ...]
val map :
(module S with type elt = 'a and type t = 'b) ->
'b -> ('a -> elt) -> t
Now, back in OCaml 3.12, this function could be written (without the nice
package-expanding pattern I've made use of), but calling it was quite a
pain, enough to invalidate the whole enterprise. One would have to type
this:
# let strs = StringSet.(add "foo" (add "bar" empty));;
val strs : StringSet.t = <abstr>
# let chrs = CharSet.map (module StringSet : Set.S with type elt =
StringSet.elt and type t = StringSet.t) strs (fun s -> s.[0]);;
val chrs : CharSet.t = <abstr>
It's much easier with the type inference changes in OCaml 4.00:
# let strs = StringSet.(add "foo" (add "bar" empty));;
val strs : StringSet.t = <abstr>
# let chrs = CharSet.map (module StringSet) strs (fun s -> s.[0]);;
val chrs : CharSet.t = <abstr>
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next reply other threads:[~2012-11-01 21:00 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-11-01 21:00 Jeff Meister [this message]
[not found] <fa.FUGAe9RTYsSPA4RwbpKO14B0oVo@ifi.uio.no>
2012-11-02 14:59 ` Radu Grigore
2012-11-02 15:11 ` Radu Grigore
2012-11-02 16:00 ` Gabriel Scherer
2012-11-02 16:21 ` Radu Grigore
[not found] <fa.yP8czrxqEGCABee05wzAlISIlN4@ifi.uio.no>
[not found] ` <fa.rve4x46ugIvZs4BizovvjOCeTG0@ifi.uio.no>
[not found] ` <fa.i9E+7rAvghoTZ1MXH4g+uL4dpCY@ifi.uio.no>
[not found] ` <fa.AX0JYJa8kXsH/YSZ08tuyBc8m+0@ifi.uio.no>
[not found] ` <fa.YAoOn2iUcpHbr53eeBfGhPTDvtM@ifi.uio.no>
2012-11-05 8:05 ` Radu Grigore
2012-11-05 10:12 ` Gabriel Scherer
2012-11-05 12:02 ` Radu Grigore
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