[Caml-list] type var in functor instance?
skaller
containing a type variable? I have a Set with t = int,
and I now need 'a * int..
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skaller
> skaller writes:
> > Is it possible instantiate a functor with a module
> > containing a type variable? I have a Set with t = int,
> > and I now need 'a * int..
> Various unsatisfying solutions have been proposed, among which
>
> - to make a copy of Set with an additional type parameter eveywhere (I
> attach such a module below. Beware: this is an old version of Set)
Hmmm. I see. I think I understand why this is needed.
Dependent typing problem? Set depends on element...
> - to make an unfunctorized version of Set that uses Pervasives.compare,
> thus polymorphic
This is already in the standard distro for Hashtbl.
Why not for Set and Map as well? Why is this
unsatisfactory?
I actually use heaps of Hashtbl of polymorphic kind
and am ready to switch some over to an functor instance
with key type int -- most of my tables use int keys
and I hope to gain some performance and reasoning ability.
The duplicate interface has let me experiment rapid
prototyping style, then lock down the typing a bit,
leaving the raw function names and semantics the same.
I think that's nice -- one reason I use Hashtbl
even when Set or Map would be better.
Daniel Bünzli
Accidentally I'm running into the same problem. I'd like to have a
polymorphic Hashtbl that uses physical equality. Using Hashtbl.Make is
not possible since the type t of the input signature HashedType has no
variable.
More precisely I would like to have a key with the following type
type 'a 'b key = 'a option * 'b option
and the following equal function,
let equal k k' = match k, k' with
| (Some x, y), (Some x', y') -> if x == x' then y = y' else false
| _, _ -> k = k'
which tests for physical equality when the first component of the key
is "Some" in both keys.
Any hints besides those already suggested ?
Daniel
Jon Harrop
> > - to make an unfunctorized version of Set that uses Pervasives.compare,
> > thus polymorphic
>
> This is already in the standard distro for Hashtbl.
> Why not for Set and Map as well? Why is this
> unsatisfactory?
You mean Hashtbl uses a polymorphic hash?
I think it would be productive to factor the tree out of set and map.
What if someone released backwards-compatible replacement modules for List,
Array, Set, Map etc. which implemented this stuff? Then package maintainers
could replace them if they liked the alternative implementations and coders
could assume their existence. Maybe.
> I actually use heaps of Hashtbl of polymorphic kind
> and am ready to switch some over to an functor instance
> with key type int -- most of my tables use int keys
> and I hope to gain some performance and reasoning ability.
IMHO, hash tables are a bit less safe than sets and maps because hash tables
make it easier to apply a structure-based polymorphic function incorrectly,
e.g. to many abstract types, when a semantically meaningful (e.g. comparison)
function should be used. For example, I don't think a hash table of sets
would be very useful...
This strikes me as an important way to screw up ML programs and, consequently,
it would be nice to address this. I've been trying to think of a solution but
all I've come up with is: you could enforce the use of an appropriate
comparison by making everything an object with its own compare method. But
then you're going back to dynamic typing. Perhaps phantom types could do
it...
Cheers,
Jon.