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[Caml-list] Functor implementation
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 Date: -- (:) From: Xavier Leroy Subject: Re: [Caml-list] Functor implementation
```> Hi all, I was curious as to how ocaml implements functors and
> finding no help with google, I thought I could ask on the
> list.

Basically, a structure is compiled like a record, and a functor like a
function.

> After giving it a little thought I was thinking they might be
> implemented as follows:
> [...]
> How far off is Ocaml's way and is the performance comparable?

It's pretty close, and the performance is comparable.
Here is pseudocode that is closest to the actual compiled code:

type 'a orderedtype = {
compare: 'a -> 'a -> comparison
}

type 'a setmodule = {
empty: 'a tree;
insert: 'a tree -> 'a -> 'a tree;
member: 'a tree -> 'a -> bool;
}

let make compare =

let node l v r = Node(l, v, r) in
let single v = Node(Empty, v, Empty) in

let rec insert t a =
match t with
Empty -> single a
| Node(l, v, r) -> match (compare.compare a v) with
LT -> node (insert l a) v r
| EQ -> t
| GT -> node l v (insert r a) in

(* other struct members similar *)

{
empty = Empty;
insert = insert;
member = member
}

let mymodule =
make { compare = fun x y -> ... }

Now, let me warn you about the following code:

> let compare x y =
let cmp = x - y in if cmp > 0 then GT else if cmp < 0 then LT else EQ

which doesn't work if x and y are sufficiently far apart
(e.g. x = max_int and y = min_int).

- Xavier Leroy

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```