Re: [Ocaml-lib-devel] Map with set operations over the keys

[John S. <sk...@us...>]

On Thu, 2005-06-09 at 13:02 -0700, Martin Jambon wrote:
> On Mon, 6 Jun 2005, Nicolas Cannasse wrote:

> The following signatures seem good to me:
> 
> (* Exception Remove can be raised by the user-given functions and means
>    "do not keep a binding to this key" for both union and inter. *)
> 
> val union :
>   ('key -> 'a -> 'c) -> ('key -> 'a -> 'b -> 'c) -> ('key -> 'b -> 'c) ->
>   ('key, 'a) t -> ('key, 'b) t -> ('key, 'c) t
> 
> val inter :
>   ('key -> 'a -> 'b -> 'c) -> ('key, 'a) t -> ('key, 'b) t -> ('key, 'c) t
> 
> val diff :
>   ('key, 'a) t -> ('key, 'b) t -> ('key, 'a) t


Just a minor comment, unrelated to the actual topic but
concerning these names: it isn't clear from the name
if 'diff' is setwise subtraction or symmetric difference.
It isn't clear in the Ocaml standard library either.
I believe 'diff' is actually subtraction, not the
symmetric difference, which makes the choice
of name extremely unfortunate. Both functions
should be provided, they're both useful and expensive
to compute with a multi-term expression eg: 

	symdiff x y = sub (union x y) (inter x y)
or
	sub x y = inter x (symdiff x y)

Of course one can easily define both with only membership
test and fold.

Perhaps if the symmetric difference was provided and named

	symdiff

it would be clear that 'diff' was the substraction.


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