Polymorphic variants question

["David Allsopp" <dra-news@metastack.com>]

Forgive the potentially obvious question --- I'm not very familiar with
polymorphic variants but I think that they're what I want in this situation!

Suppose I'm dealing with three constructors `A, `B and `C and I have a
function f that's supposed to take either `A or `C and return any of `A, `B
or `C. If I write:

let f x = if x = `A then (true, `B) else (false, x)

then I get the type

val f : ([> `A | `B] as 'a) -> bool * 'a

Now, if I try to constrain it to what I'm after with

let (f : [`A | `C] -> bool * [`A | `B | `C]) = fun x -> ...

then I get a type error unless I change
	(false, x)
to
	(false, id x)
with 
	let id = function `A -> `A | `C -> `C

Is there a better way of writing this? I'm using this in the context of
several interrelated lexers where `A, `B and `C are high-level states and
certain lexers can only be called in a subset of those states but each lexer
may yield any value for the next-state. I'd quite like to eliminate the id x
bit since it's only there to "separate" x from the return value for the
type-checker.

Thanks!


David