> Why don't you use the ``when'' clauses of Caml Light 0.7?
>
> let f x =
>   match read x with
>    C (x1, x2) when
>     (match read x1, read x2 with
>        C _,C _ -> true
>      | _ -> false) ->
>      ...code2...
>  | _ -> ...code1...
>;;

This is ok in this example, but the problem is that "when" is not a binder !

So if I want to write 

let f x =
  match read x with
    C (x1, x2) when
     (match read x1, read x2 with
        C (x3,x4),C (x5,x6) -> true
      | _ -> false) ->
      ...code2...
  | _ -> ...code1...
;;

x3,x4,x5,x6 are not bound in ...code2... too bad !

With the "where match" match guard it will work and look nicer !

let f x =
  match read x with
    C (x1, x2) where
      match read x1, read x2 with
        C (x3,x4),C (x5,x6) -> ....code2....
  | _ -> ...code1...
;;

The "where match" is in fact strictly more general than the when:

   pat when exp ->

is equivalent to

   pat where match exp with true ->

but the when is weaker because it does not bind any variable.


Christophe Raffalli.