Re: null values and sentinels

[Anton Moscal <msk@post.tepkom.ru>]

On Tue, 12 Jan 1999, Jerry Jackson wrote:

> Hello,
> 
> I'd like to know what the recommended solution to the following 
> situation is...
> 
> I'd like to create a type like:
> 
> # type foo = {xxx: int; mutable yyy: foo};;
> 
> so that I can add nodes by mutating the "yyy" slots.  Unfortunately,
> it seems that I must create a sentinel value of type "foo" to stand
> for a null pointer.  This works but I'd like to be able to use pattern
> matching on the value of yyy so as to get compile-time warnings if I
> forget to check for null.

The only known to me solution is the following:

declare

    let rec foo_nil = {xxx=1; yyy = foo_nil};

and then use foo_nil as Nil constant. The main problem is the
following: foo_nil can't be used in the matching.
 
Anton E.Moscal