null values and sentinels

From: Jerry Jackson (
Date: Tue Jan 12 1999 - 16:50:19 MET

From: Jerry Jackson <>
To: "''" <>
Subject: null values and sentinels
Date: Tue, 12 Jan 1999 08:50:19 -0700


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.

So, I tried this:

# type foo2 = {xxx: int; mutable yyy: bar} and bar = Empty | Pointer of

This works (i.e. I can match on yyy in the way that I want), but
it _seems_ like this will create an extra indirection object in the
yyy slot. Whether it does or not seems to be a compiler question to
me... "Pointer of foo2" could be collapsed into foo2 by the
compiler. Is it? How should I approach this situation?

Thanks for any help,

Jerry Jackson

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