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To: tim@fungible.com (Tim Freeman)
Cc: caml-list@inria.fr
Subject: Re: [Caml-list] Polymorphic variants as type parameters
References: <20020830155016.90B527F63@lobus.fungible.com>
From: John Prevost <j.prevost@cs.cmu.edu>
Date: 30 Aug 2002 12:27:16 -0400 (68.560 UMT)
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>>>>> "tf" == Tim Freeman <tim@fungible.com> writes:

    tf> Here's some code that I'd like to be able to compile:

           type eventtype = [`VECTOR | `SCALAR | `UNIT]
           type +'a eventid
           constraint 'a = [> eventtype ]
           type scalar_eventid = [`SCALAR] eventid

    tf> Unfortunately it gives me an error:


The problem is that when you say [> `X | `Y | `Z] you describe a type
that contains at *least* `X, `Y, and `Z, and possibly others.  This
type is typical for an argument type to a function.  Think of this as
"greater or equal to the type [`X|`Y|`Z]".

When you want a type that may contain possibly `X or `Y or `Z but
nothing besides those, you should write [< `X | `Y | `Z].  This is
like "less than or equal to the type [`X|`Y|`Z]".

Using this I get:

# type eventtype = [`VECTOR | `SCALAR | `UNIT]
  type +'a eventid constraint 'a = [< eventtype ]
  type scalar_eventid = [`SCALAR] eventid;;
type eventtype = [ `VECTOR | `SCALAR | `UNIT]
type +'a eventid constraint 'a = [< eventtype]
type scalar_eventid = [ `SCALAR] eventid

John.
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