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[Caml-list] Polymorphic variants as type parameters
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Date: -- (:)
From: tim@f...
Subject: [Caml-list] Polymorphic variants as type parameters
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

Unfortunately it gives me an error:

   Characters 124-133:
        type scalar_eventid = [`SCALAR] eventid
                              ^^^^^^^^^
   This type [ `SCALAR] should be an instance of type
     [> eventtype] = [> `SCALAR | `UNIT | `VECTOR]
   The first variant type does not allow tag(s) `UNIT, `VECTOR

This feels like a bug to me: I said the type variable was covariant,
and the type I'm substituting for the type variable is more specific
than the old one, so I had expected it to be happy with that.

The present workaround I'm using is to leave out the type constraint
on the type argument to eventid:

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

This doesn't say what I want because it only makes sense for the type
argument to eventid to be some subset of the set {`VECTOR, `SCALAR, `UNIT}.

Another approach to getting the code to compile is to give
scalar_eventid a type parameter.  This compiles but it doesn't say
what I want to say because scalar_eventid shouldn't have a type
parameter; that would force my code to grow useless extra type
variables in lots of places.

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

Is there any way to say what I want to say in OCAML?  Is this as a
bug?

-- 
Tim Freeman       
tim@fungible.com
GPG public key fingerprint ECDF 46F8 3B80 BB9E 575D  7180 76DF FE00 34B1 5C78 
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