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Polymorphic variant as a witness?
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Date: 2008-06-21 (23:08)
From: David Teller <David.Teller@u...>
Subject: Polymorphic variant as a witness?
      Dear list,

 I have been thinking for some time about using polymorphic variants to
encode some aspects of a types-and-effects type system in OCaml using
Camlp4. While the idea is still quite fuzzy, I have the feeling that, if
I could have a value (let's call it "witness") with type 
  [> ] ref
which I could "touch" into becoming 
  [> `A] ref
 [> `A | `B] ref
etc. as effects `A, `B, etc. appear in the program, it could provide
interesting information on the effects of the program. 

 Now, of course, I can't define a value with type ref [> ] or even with
type ref [> `dummy]. That is, when compiling a module consisting only in
a declaration such as
   let witness = ref `dummy
I'm faced with the good old "cannot be generalised" error message. This
strikes me as normal -- I'm sure that, with the right modifications on
witness, I could cause runtime type inconsistencies for any client
attempting to read the value of witness. However, in this case, I'm not
going to read any value from witness, ever. I only want to "touch" it
into becoming something a tad more complex, which I could then look at
with ocamlc -i or such.

My question is: is there a way to hijack polymorphic variants into doing
what I wish? Or to encode this behaviour somehow?


David Teller
 Security of Distributed Systems
 Angry researcher: French Universities need reforms, but the LRU act brings liquidations.