Date: Tue, 12 Oct 1999 13:33:25 +0200
Message-Id: <199910121133.NAA27629@lsun565.lannion.cnet.fr>
From: Jean-Francois Monin <JeanFrancois.Monin@cnet.francetelecom.fr>
To: skaller <skaller@maxtal.com.au>
Subject: Re: Proposal for study: Add a categorical Initial type to ocaml
In-Reply-To: <38002EBF.DF0EF779@maxtal.com.au>
<37FC6572.719AD12D@tsc.uc3m.es>
I'm not sure that category theory helps so much here.
My own background in cat. th. is weak, here is my understanding:
- unit is final because there is one & only one function from any type
to unit, namely fun _ -> ()
- an initial type, say ini, is a type s.t. we have one & only one function
from ini to any type;
this should be the empty sum with no contructor:
type emp = ;;
The initial function would be
let ini (x: emp) = match x with ;;
Note that this is syntactically not allowed in ocaml. I don't think
there is a theoretical problem to add it (at least there are
extensions of caml type system allowing this) , however such a type would be
intrinsically useless (without real use). In particular your '$' seems
inconsistent to me. The only way to "get" such a value is to
introduce it locally in the context, e.g. fun x -> x, or in your case
let f dollar = let x = { data = ini dollar }
which will never help !
[John Prevost <prevost@maya.com> wrote:]
> I would like to propose adding a new special type to ocaml,
> a categorical initial type. This type is the categorical dual
> of the categorical terminal type, unit.
>
> There proposal is for a syntactic designator (say '$') for the
> non-existant value of the initial type, which can
> be bound to a variable of any type.
> [You could say it has type 'a, as does 'raise SomeException']
>
> The effect of attempting to read this value from any type
> should be to raise the exception Uninitialised_value.
>
> Example:
>
> type A = { data: t }
> let x = { data = $ }
> in x.data (* raises exception *)
Jean-Francois Monin
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