Jacques Garrigue wrote:
From: Jeremy Yallop <[EMAIL PROTECTED]>
Dario Teixeira wrote:
type ('a, 'b) t = private 'a constraint 'a = [< super_node_t ]
I don't think this is quite what you want yet, although it's getting
close!
The first problem is that phantom types must be implemented in terms
of abstract (or at least generative) types.
This is actually the other way round: abstract and private types allow
phantom types, but abbreviations and normal datatypes (generative
ones) don't.
Thanks for the correction! I haven't yet properly internalized private
rows. It seems that private rows allow phantom types because there is
abstraction (and hence generativity) involved.
However, it seems to me that normal (generative) datatypes also allow
phantom types. If `t' is a unary generative datatype constructor then
`int t' and `unit t' are not unifiable, even if the type parameter does
not occur on the rhs of the definition of `t'. For example:
type 'a t = T
let f ((_ : int t) : unit t) = () (* Wrong. *)
So the above code really defines a phantom type.
Yes.
On a related note, does Dario's declaration above become ambiguous with
the introduction of private type abbreviations? The following program
passes typechecking in 3.10.2, but not in 3.11+dev12, perhaps because
`t' is interpreted as a private type abbreviation rather than as a
private row type.
type super_node_t = [`S]
module M :
sig
type ('a, 'b) t = private 'a constraint 'a = [< super_node_t ]
end =
struct
type super_node_t = [`S]
type ('a, 'b) t = 'a constraint 'a = [< super_node_t ]
end
let f x = (x : [<super_node_t] :> ([< super_node_t], _) M.t)
Jeremy.
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