Re: [Caml-list] Troublesome nodes

[Dario Teixeira <darioteixeira@yahoo.com>]

Hi,

After some trial and error in the top-level, I think I got the hang of the
'constraint' declaration (is there other documentation for it besides the
couple of lines in the Reference Manual?).  Note that besides the already
discussed link/nonlink distinction between nodes, there is now also an
orthogonal distinction between basic/complex nodes (all nodes except for
Mref are basic; any document that uses Mref thus becomes "contaminated"
and cannot use basic-only functions; the inverse, however, is not true).
I took advantage of Jon's suggestion of boxing all return types into a
single Node.t to implement this latter distinction using phantom types
(is there even an alternative technique?).

Now, the code below seems correct, and running "ocamlc -i node.ml" produces
the type signatures you would expect, and without error messages.  However,
running "ocamlc -c node.ml" fails with a "The type of this expression contains
type variables that cannot be generalized" error on the declaration of foo3.
Moreover, if you comment out the top-level declarations and try to compile
only the module declaration, you will also get different results whether you
run "ocamlc -i" or "ocamlc -c".  The former displays the module's signature
as expected, but the latter produces this error message:

 Type declarations do not match:
   type ('a, 'b) t = ('a, 'b) Node.t constraint 'a = [< super_node_t ]
 is not included in
   type ('a, 'b) t = private 'a constraint 'a = [< super_node_t ]


I find this behaviour a bit odd.  What is going on?

Best regards,
Dario Teixeira


module rec Node:
sig
    type nonlink_node_t = [ `Text of string | `Bold of Node.super_node_t list ]
    type link_node_t = [ `See of string | `Mref of string * nonlink_node_t list ]
    type super_node_t = [ nonlink_node_t | link_node_t ]
    type ('a, 'b) t = private 'a constraint 'a = [< super_node_t ]

    val text: string -> ([> nonlink_node_t], [> `Basic]) t
    val bold: ([< super_node_t], 'a) t list -> ([> nonlink_node_t], 'a) t
    val see: string -> ([> link_node_t], [> `Basic]) t
    val mref: string -> (nonlink_node_t, 'a) t list -> ([> link_node_t], [> `Complex]) t

    val make_basic: ([< super_node_t], [< `Basic]) t -> (super_node_t, [`Basic]) t
    val make_complex: ([< super_node_t], [< `Basic | `Complex]) t -> (super_node_t, [`Complex]) t
end =
struct
    type nonlink_node_t = [ `Text of string | `Bold of Node.super_node_t list ]
    type link_node_t = [ `See of string | `Mref of string * nonlink_node_t list ]
    type super_node_t = [ nonlink_node_t | link_node_t ]
    type ('a, 'b) t = 'a constraint 'a = [< super_node_t ]

    let text txt = `Text txt
    let bold seq = `Bold (seq :> super_node_t list)
    let see ref = `See ref
    let mref ref seq = `Mref (ref, seq)

    let make_basic n = (n :> (super_node_t, [`Basic]) t)
    let make_complex n = (n :> (super_node_t, [`Complex]) t)
end


open Node

let foo1 = text "ola"
let foo2 = bold [text "ola"]
let foo3 = mref "ref" [foo1; foo2]

let basic1 = make_basic foo1
let basic2 = make_basic foo2

let complex1 = make_complex foo1
let complex2 = make_complex foo2
let complex3 = make_complex foo3



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