Re: [Caml-list] Types look compatible, but they aren't?

[Anthony Tavener <anthony.tavener@gmail.com>]

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Ohhh... that is interesting. (TL;DR: problem solved, and it was from
inappropriate Oo.id use.)

Modifier.attach is actually implemented as a function of one argument which
does some stuff,
returning a function of two arguments, to avoid redundant lookups in the
case of multiple "attach"
to the same "id".

When I remove the let m = ... and just inline "Modifer.attach id ..." the
type of Modifier.attach changes to:

  Db.key -> int * (((< _.. > as 'a) list -> exn) * (exn -> 'a list) -> 'a
-> Modifier.deleter

So, 'a becomes: (< _.. > as 'a) -- I get some monomorphic... object?

As I wrote this I had an idea and found the problem:

  ...
  (* return (tbl -> unit) function which deletes this specific function *)
  let del_id = Oo.id fn in
  (fun tbl ->
    let lst = List.filter (fun e -> Oo.id e <> del_id) (fn_list tbl) in
    Hashtbl.replace tbl tag (inj lst))


Here, "fn" is the provided function, and I want an easy way to remove such
functions uniquely from the
mess of Hashtbl, universal embedding, and list. I tried a trick I once read
Alain suggest for getting a
unique id using the object module... and I guess that brought in this <..>
thing I was unfamiliar with. :)
Instead of Oo.id I'm using Hashtbl.hash now, which is normally what I'd
do... not sure why I
half-remembered some trick with Oo.id.

Thank-you for looking at this, both of you. It helped me dig in the right
direction!


On Sat, Apr 13, 2013 at 1:33 AM, Gabriel Scherer
<gabriel.scherer@gmail.com>wrote:

> This looks like a value restriction issue with
>
>   let m = Modifier.attach id
>
>   "A function obtained through partial application is not polymorphic
> enough"
>   http://caml.inria.fr/resources/doc/faq/core.en.html#eta-expansion
>
> If this is indeed the source of your error, you can regain
> type-checking by using instead
>
>   let m total = Modifier.attach id total
>
> Note that this may change the semantics of your code if
> (Modifier.attach id) does a side-effect before getting its next
> parameter: if would have been effected only once with your previous
> definition, and will be effected at each call of 'm' with the new
> definition.
>
> On Sat, Apr 13, 2013 at 8:56 AM, Kakadu <kakadu.hafanana@gmail.com> wrote:
> > Maybe function type (int * int -> int * int) is incompatible with object
> > type <..>?
> >
> > Kakadu
> >
> >
> > On Sat, Apr 13, 2013 at 10:50 AM, Anthony Tavener
> > <anthony.tavener@gmail.com> wrote:
> >>
> >> File "virtue.ml", line 462, characters 12-24:
> >> Error: This expression has type
> >>          int * ((int * int -> int * int) list -> exn) *
> >>          (exn -> (int * int -> int * int) list)
> >>        but an expression was expected of type
> >>          int * ((< .. > as 'a) list -> exn) * (exn -> 'a list)
> >>
> >> The code in question:
> >>
> >>   (fun id ->
> >>     let m = Modifier.attach id in
> >>       [ m Cast.total'k (fun (v,b) -> (v, max 1 (b-3)))     (* <-- line
> 462
> >> *)
> >>       ; m Lab.total'k (fun (v,b) -> (v, max 1 (b-3))) ])
> >>
> >> For reference, the signature of Modifier.attach:
> >>   Db.key -> int * ('a list -> exn) * (exn -> 'a list) -> 'a ->
> >> Modifier.deleter
> >>
> >> OCaml version is 4.00.0 -- I know I should upgrade. Keep meaning to, I
> >> guess I will if I wake up and there's no helpful soul explaining what
> >> could
> >> be wrong here. :)
> >>
> >> Thank-you for any help. My eyes are starting to bug-out looking at this.
> >>
> >>  -Tony
> >>
> >>
> >
>

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