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

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

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I forgot to note, that the interesting thing was how the type inferred for
Modifier.attach when it had
one argument applied did not show the < _.. > monomorphic object
constraint. Modifier.attach
is actually a: fun id -> (key -> fn -> deleter), rather than a
straightforward three-argument function. Once
the (key -> fn -> deleter) function would come into play, the "object" was
revealed.


On Sat, Apr 13, 2013 at 10:07 AM, Anthony Tavener <anthony.tavener@gmail.com
> wrote:

> 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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