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

[Gabriel Scherer <gabriel.scherer@gmail.com>]

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

Just a small thing, (< .. > as 'a) is not monomorphic, it is still a
polymorphic type, that may be instantiated with any object type. It is
"less polymorphic" than 'a (can be instantiated with anything).

On Sat, Apr 13, 2013 at 6:15 PM, Anthony Tavener
<anthony.tavener@gmail.com> wrote:
> 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
>>> >>
>>> >>
>>> >
>>
>>
>