Re: [Caml-list] Generic usage of Hashtbl.S with first class module

["Rémy El Sibaïe Besognet" <remy.el-sibaie@lip6.fr>]

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@Phil Eaton :
I already tried this solution but it just moves the pattern matching from
parameter to a let binding and does not fix the extrusion scope error.
Thank anyway.



Le mar. 16 juin 2015 à 16:30, Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>
a écrit :

> Dear Rémy,
>
> The trouble here is that Hashtbl.S defines an abstract type for hash
> tables, and that each time you open this module, using the (module H :
> Hashtbl.S) pattern, your are talking about a fresh abstract type,
> incompatible with anything from the outside world. In particular, there is
> no way the parameter ht could have this type, and this causes an error.
>
> Contrary to real functors, first-class modules do not let you connect
> abstract parameterized types with the outside world. So in this case, the
> only solution I see would be to pack your hash table inside the module
> itself, like for an object, so that you don't need to receive ht as a
> separate argument.
>
> Jacques Garrigue
>
Thank you. This was the kind of answer I expected.
During this time I tried my solution without the `ht` parameter which
solves my original problem. But your solution of giving the `ht` value in
the module is way more general.


> 2015/06/16 21:16 "Rémy El Sibaïe Besognet" <remy.el-sibaie@lip6.fr>:
>
> Hi all,
>>
>> Considering this function
>>
>> let find ht x =
>>   Hashtbl.find ht x
>>
>>
>> I would like to write a more general `find` function which takes the
>> module
>> as first class module in parameter with the possibility to use it on
>> different implentations of Hashtbl.S.
>>
>> let find (module H : Hashtbl.S) ht x =
>>   H.find *ht* x
>>
>>
>> but it failed with this error (on the underlined identifier) :
>>
>> *Error: This expression has type 'a H.t but an expression was expected of
>> type         'a H.t       The type constructor H.t would escape its scope*
>>
>>
>>
>> I also tried by specifying the implementation but had the same problem:
>>
>> let find (type a) (module H : Hashtbl.S with type key = a) (ht : 'a Ht.t) (x : a) =
>>
>> H.find ht x
>>
>> Anyone has an idea how to achieve this thing ?
>>
>> Thanks,
>> - Rémy El Sibaïe, LIP6
>>
>>
>>
>> PS: at the begining, I was trying on a memoization function which has
>> more interest than the previous function :
>>
>> let memo_rec (module H : Hashtbl.S) h f =
>>   let rec g x =
>>     try H.find h x with
>>     | Not_found ->
>>       let y = f g x in
>>       H.add h x y; y
>>   in g
>>
>>

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