[Caml-list] How to add a hook to Stream.junk?

[Caml-list] How to add a hook to Stream.junk?

[bob zhang]

Hi List,
   Does anyone know any trick to add a hook to Stream.junk,  I have
tried different ways,  but did not find any solution, yet
I want to trigger an action each time when I junk a token from the
stream. (copy the whole file from stdlib is fine, but I don't
know how to make the type checker happy :-()
-- 
Regards
-- Bob

Re: [Caml-list] How to add a hook to Stream.junk?

[Wojciech Meyer]

bob zhang <bobzhang1988@gmail.com> writes:

> Hi List,
>    Does anyone know any trick to add a hook to Stream.junk,  I have
> tried different ways,  but did not find any solution, yet
> I want to trigger an action each time when I junk a token from the
> stream. (copy the whole file from stdlib is fine, but I don't
> know how to make the type checker happy :-()
> --
> Regards
> -- Bob

You can parametrise the Stream.t type with your desired type and the
hook.

Then you can substitute type using destructive substitution of a type:

module Stream : module type of Stream with type 'elt t := ('hook, 'elt)
CustomStream.t = Stream

Finally you can implement your own CustomStream.junk function that
operates on the parametrised type and include the CustomStream module.

This should work, but I've not tested or compiled it.

--
Wojciech Meyer
http://danmey.org

[Caml-list] Re: How to add a hook to Stream.junk?

[bobzhang]

On 11/20/12 8:35 PM, Wojciech Meyer wrote:
> bob zhang <bobzhang1988@gmail.com> writes:
>

>> Hi List,
>>     Does anyone know any trick to add a hook to Stream.junk,  I have
>> tried different ways,  but did not find any solution, yet
>> I want to trigger an action each time when I junk a token from the
>> stream. (copy the whole file from stdlib is fine, but I don't
>> know how to make the type checker happy :-()
>> --
>> Regards
>> -- Bob
>
Greetings,
> You can parametrise the Stream.t type with your desired type and the
> hook.
>
> Then you can substitute type using destructive substitution of a type:
>
> module Stream : module type of Stream with type 'elt t := ('hook, 'elt)
> CustomStream.t = Stream
>
> Finally you can implement your own CustomStream.junk function that
> operates on the parametrised type and include the CustomStream module.
>
> This should work, but I've not tested or compiled it.
>
Stream specialized on char stream :-(

> --
> Wojciech Meyer
> http://danmey.org
>

Re: [Caml-list] How to add a hook to Stream.junk?

[Francois Berenger]

On 11/21/2012 10:35 AM, Wojciech Meyer wrote:
> bob zhang <bobzhang1988@gmail.com> writes:
>
>> Hi List,
>>     Does anyone know any trick to add a hook to Stream.junk,  I have
>> tried different ways,  but did not find any solution, yet
>> I want to trigger an action each time when I junk a token from the
>> stream. (copy the whole file from stdlib is fine, but I don't
>> know how to make the type checker happy :-()
>> --
>> Regards
>> -- Bob
>
> You can parametrise the Stream.t type with your desired type and the
> hook.
>
> Then you can substitute type using destructive substitution of a type:
>
> module Stream : module type of Stream with type 'elt t := ('hook, 'elt)
> CustomStream.t = Stream
>
> Finally you can implement your own CustomStream.junk function that
> operates on the parametrised type and include the CustomStream module.

Why the need for module type of and all?

Why not simply:

module MyStream = struct
   include Stream
   let junk =
     failwith "put your code here"
end;;

Which I can read as "MyStream is a Stream where I redefined the junk 
operation".

> This should work, but I've not tested or compiled it.
> --
> Wojciech Meyer
> http://danmey.org
>

Re: [Caml-list] How to add a hook to Stream.junk?

[Wojciech Meyer]

Francois Berenger <berenger@riken.jp> writes:

> Why the need for module type of and all?
>
> Why not simply:
>
> module MyStream = struct
>   include Stream
>   let junk =
>     failwith "put your code here"
> end;;

Yes you could also do through a functor application.

module MyStream(H : sig val junk_hook : unit -> unit end) = 
struct
  include Stream
  let junk stream =
     H.junk_hook ();
     junk stream
end


--
Wojciech Meyer
http://danmey.org

[Caml-list] Re: How to add a hook to Stream.junk?

[Wojciech Meyer]

bobzhang <bobzhang1988@gmail.com> writes:

> Stream specialized on char stream :-(

Please see then functor application idea, I just posted, inspired by
Francois Berenger's idea of ad-hoc inclusion of the module.

--
Wojciech Meyer
http://danmey.org

[Caml-list] Re: How to add a hook to Stream.junk?

[bobzhang]

On 11/20/12 10:34 PM, Wojciech Meyer wrote:
> Francois Berenger <berenger@riken.jp> writes:
>
>> Why the need for module type of and all?
>>
>> Why not simply:
>>
>> module MyStream = struct
>>    include Stream
>>    let junk =
>>      failwith "put your code here"
>> end;;
>
> Yes you could also do through a functor application.
>
> module MyStream(H : sig val junk_hook : unit -> unit end) =
> struct
>    include Stream
>    let junk stream =
>       H.junk_hook ();
>       junk stream
> end
neither works.
functor approach, as I pointed out, stream specialized on char.
overshadowing junk does not work
   a. it's not late binding, existing definitions will not be shadowed
   b. type does not check...
>
>
> --
> Wojciech Meyer
> http://danmey.org
>

[Caml-list] Re: How to add a hook to Stream.junk?

[Wojciech Meyer]

bobzhang <bobzhang1988@gmail.com> writes:

>   a. it's not late binding, existing definitions will not be shadowed

If the existing code uses junk then you have no other choice as to copy
paste the Stream module and customise it.

--
Wojciech Meyer
http://danmey.org