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From: Wojciech Meyer <wojciech.meyer@googlemail.com>
To: bob zhang <bobzhang1988@gmail.com>
Cc: Caml List <caml-list@inria.fr>
References: <CANcqPu5QtxenJWnkasNNy16bB1QSfdWjMOeOudQ9e0r6cWASrg@mail.gmail.com>
Date: Wed, 21 Nov 2012 01:35:15 +0000
In-Reply-To: <CANcqPu5QtxenJWnkasNNy16bB1QSfdWjMOeOudQ9e0r6cWASrg@mail.gmail.com>
	(bob zhang's message of "Tue, 20 Nov 2012 20:07:53 -0500")
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Subject: Re: [Caml-list] How to add a hook to Stream.junk?

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