Jacques GARRIGUE wrote:

> Interesting, because the example you describe here is precisely the
> reason it is not allowed (at least as Jerome Vouillon explained to
> me.)
> That is, you intend the instance variable lexbuf to be the one
> associated to the current (mutable) chan, but if you change chan this
> will no longer be true.
> So, in order to avoid this kind of ambiguity, you have to use let
> defined variables. For instance:
> 
> class lexstack top_filename =
>   let init_chan = open_in top_filename in
>   object
>     val mutable filename = top_filename
>     val mutable chan = init_chan
>     val mutable lexbuf_chan = init_chan
>     val mutable lexbuf = Lexing.from_channel init_chan
>     method lexbuf =
>       if chan == lexbuf_chan then lexbuf else
>       (lexbuf <- Lexing.from_channel chan; lexbuf_chan <- chan)
>     ...
> 
> Note that this restriction applies also to immutable instance
> variables, because you can modify them through functional update
> (the {< chan = ... >} notation.)

Thanks for the excellent explanation Jacques.

I must admit, this is one of my first real forays into OO side of
ocaml, but I quickly found that my aims could be acheived just as
easily using the technique I posted.

Erik
-- 
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Erik de Castro Lopo
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"A programming language is low level when its programs require
attention to the irrelevant." -- Alan Perlis