On Mon, Oct 03, 2016 at 10:39:08AM +0100, Graham Bloice wrote:
> On 3 October 2016 at 00:51, Peter Wu <[email protected]> wrote:
>
> > Hi,
> >
> > To fix bug 12968 (__gc being called for tables, resulting in lua_error
> > while exiting), I modified the way how classes are registered in
> > https://code.wireshark.org/review/18026
> >
> > In these changes I also reduced the use of macros, following Joãos
> > concerns. Macros like WSLUA_META should most likely become typedefs.
> > Below I will give a motivation for the changes (feedback welcome!).
> >
> >
> > Consider class Address with function Address.ip and metamethods like
> > address.__tostring. Previously you could have these strange invocations:
> >
> > -- note: "Address" is the global class, "address" is an instance.
> > tostring(Address) -- invokes metamethod __tostring (error!)
> > address.ip("foo") -- invokes "static" class function
> > tostring(FileHandler) -- error
> > filehandler.new(...) -- huh?
> >
> > In the proposed change, there will be different method tables and
> > metatables for the class ("Address") and its instances ("address").
> > Further modification could disable the above badness, allowing just:
> >
> > tostring(address)
> > Address.ip("foo")
> > tostring(filehandler)
> > FileHandler.new(...)
> >
> > This might break some dissectors that use non-documented invocations,
> > but enables use of separate __call functions for example:
> >
> > field = Field("x") -- __call in metatable of class Field
> > fields = field() -- __call in metatable of instance of Field
> >
> > Also changed is that attributes are no longer visible on the class, only
> > its instances:
> >
> > filehandler.read_open = x -- ok, sets callback attribute
> > -- previously failed in the setter callback out because "self" is
> > -- not a FileHandler instance. Now it will already fail while
> > -- looking up the attribute (in the __newindex metamethod).
> > FileHandler.read_open = x
> >
> >
> >
> > Distinct metatables are good, but what about different method tables?
> > The proposal will also disable invocations like Tvb.len(tvb) assuming
> > that nobody wants to do it (tvb:len() is saner).
> >
> > Any objections with removing __setters/__getters/__methods?
> > I doubt that dissectors use this. Users can read the WSDG and study the
> > source code to discover available functions.
> >
> >
> > Future work, in order (assuming the current proposal):
> > - grep for WSLUA_REGISTER_ATTRIBUTES, change their ClassName_register
> > functions to use the new wslua_class definition mechanism, including
> > setting the "attrs" member.
> > - Remove wslua_reg_attributes and macros.
> > - grep for WSLUA_REGISTER_META and WSLUA_REGISTER_CLASS to change the
> > remaining ClassName_register files.
> >
> > If you feel that the Lua core code needs some love, help is appreciated
> > in these tasks :-)
> > --
> > Kind regards,
> > Peter Wu
> >
>
> As a non-lua programmer most of that goes straight over my head, but I'm
> intrigued\concerned about methods that are class based (i.e. similar to C++
> class static methods ??) and instance based that differ only by the case of
> the initial method letter.
>
> Is this normal for Lua so won't be a surprise for users, or is it likely to
> surprise\confuse users and cause lots of issues and support queries?
>
> --
> Graham Bloice
So far all methods have lowercase letters which seems to be a fine
convention. If by "case" you refer to the "field" and "Field" examples
above, there is a class "Field" that results in an instance "field" on
invocation.
In C++ you can invoke the static and non-static (instance) methods on an
instance, but instance methods cannot be invoked on the class itself:
C c;
c.static_fn(); // note: in the Lua proposal this is also forbidden
c.normal_fn();
C::static_fn();
C::normal_fn(); // error: cannot call member function without object
The Lua proposal seems to match Ruby which also distinguishes class
methods from normal methods on instances. Unfortunately I was not able
to find examples of Lua classes where they class methods in addition to
instance methods. What would be intuitive to you?
--
Kind regards,
Peter Wu
https://lekensteyn.nl
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