Em dom., 12 de jun. de 2022 às 03:41, Andrew <ajc@gmx.net
<mailto:ajc@gmx.net>> escreveu:
I've just fixed an error in my patch. Github has been updated.
https://github.com/geneas/lua/blob/master/eqval544.p
<https://github.com/geneas/lua/blob/master/eqval544.p>
-a
On 08/06/2022 23:06, Andrew wrote:
> Hello All,
>
> there is currently a small but annoying problem when constructing
> numerical objects based on tables or userdata.
>
> To illustrate this, here is a little multi-precision integer
library in
> pure Lua that I've been working on (https://github.com/geneas/lua
<https://github.com/geneas/lua>, see
> file mpi.lua).
>
> By defining appropriate metamethods we can use these objects in
> expressions just like native Lua numbers:
>
> local mpi = require "geneas.mpi"
> local a = mpi(1)
> local b = mpi "777777777777777777777777777777"
> local c = a + b -- metamethod __add
> print(c) -- metamethod __tostring
> [ 777777777777777777777777777778 ]
> if a < 2 then -- metamethod __lt
> print "true"
> else
> print "false"
> end
> [ true ]
>
> But there's a problem when comparing for equality, because the __eq
> metamethod is only called when _both_ operands are tables or
userdata!
>
> if a == 1 then -- metamethod __eq
> print "true"
> else
> print "false"
> end
> [ false ]
>
> This is annoying, because we have to write "if a == mpi(1)", or maybe
> even "if mpi(a) == mpi(1)", if we are not sure whether a is
actually an
> mpi object or a Lua number!
>
> So we can't write "natural" code that works for both Lua numbers and
> numerical objects.
>
> My proposed solution is to add an extra metamethod __eqval (suggested
> name) which is called by the equality operators when exactly one
of the
> operands is a table or userdata, ie, in all those cases where the
other
> arithmetic and comparison metamethods would be called but __eq is
not.
>
> The __eqval metamethod should perform the same function as __eq
(maybe
> with some extra checking of the operand types). As a result the
equality
> operators '==' and '~=' now behave the same for objects of the
numerical
> class as they do for Lua numbers.
>
> No existing Lua code is impacted, since the semantics of the __eq
> metamethod remain unchanged and no __eqval metamethod will be
defined.
>
> I've made a patch for Lua 5.4.4 which is available here:
> https://github.com/geneas/lua/blob/master/eqval544.p
<https://github.com/geneas/lua/blob/master/eqval544.p>
>
> Perhaps this functionality could be considered for inclusion in a
future
> mainline release?
>
> Andrew
