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Hi Eduardo,

Nathan's patch modifies the behaviour of the __eq metamethod so it might
(and almost certainly will) break some existing code. That's why I
suggest adding a new metamethod to 'fill in the gap'.

This way, the change is completely backwards compatible. There should
not be any code which puts an entry called '__eqval' into the metatable,
since all identifiers starting with '__' are reserved for use by Lua.

My patch introduces a slight performance penalty on all comparisons with
a constant, and an extra search for the __eqval metamethod iff one of
the operands to '==' or '~=' is a table or userdata.

I'm not very familiar with the internals of the VM, so it's probably
possible to implement the feature more efficiently.

-a

On 09/06/2022 06:20, Eduardo Bart wrote:

 > So we can't write "natural" code that works for both Lua numbers and
numerical objects.

As the author of lua-bint (Arbitrary precision integer arithmetic
library in pure Lua), I also wish there was some solution for this
problem in a future Lua release, because this is the only corner case
where I can't mix lua numbers.

This problem has been mentioned before by Nathan Page and he has also
come with a smaller patch for Lua 5.4 in
http://lua-users.org/lists/lua-l/2022-01/msg00140.html
<http://lua-users.org/lists/lua-l/2022-01/msg00140.html> .

---
Eduardo Bart


Em qua., 8 de jun. de 2022 às 10:07, Andrew <ajc@gmx.net
<mailto:ajc@gmx.net>> escreveu:

    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