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, 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
Perhaps this functionality could be considered for inclusion in a future
mainline release?
Andrew