Re: Custom "comparison for equality" in Lua ?

["Wim Couwenberg" <w.couwenberg@...>]

Hi,

> How can I let the lua interpreter make :

[... snip snip ...]

> if a==b then print("equality") end
>
> run as expected  (i.e. print "equality") ?

In Lua 4 there is no tagmethod to redefine ~= or ==.  Instead, you can write
(in)equality functions or member functions, like:

function equal(c1, c2)
    return c1.r == c2.r and c1.i == c2.i
end

etc.

A nice way to implement member functions in general is by redefining the
"index" event like so:

local complex_tag = newtag()
local complex_meta = {}  -- shared members of complex type go here

settagmethod(complex_tag, "index", function(_, index) return
%complex_meta[index] end)

function complex_meta:equal(other)
    return self.r == other.r and self.i == other.i
end

-- a simpleton complex number constructor...
function complex(r, i)
    local c = {r=r, i=i}
    settag(c, %complex_tag)
    return c
end

c1 = complex(1,2)
c2 = complex(1,2)

-- test for equality
if c1:equal(c2) then print "equal!" end

Hope this helps,

Wim