A ping for __tonumber

[Andrew Starks]

I've gone over the archives and read past posts on the __tonumber
metamethod idea. In light of work on 5.3, I wanted to throw out that
__tonumber would be welcomed.

Within my own Lua code, I often want to turn an object into a plain
number. The meta-methods can effectively do most of what I want, but
not when I'm dealing with relational operators when one side of is an
actual number.

`tonumber` might be more appropriate than going through the work of
defining every arithmetic metamethod that I may or may not need for
that object (it's tedious).

Finally, __tonumber just feels... missing. I even had to triple check
that it wasn't in 5.2, lest I embarrass myself. :)

Of course, I'm happy to monkeypatch tonumber (and may apply the power
patch that I found in my research). My opinion is that it would be
generally helpful, especially when/if coercion of strings to numbers
should go away[1].

-Andrew

[1]I'm in the habit of calling tonumber whenever I need a number, in
case I get a string. I use it as a silent protest that Lua coerces
strings without me asking.

[Daurnimator]

Post by Andrew Starks
Finally, __tonumber just feels... missing. I even had to triple check
that it wasn't in 5.2, lest I embarrass myself. :)

I agree, this would allow for a best-effort conversion of various
numerical libraries (e.g. big nums, vectors, etc)
It also provides a good duality when compared to __tostring.

FWIW, luajit supports tonumber on numerical cdata objects (e.g. I can
tonumber() a uint32_t)

Though I wonder, with the addition of integers, do we need a
tofloatingpoint() and tointeger() (where tonumber would be a alias for
the former)

D

[Philipp Janda]

Post by Andrew Starks
Within my own Lua code, I often want to turn an object into a plain
number. The meta-methods can effectively do most of what I want, but
not when I'm dealing with relational operators when one side of is an
actual number.
Finally, __tonumber just feels... missing. I even had to triple check
that it wasn't in 5.2, lest I embarrass myself. :)

But Lua 5.2 *does* have support for mixed arguments in relational
operations :-p

local o = { n = 1 }
local mt = {
__lt = function( a, b )
if type( a ) == "table" then a = a.n end
if type( b ) == "table" then b = b.n end
return a < b
end,
}
setmetatable( o, mt )

if o < 2 then
print( "o < 2" )
end
if o >= 1 then
print( "o >= 1" )
end


Philipp

[Thomas Jericke]

Post by Philipp Janda

Post by Andrew Starks
Within my own Lua code, I often want to turn an object into a plain
number. The meta-methods can effectively do most of what I want, but
not when I'm dealing with relational operators when one side of is an
actual number.
Finally, __tonumber just feels... missing. I even had to triple check
that it wasn't in 5.2, lest I embarrass myself. :)

But Lua 5.2 *does* have support for mixed arguments in relational
operations :-p

Unless you want to test for equality. And at some point down the road
you will have to test for it. I would actually never define the
comparison metamethods for mixed types because once you test for
equality it will always be confusing.

[John Hind]

Yes - let me add a 'ping' for that too!

I'm just completing a bitfield library which would benefit from this. I have
a 'tostring' which works either as a method or as a metamethod, but the
corresponding 'tonumber' only works as a method:

s = bf:tostring() -- good
s = tostring(bf) -- good
n = bf:tonumber() -- good
n = tonumber(bf) -- bad

It would also be useful if the manual had a complete list of metamethods
somewhere for reference. Either list all 'the other' metamethods at the end
of the metatables section with links to the library function that uses them,
or just make sure they are all in the index (conveniently the double
underscore prefix will make sure they all get sorted together).

Date: Fri, 26 Jul 2013 10:17:26 -0500
Subject: A ping for __tonumber
I've gone over the archives and read past posts on the __tonumber
metamethod idea. In light of work on 5.3, I wanted to throw out that
__tonumber would be welcomed.
Within my own Lua code, I often want to turn an object into a plain
number. The meta-methods can effectively do most of what I want, but
not when I'm dealing with relational operators when one side of is an
actual number.
`tonumber` might be more appropriate than going through the work of
defining every arithmetic metamethod that I may or may not need for
that object (it's tedious).
Finally, __tonumber just feels... missing. I even had to triple check
that it wasn't in 5.2, lest I embarrass myself. :)
Of course, I'm happy to monkeypatch tonumber (and may apply the power
patch that I found in my research). My opinion is that it would be
generally helpful, especially when/if coercion of strings to numbers
should go away[1].
-Andrew
[1]I'm in the habit of calling tonumber whenever I need a number, in
case I get a string. I use it as a silent protest that Lua coerces
strings without me asking.