tostring and 0

[Valeriy Skurikhin]

Hi, list!

Recently I found, that results of tostring(0) and tostring(-0) may
depend on call order.

Lua 5.1.4 Copyright (C) 1994-2008 Lua.org, PUC-Rio

return (tostring(0) .. tostring(-0))

00

return (tostring(-0) .. tostring(0))

-0-0

A bit confusing behaviour. And it was chnanged in Lua 5.2:

Lua 5.2.0 Copyright (C) 1994-2011 Lua.org, PUC-Rio

return (tostring(0) .. tostring(-0))

0-0

return (tostring(-0) .. tostring(0))

-00

I believe this feature is known to the community, but I didn't manage
to find any clear anwser, whether it should be considered as a bug or
not. Could someone give me any reference, please?

Regards,
Valeiry

[Drake Wilson]

Post by Valeriy Skurikhin
Recently I found, that results of tostring(0) and tostring(-0) may
depend on call order.
Lua 5.1.4 Copyright (C) 1994-2008 Lua.org, PUC-Rio

return (tostring(0) .. tostring(-0))

00

return (tostring(-0) .. tostring(0))

-0-0

It looks like Lua is merging positive and negative zero in the
constant vector:

| $ cat negzero.lua
| print(tostring(0)..tostring(-0))
| print(tostring(-0)..tostring(0))
| $ lua negzero.lua
| 00
| 00
| $ luac -v
| Lua 5.1.5 Copyright (C) 1994-2012 Lua.org, PUC-Rio
| $ luac -l negzero.lua
|
| main <negzero.lua:0,0> (19 instructions, 76 bytes at 0x17ce530)
| 0+ params, 4 slots, 0 upvalues, 0 locals, 3 constants, 0 functions
| 1 [1] GETGLOBAL 0 -1 ; print
| 2 [1] GETGLOBAL 1 -2 ; tostring
| 3 [1] LOADK 2 -3 ; 0
| 4 [1] CALL 1 2 2
| 5 [1] GETGLOBAL 2 -2 ; tostring
| 6 [1] LOADK 3 -3 ; 0
| 7 [1] CALL 2 2 2
| 8 [1] CONCAT 1 1 2
| 9 [1] CALL 0 2 1
| 10 [2] GETGLOBAL 0 -1 ; print
| 11 [2] GETGLOBAL 1 -2 ; tostring
| 12 [2] LOADK 2 -3 ; 0
| 13 [2] CALL 1 2 2
| 14 [2] GETGLOBAL 2 -2 ; tostring
| 15 [2] LOADK 3 -3 ; 0
| 16 [2] CALL 2 2 2
| 17 [2] CONCAT 1 1 2
| 18 [2] CALL 0 2 1
| 19 [2] RETURN 0 1

Whether this is incorrect depends on whether Lua numbers are
semantically required to have positive and negative zero be distinct
when using doubles underneath. A Lua with a different numeric type
might well have only a single zero anyway.

---> Drake Wilson

[Luiz Henrique de Figueiredo]

That is correct. It has been reported here before.

[Valeriy Skurikhin]

Post by Luiz Henrique de Figueiredo
That is correct. It has been reported here before.

I found the post, where it was mentioned. Not an obvious behaviour. Thank you.

Regards,
Valeriy

[Tony Finch]

Post by Valeriy Skurikhin
Recently I found, that results of tostring(0) and tostring(-0) may
depend on call order.

Actually what you are observing is the way Lua stores literal constants.
Since +0 and -0 compare equal, only one of them is retained when the
script is compiled.

Tony.