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 Subject: Re: The 4 negations of Lua
 From: Russell Haley <russ.haley@...>
 Date: Sat, 17 Sep 2016 23:54:02 0700
Hi Sean/List,
I was trying to understand your awesome explanation and I needed to
remove the reference to C as it obscured the intent to inform about
Lua negation. This is where I ran into my previous question. I copied
the edited version to pastebin in case anyone cared (i.e. to put it on
the wiki if something isn't there? I don't know!). I'll paste it
below as well. I would be very grateful if you could confirm it has
not lost fidelity from the original?
Soni, I have also added your original comment about overriding if you
would like to and more detail?
I also don't have enough background to be able to show tests for the
bitwise operations (all that computer sciencey stuff I didn't pay
enough attention too).
http://pastebin.com/8BSXeE8B
Cheers,
Russ
Equality in Lua is denoted as
==
> 5==5
true
> 5==6
false
> "Mary" == "Mary"
true
> "Mary" == "mary"
false
Lua has 4 forms of negation:

~
not
~=
Yet only 2 of them can be overloaded. (Needs more explanation)
""
'' is numeric negation. At the CPU level, this is implemented via the
NEG instruction (CPUs that support floating point have a separate
instruction for this). On the x8664 systems, you can negate 8 bit, 16 bit,
32 bit and 64 bit quantities.
> string.format("%X",5)
FFFFFFFFFFFFFFFB
> string.format("%X",0)
0
"~"
'~' is bitwise negation. This flips each bit of an integer, and is
implemented by the NOT instruction (some CPUs name this COM, for
"complement"also note that you can't NOT a floating point value).
Also note that for a given integer A:
A is not equal to ~A [1]
> string.format("%X",~5)
FFFFFFFFFFFFFFFA
> string.format("%X",~0)
FFFFFFFFFFFFFFFF
"not"
'not' is boolean negation
In Lua, 'not', 'and'
and 'or' are defined for booleans:
not a == b
is true if a is not equal to b.
> not 5 == 5
false
> not "Mary"== "Mary"
false
> not 5==6
false
> not "Mary"=="mary"
false
> not "Mary"=="Mary"
false
> not "Mary"=="mary"
false
Also, because only nil and false are false
in Lua:
not 3 returns false
not nil returns true
not false returns true
not true returns false
"~=" is shorthand for "not =="
Filling this out:
not == ~=
not < >=
not <= >
not > <=
not >= <
Bitwise Operations
In Lua 5.3, we now have operators for bitwise operations like and, or, xor
and not. These are
& bitwise and
 bitwise or
~ bitwise xor (in context)
~ bitwise not (in context)
[1] For any system you will probably encounter today. There were
systems where A does equal ~A, but the chances of coming across
such a system are very slim these days, and probably only in a
museum.
On Fri, Sep 16, 2016 at 11:23 PM, Sean Conner <sean@conman.org> wrote:
> It was thus said that the Great Soni L. once stated:
>> Lua has 4 forms of negation:
>>
>> 
>> ~
>> not
>> ~=
>>
>> Yet only 2 of them can be overloaded.
>>
>> It's cool that Lua has 4 forms of negation tho.
>
> '' is numeric negation. At the CPU level, this is implemented via the
> NEG instruction (CPUs that support floating point have a separate
> instruction for this). On the x8664 systems, you can negate 8 bit, 16 bit,
> 32 bit and 64 bit quantities.
>
> '~' is bitwise negation. This flips each bit of an integer, and is
> implemented by the NOT instruction (some CPUs name this COM, for
> "complement"also note that you can't NOT a floating point value).
>
> Also note that for a given integer A:
>
> A is not equal to ~A [1]
>
> 'not' is boolean negation (in C, this is '!'). Yes, this is synthesized
> by the language out of bitwise negation, but with caveats. In C,
>
> !5
>
> is 0, not 0xFFFFFFFB (numeric negation) or 0xFFFFFFFA (bitwise negation).
> Conversely,
>
> !0
>
> is 1, not 0x00000000 (numeric negation) or 0xFFFFFFFF (bitwise negation).
> This is because of the defintion of a boolean in C. In Lua, 'not', 'and'
> and 'or' are defined for booleans:
>
> not a == b
>
> is true if a is not equal to b. Also, because only nil and false are false
> in Lua:
>
> not 3 returns false
> not nil returns true
> not false returns true
> not true returns false
>
> Also,
>
> ~=
>
> is shorthand for
>
> not ==
>
> Filling this out:
>
> not == ~=
> not < >=
> not <= >
> not > <=
> not >= <
>
> In Lua 5.3, we now have operators for bitwise operations like and, or, xor
> and not. These are
>
> & bitwise and
>  bitwise or
> ~ bitwise xor (in context)
> ~ bitwise not (in context)
>
> spc
>
> [1] For any system you will probably encounter today. There were
> systems where A does equal ~A, but the chances of coming across
> such a system are very slim these days, and probably only in a
> museum.
>
>