Re: The 4 negations of Lua

[Russell Haley <russ.haley@...>]

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 x86-64 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.

Hi Sean. My result differed (Lua prompt changed for inline formatting) :

Lua 5.3.3  Copyright (C) 1994-2016 Lua.org, PUC-Rio
% not 5 == 5
false
% not 5 == 6
false

%  not "Mary"=="Mary"
false
% not "Mary"=="mary"
false

This was run on FreeBSD 10.3

Thanks,

Russ

> 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.
>
>