Re: The 4 negations of Lua

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

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 x86-64 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 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.  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.
>
>