World: r3wp

The modulus operation from math is not defined for negative numbers 
at all. Most programming languages with a modulus operation have 
extended it to cover negative numbers, but there is no agreed definition 
for it. This is why *all* programming languages and math processors 
that have modulus of negative numbers defined, are using an arbitrary 
platform-dependent definition for it, *even Wolfram Alpha*.

The "correct answer" is NaN or to trigger an error, but since that 
is not useful, to pick a definition that is useful.

>> lesser? 'a 6
== false
>> lesser? 6 'a
** Script Error: Expected one of: integer! - not: word!
** Near: lesser? 6 'a

how does SORT do it without errors?

(the lesser? is from a sort/compare)

The correct answer is -2.
 - that is false, you need to read the help string

From HELP MOD: "Compute a nonnegative remainder of A divided by B."
MOD can produce negative results as seen above.

From HELP MODULO: "Wrapper for MOD that handles errors like REMAINDER."

So REMAINDER must give wrong result in some cases. What does REMAINDER 
say:

From HELP REMAINDER: "Returns the remainder of first value divided 
by second."

That's what I expect a modulo operation to do. A modern definition 
is given by Knuth in "The Art of Computer Programming" using floored 
division, and this seems to be also the definition, Wolfram Alpha 
use. So I would say, REMAINDER give the correct answer in the second 
case, but not in the first.


As I see it, REBOL have 3 modulo functions, and none of them operate 
as expected, if Knuth's definition is used.

would be nice if such discussion could be looked up when someone 
is curious why something has been implemented in a certain way... 
the rebol3 blog is a kind of raw material for such a "background 
documentation"...

none uses Knuths definition

BTW, this was discussed and implemented in ALtMe some time ago

none uses Knuths definition
 Ladislav, please!


Check mod in these languages: Clojure, Common Lisp, Filemaker, Fortran 
(modulo), Lua, Mathematica, MATLAB, R, Ruby (remainder), Scheme (modulo), 
Smalltalk (\\), VHDL


and maybe a few more. They all give the same result as from Knuth 
definition.

none uses Knuths definition

 - you should try to comprehend. None of the functions you mentioned 
 uses the Knuths definition.

In case you again try to misunderstand, then it is "none of the Rebol 
functions you mention"

And, moreover, you can easily redefine the MOD and MODULO, since 
they are mezzanines

Ladislav: I misunderstood it too as "no fucking body is using knuths 
definition"

s/is using/uses/

come on. If a function does not use a definition, then you cannot 
claim the function has a bug when differing from the definition in 
question.

Oh, sorry. None of the REBOL functions use Knuth's definition. Got 
it. :)

Due to specifics in REBOL arithmetic it was deemed desirable to remain 
compatible with some of them when defining more arithmetic functions

In essence, the MOD function is a "helper" for ROUND. It uses the 
Boute's definition, but assuming that the divisor is positive. If 
you do want to use a more comfortable variant, you can use the MODULO 
function, which does not make such an assumption.

Still, there are arithmetic specifics, regarding rounding.

A bug?

>> cur: what-dir

== %/Users/peter/

>> cd %Code/Rebol

== %/Users/peter/Code/Rebol/
>> cd cur

** Access Error: Cannot open /Users/peter/Code/Rebol/cur/

** Near: change-dir to-file path

>> change-dir cur

== %/Users/peter/
Though this works:

>> cd :cur
== %/Users/peter/

>> ? cd 
USAGE:
    CD 'dir


Notice the non-evaluated Literal Argument. So when you use CD, the 
argument isn't looked up.

>> ? change-dir
USAGE:
    CHANGE-DIR dir


With CHANGE-DIR, it is looked up. It is expected behaviour, but maybe 
confusing, that CD and CHANGE-DIR are different.

If I remember correctly, CHANGE-DIR came first, and it's suited for 
being used in scripts. CD is suited for using at the prompt.

<tab> completion of directories and files works, if you give argument 
as a file! datatype, but not when giving word! argument for CD. I 
believe, this is hard to get by.

And then, maybe if CD were constructed like this:

	cd: func [
		'dir [file!]
	][
		...
	]

What does the PATH function do?

>> blk: [a b c]
== [a b c]
>> path blk 'a
>> blk
== [a]
>> blk: [a b c]
== [a b c]
>> path blk 'b 
>> blk         
== [a b]
>> blk: [a b c]
== [a b c]
>> path blk 'c 
** Script Error: Invalid path value: c

Maybe for some internal use?

From group Core-old:


A: the PATH action is what the interpreter uses to evaluate VALUE/selector 
expressions for each datatype. It is an internal action and has no 
external purpose in programs. These kinds of words often appear as 
a sort of 

side-effect" from how REBOL is structured.  Datatypes are implemented 
as a sort of object class, where the interpreter "sends messages" 
to the class to evaluate expressions. The PATH action is a message 
that tells the datatype to perform a pick-like or poke-like internal 
function."

interesting, thanks

It seems to original come from a post in group "RT Q&A" dated 11-Dec-05.

yep, that's supposed to be unset but it somehow has been leaking 
out for a while.

is there any official way of using SORT/COMPARE on a block containing 
a mix of different datatypes?

or perhaps more correctly, a method to use LESSER? or GREATER? on 
a mix of different datatypes?

Looks like SORT uses this datatype map internally:


[unset! datatype! native! action! function! object! word! set-word! 
get-word! lit-word! refinement! none! logic! integer! decimal! money! 
time! date! char! pair! event! tuple! bitset! string! issue! binary! 
file! email! url! tag! image! block! paren! path! get-path! set-path! 
lit-path! hash! list!]

how do u know?

I made a block of all datatypes and sorted it. That was the outcome.

Nevertheless, it seems the method is going to be complicated and 
slow for /COMPARE as I don't know how it handles periferal cases 
like:

sort reduce [true false]
== [false true]

which cannot be compared outside SORT using:

lesser? true false
** Script Error: Cannot use lesser? on logic! value


and I don't know and should not need to know how many periferal cases 
there are.

the solution to me would be that comparison functions use the same 
logic as SORT.

Since you can sort a block of logic! values, then <, > etc. should 
work on them, so that's missing. Doing to-integer on logics first 
solve it. And you have to check all the other datatypes by hand first 
to find out, where possible problems are, I guess.

Sorting pairs. hmm...

>> sort [1x2 2x1]
== [2x1 1x2]

Doesn't make sense.

It shouldn't be possible to order pairs, like complex numbers. (Unless 
you define your own rules.) :)

But as pairs are used to make GUIs, it maybe makes sense to sort 
them in this case. Top of screen is before bottom.

SORT seems to sort anything that you throw at it and I think that 
makes sense, when making GUI lists. Right now I have a problem in 
that I can't control the input datatype and must sort anyway. The 
structure of the data is currently so that SORT/COMPARE is best to 
use, but LESSER? and GREATER? prevent this from being simple.

Today's Moment of REBOL Zen:

>> forever [prin now/time // 7 wait 1 loop 20 [prin bs]] 
0:00:00.000000002

Very cool John.

:)) i didn't know about bs until now...

forever [prin now/time // 7 wait 1 prin cr]

(remainder on time is funny)

Yeah, looks like a bug.

onetom:

>> ? char!
Found these words: 
   backslash       char!     #"\" 
   backspace       char!     #"^H" 
   bs              char!     #"^H" 
   cr              char!     #"^M" 
   escape          char!     #"^[" 
   lf              char!     #"^/" 
   newline         char!     #"^/" 
   newpage         char!     #"^L" 
   null            char!     #"^@" 
   slash           char!     #"/" 
   tab             char!     #"^-"