World: r3wp

compose strips the outer block.

the block from (vals) ?

reduce simply evaluates each value in the block and inserts the result

i see

yep

this is not even mentioned in the tutorial i read... do more operators 
behave like that?

a common trick is to do this to unify a series as a block! which 
might also accept a string! :

val: compose [(val)]


this way, if val was originally a block, it stays that way, but if 
it was a string, its then inserted within one.  Note that the above 
actually creates a new block... so that the original val (if it was 
a block) and the new one are not the same

nope, its a special case for compose... and is one of its differentiating 
features.

also note that compose has a /deep refinement  which is very handy

ex:


a-big-view-block:  [ button "press me"  [print rejoin ["You have 
pressed " (val)]]]

val: "the button"

view layout compose/deep a-big-view-block

neat (and important)

im using rebol on sparc/solaris (for automation). not even perl installed 
on these boxes .-)

note that in the above, if you didn't compose the block it would 
still work, since val would be evaluated on the fly... BUT if you 
have several windows opened at the same time and each new window 
displayed a different caption, then the above is necessary... otherwise 
changing the value of val will change ALL windows at the same time 
 :-)   which is a common error we all make at some point.

so the callback of the button will be bound to the value of 'val' 
at compose/deep time .. ? ok.

it will replace the (val) by its value at specific time

another very good use of compose:

time: now
my-object: make object! compose [
	name: "me"
	time: (time)
]

without the compose, my-object/time will be none

actually it will raise an error since time within the context of 
the object, is not yet set.

hm, isn't:

my-object: make object! [
  name: "me"
  time: now
]

equivalent?

ah, i see.

yes... obviously, but sometimes, the value you want to set within 
your object is already used within the code that is populating it... 
it was just an example  :-)

yeah, i see the difference. but i think the behaviour is a bit weird

why?

i would expect that time is identical to now after

time: now

but it's not.

forget it.

time: now
my-object: make object! [
  name: "me"
  mtime: time
]

behaves the same ...

time: time was the problem

yes exactly.  that's what the compose allows.

obviously one can say well, just use other words... but... its not 
always pretty... actually it rarely is.

one last question:

a: [ "a" "b" ]
how to write/append %to.txt a
so that
a b <- with whitespace inbetween 
gets written?

btw, Ladislav has another more flexible function similar to compose... 
 its called 'BUILD   ( http://www.fm.vslib.cz/~ladislav/rebol/)

write/append %to.txt form a

woot! thx

glad I can help  :-)

should have read strings, not blocks in the reference ....

Thanx for you help Maxim. Have to drive home now ...

Dirk, check out  compose/ONLY, eg:
>> blk: [1 2 3] compose/only [hello (blk)]
== [hello [1 2 3]]

I want to speed up access to a block of objects (unassociated) for 
a search algorithm.
Should I use LIST! or HASH! ?

It's a growing list of visited objects, and I'm searching it each 
time to see if the currently visited object has already been visited.

It looks like I should use HASH!, as it is designed to make lookup 
faster.
LIST! is designed for faster modifications.

I wonder what the conversion time is between BLOCK!, LIST! and HASH! 
there must be some kind of penalty there.

perhaps the objects could just contain a  visited field

No, I'm searching the system object; I can't modify the objects.

ah

what are you keying off  of?

Search term is a word, but I want to support strings as well.

If lookup speed is primary, use hash!, if you're doing a lot of inserts 
and removes, use list! There is overhead to has items for lookup, 
but the speedup is enormous on lookups..

also remmember that lists are at their tails after inserts and append... 
this is usefull, but quite frustrating to discover when you don't 
know about it.

A quick test looks like using a hash didn't help in my case, the 
major work must be somewhere else.

Yes, I am aware of the index position confusion with list!.

Is it possible to turn off: ** Math Error: Math or number overflow 
?

My gcc compiler here warn me of integer overflow, but I can run the 
C program, and bits are lost. But that can be ok in some cases, for 
example to calculate noise. I'm wondering, if I can do that in a 
fast way with REBOL. Example:
46341 * 46341

will produce an integer overflow. I just want the result with the 
high order bits lost.

Good question. I would like to avoid this error as well. A overflow 
would be OK in most cases.

Youd could bypass it with something like:
(1. * 46341 * 46341) // (2 ** 31)

How do you do this when you have a lot of claculation that use user 
input?