World: r3wp

graham, the paren is not needed there.

chris: a trick i have seen:

if not all [
   msg: "rebol.exe does not exist"
   exists? %rebol.exe
   msg: "rebol.exe is empty"
   0 < size? %rebol.exe
] [make error! msg]

anyway, assert seems a good name to me.

Gabriele, yes I've used this trick, but it's a little uncomfortable.

Hi, syntax question: 

i want to insert a row into a mysql db:

string-block: [ "value1" "value2"
insert db [  "insert into table values (?,?)" string-block ]


this fails (string-block is not evaluated i guess), but i dont know 
how to generate the following

insert db [  "insert into table values (?,?)" "value1" "value2" ]

(which works) using rejoin, remold, join, .. whatever.

merrry christmas  :-)


insert db compose [  "insert into table values (?,?)" (string-block) 
]

and happy xmas to you! 


this works! how to factor out the "insert into .." string into a 
variable?

stmt: "insert into .."
insert db compose [ stmt (vals) ]

does not work, neither does 

insert db reduce [ stmt vals ]

which puzzles me ...

btw. is this the right place to ask?

this is the right place indeed  :-)

like so?

insert db reduce compose [ stmt (vals) ]
== ["insert into .."  "value1" "value2" ]

in REBOL since string datatypes evalutate to themselves, the result 
of the compose (which removes the outer block) will simply stay where 
they are .

I understand your issues trying to sort out all of the variations. 
 remember that most series handling functions return the series at 
some point (its head, its tail, or somewhere in between).

that becomes the starting point for the next function "in the chain"

most series handling functions modify the series "in place" instead 
of copying them.  This simplifies series manipulation by breaking 
down each step into little functions.

hm, but:

>> a

== ["abc" "def"]

>> compose [ "abc" (a) ]
== ["abc" "abc" "def"]

>> reduce [ "abc" a ]
== ["abc" ["abc" "def"]]


why is a block returned in reduce, but two strings (which i need) 
in compose. should be the same imho...

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.