World: r3wp

a 10MB xml file loaded from the net and loaded with xml2rebxml  took 
about 100mb of ram.  the same file loaded in firefox  took up 600mb 
of ram.  I was pretty shocked !

10mb of string becomes 600MB of binary xml objects... this is just 
insane.

Maxim, "rethrow": I would say "recursing into" myfunc or maybe "re-calling" 
myfunc. "Rethrow" is more for error handling.

... error handling or "quick exit" flow control.

that's what I was wondering... somehow rethrow sounded cooler  ;-)

juggling?

I have question WRT to error handling:

If I use

make error! "problem"

Immediatly an error message is printed to console. How can I catch 
this? I just want to return an error that I can catch an DISARM to 
get an error-object. Without anything printed to the console.

Wrap the target code in an attempt:

   attempt [oops: make error! "problem"]
   if error? oops [probe disarm oops]

ah, I tried it with TRY etc.

The MAKE error stuff is return from a function.

For example I call:
	a-function 1


and if this function has an error, it calls an error-function that 
ends with: make error! ...

I can't do: attempt [a: a-function] to getback the error.

This works for me:
a-function: func [] [make error! "problem"]
error? a: try [a-function]
***

If that's not quite what you have in mind, take a look at 'throw 
and 'catch -- they may be more suited.

Ok, is this error than gone? It looks like within the error-object/near 
there is an other error. So I can handle the first one but as soon 
as I do something, I get the next output...

Ah, damn, there was an other error in the code...

Happens to me all the time......:-(

Robert: the following approach is better:

a-function: func [] [return make error! "problem"]

error? a-function ; == true

or: error? the-error: a-function

etc.

I want to create a little spreadsheet application where users have 
access to the mathematical functions in rebol and nothing else.

Is there some way to constrain 'do to work within a specific context 
of some mathematical functions, and nothing else?

Or, do I have to write a little mini parser ?

There's a little parser here http://www.rebol.com/docs/core23/rebolcore-15.html

you don't have to write a parser. The approach using my SAFE-EVAL 
function may be faster:

safe-eval: func [
	stm-block [block!]
	/local stm
] [
	stm: make block! (length? stm-block) * 2 + 1
	insert stm first stm-block
	while [stm-block: next stm-block not tail? stm-block] [
		insert tail stm :first
		insert/only tail stm stm-block
	]
	first do/next stm
]

(before using SAFE-EVAL you should test the first element of the 
STM-BLOCK to find out whether it is one of the functions you are 
willing to evaluate)

so, use example?

Do I have to check each value in the block for safety?

error? try [safe-eval [square-root print "gotcha!"]] ; == true

...check each value in the block for safety?
 - no

just the first one

Ok, I'm going to use this example first : 0.56 * sqrt(tender28) + 
0.28 * sqrt(swollen28) + 0.70 * ln(ESR) + 0.014 * GH

turn that into Rebol first ... and then only check the first value?

it looks, like you want to use any expression, not just function 
evaluation?

yes ..

then it is not for you, this was just for a function evaluation

Ok, so looks like it will have to be a parser

yes

but it would be nice to be able to use the rebol parser do evaluate 
words within a mathematical context only

when using a parser, you can even use operator precedence rules and 
such

A non parse based solution ( ie. parse = too hard! )

eval-math: func [ exp [string!]
	/local t allowed okay
][
	allowed: [ + - / square-root log-e * ]
	t: to block! exp
	bind t '+
	okay: true
	allowed: [ + - / square-root log-e * ]
	foreach e t [
		switch/default type?/word e [
			paren! [ eval-math form e ]
			word! [ if not find allowed e [ okay: false break ]]
			decimal! []
			integer! []
		][ okay: false break ]
	]
	either okay [
		do t
	][ none ]
]

Hmm.  Needs debugging

eval-math: func [ exp [string!]
	/local t allowed
][
	allowed: [ + - / square-root log-e * ]
	t: to block! exp
	bind t '+
	foreach e t [
		switch/default type?/word e [
			paren! [ if none? eval-math form e [ return none ]	]
			word! [ 
				if not find allowed e [ 
					print [ "not allowed is : " e ]
					return none
				]
			]
			decimal! []
			integer! []
		][ return none ]
	]
	do t
]

How does one access the parts of a function?

eg; func [face /local var][print face/text] 
how do I get to the [print face/text ] part?