[REBOL] Ifs Re:(5)
From: joel:neely:fedex at: 12-Oct-2000 11:53
Hi, Ladislav,
That was severely subtle! (And AFAIAC sounded the death knell on the notion
that REBOL is a simple language for non-programmers! ;-)
[lmecir--geocities--com] wrote:
> Hi,
>
> a stupid example:
>
replace read previous-message-in-thread "stupid" "challenging"
> f: func [x [any-type!]] [1]
> b: to paren! [to paren! [:f]]
> ifs-for-dummies-who-play-with-fire b [positive: [print "positive"] negative:
> [print "negative"] zero: [print "zero"]]
>
> The result:
>
> zero
> == 1
>
Even knowing the dynamic, code=data=code=data... nature of REBOL, I hadn't
thought of the possibility of supplying a value that totally changes the
evaluation pattern of another bit of code. Of course, this creates a whole
new level of threat models for trying to write defenses against potentially
hostile external code.
At any rate, the following mod seems to close the wormhole:
signed-choice: make object! [
positive: []
negative: []
zero: []
selector: 0
compute: func [[throw] selexpr] [
selector: (unravel selexpr)
either positive? selector [
do positive
][
either negative? selector [
do negative
][
do zero
] ] ] ]
...allowing...
ifs-for-dummies-who-play-with-fire b [
positive: ["+"] negative: ["-"] zero: ["0"]
]
== "+"
ifs-for-dummies-who-play-with-fire b [
positive: ["+"] negative: ["-"] zero: ["0"]
]
== "+"
Thanks!
-jn-
