[REBOL] Re: On mutability and sameness
From: lmecir:mbox:vol:cz at: 6-Jun-2001 7:47
Hi Colin,
> No worries. I think the only point I was making was that Joel *may* be
> mistaking a bug for an esoteric, and hard-to-model, feature.
>
> I don't even have much of a problem knowing Rebol has bugs in it (or
> "confusing quirk" to use the term Feedback used to describe why
face/data:
> copy []
won't clear a VID text-list---you have to write "clear
face/data").
> All software has bugs, and given the speed the Rebol guys are moving, it's
> astounding there are so few....They write quality code with a vengeance!
>
> But I would appreciate (and I know I'm not the first to ask) a bug
> list.....I've spent an embarrassing amount of time "debugging" (or
stumbling
> over) things like Poking a Tuple or clearing a Text-list or DOing a
string. A
> simple, searchable database of reported problems and known workarounds
would
> help us all.
>
> --Colin.
I am pretty sure, that the feature you are describing is not a bug. The
reason for the behaviour is pretty simple. Rebol values of type TUPLE! DATE!
INTEGER! and some other types are immutable - Mark Dickson uses the word
constant to express the same feature. What does the strange word mean? No
Rebol function can change these values. Example:
try-to-change1: func [
value [tuple!]
] [
either value = 1.1.1 [value: 1.1.2] [value: 1.1.1]
value
]
a: 1.1.2 ; == 1.1.2
try-to-change1 a ; == 1.1.1
a ; == 1.1.2
As you can see, the function I wrote wasn't able to change the supplied
value, instead it computed a new different value.
How about another function:
try-to-change2: func [
'value [word!]
] [
either (get value) = 1.1.1 [set value 1.1.2] [set value 1.1.1]
get value
]
a: 1.1.2 ; == 1.1.2
try-to-change2 a ; == 1.1.1
a ; == 1.1.1
Here it looks like I succeeded to change the supplied value 1.1.2. In fact I
didn't change the value 1.1.2. I computed a new value 1.1.1 instead and
stored it to the supplied word 'a , which now contains a different value
than before.
the same effect I can achieve using a set-path:
a: 1.1.2 ; == 1.1.2
a/3: 1 ; == 1
a ; == 1.1.1
Regards
Ladislav
