In another question, I saw the following syntax:
#[unset!]
What is that? If I say type? #[unset!] in R3, it tells me unset!, but it doesn't solve the mystery of what #[] is.
So curious.
views:
74answers:
4
+1
A:
Looks like it's the value-construction syntax for an unset instance, as opposed to the word unset!:
>> comparison: [unset! #[unset!]]
== [unset! unset!]
>> type? first comparison
== word!
>> type? second comparison
== unset!
>> second comparison
>> first comparison
== unset!
If you're in a programmatic context you could do this with to-unset, but having a literal notation lets you dodge the reduce:
>> comparison: reduce ['unset! to-unset none]
== [unset! unset!]
>> second comparison
>> first comparison
== unset!
Looks like they've reserved the #[...] syntax for more of these kinds of constructors.
Hostile Fork
2010-06-18 07:16:14
BTW, check out some of my nuts Rebol-vs-Perl competition in Code Golf. :) http://stackoverflow.com/questions/2527477/code-golf-connecting-the-dots/3066878#3066878
Hostile Fork
2010-06-18 07:18:51
Rebmu looks pretty cool! What a great use of REBOL!
Gregory Higley
2010-06-18 07:36:08
It's getting easier, but here's one I did last night before that dot connection one that I got burned out on and didn't feel like pushing forward to the point of "winning"... sure these can be improved, if you want to play :) http://stackoverflow.com/questions/3034331/code-golf-rotating-maze/3060396#3060396
Hostile Fork
2010-06-18 08:47:12
+1
A:
#[] is the serialized form for values. Play with MOLD versus MOLD/ALL in the console to get a feel for it.
Gregg Irwin
2010-06-21 16:20:50
+1
A:
the #[] "syntax" is actually not a syntax as such (not legal syntax, if you try it), only special cases of such constructs are "legal", like the #[unset!] syntax, #[true] syntax, #[false] syntax, #[none!] syntax, or #[datatype! unset!] syntax.
What is even more interesting, is, what the #[unset!] value actually is. It happens to be the value every "uninitialized" variable in REBOL has (not in functions, though, function local variables are initialized to #[none!]), as well as the result of such expressions like print 1, do [], (), etc.
Regarding the "function local variables ... initialized to #[none!]" I should add, that only the variables following an "unused refinement" (i.e. the one not used in the actual call) are initialized to #[none!] together with the refinement variable.
To explain the issue further, the syntactic (Data exchange dialect) difference between true and #[true] is, that the former is a word, while the latter is a value of the logic! type. Seen from the semantic (Do dialect) point of view, the difference is smaller, since the (global) word is interpreted as a variable, which happens to refer to the #[true] value.
Ladislav
2010-07-17 12:15:06
A:
Look at the example below:
trance on
>> e: none
Trace: e: (set-word)
Trace: none (word) ;<<<<<
>> e: #[none]
Trace: e: (set-word)
Trace: none (none) ;<<<<<
none is a word in the first one, but in the second one, #[none] is not a word, it is the none's itself. same for other values like unset, true, false.
endo64
2010-08-03 07:33:13