1

It is easy to create GLOBAL variables dynamically in rebol / red with set like

i: 1
myvarname: rejoin ["var" i]
set to-word myvarname 10
var1

but then var1 is global. What if I want to create var1 dynamically inside a function and make it LOCAL so as to avoid collision with some global variables of same name ?

In javascript it is possible: How to declare a dynamic local variable in Javascript

Not sure it is possible in rebol/red ?

user310291
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2 Answers2

2

In Rebol, there is USE:

x: 10
word: use [x] [
   x: 20
   print ["Inside the use, x is" x]
   'x ;-- leak the word with binding to the USE as evaluative result
]
print ["Outside the use, plain x is" x]
print ["The leaked x from the use is" get word]

That will give you:

Inside the use, x is 20
Outside the use, x is 10
The leaked x from the use is 20

One should be forewarned that the way this works is it effectively does a creation like make object! [x: none]. Then it does a deep walk of the body of the USE, looking for ANY-WORD! that are named x (or X, case doesn't matter)...and binds them to that OBJECT!.

This has several annoying properties:

  • The enumeration and update of bindings takes time. If you're in a loop it will take this time each visit through the loop.

  • The creation of the OBJECT! makes two series nodes, one for tracking keys (x) and one for tracking vars (20). Again if you are in a loop, the two series nodes will be created each time through that loop. As the GET outside the loop shows, these nodes will linger until the garbage collector decides they're not needed anymore.

  • You might want to say use [x] code and not disrupt the bindings in code, hence the body would need to be deep copied before changing it.

The undesirable properties of deep binding led Red to change the language semantics of constructs like FOR-EACH. It currently has no USE construct either, perhaps considered best to avoid for some of the same reasoning.

(Note: New approaches are being investigated on the Rebol side for making the performance "acceptable cost", which might be good enough to use in the future. It would be an evolution of the technique used for specific binding).

HostileFork says dont trust SE
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  • I know use but I can't see how to add variables inside use block dynamically (I'm in the context of a code generator): maybe it's not even possible ? – user310291 Oct 28 '17 at 07:15
  • @user310291 You might find [this answer](https://stackoverflow.com/questions/46350543/use-strings-to-create-words-and-paths-in-red-language/46352390#46352390) instructional on that point. – HostileFork says dont trust SE Oct 28 '17 at 07:55
2

In Red you have function, in Rebol2 you have funct. Both create local variable words automatically. Here an example for Rebol2

>> for num 1 100 1 [ 
[      set to-word rejoin ["f" num] funct [] compose/deep [
[            print [ "n =" n:  (num) ]
[          ]
[    ]
>> f1
n = 1
>> f2
n = 2
>> n
** Script Error: n has no value
** Near: n

How it is done, you can see with source funct

sqlab
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