Mailing List Archive: 49091 messages

## [REBOL] Re: Monday Puzzle

### From: larry:ecotope at: 24-Jul-2001 12:05

```
Hi Michael, Anton,

Excellent! The manual method you give below is in fact exactly how I created
the test block! Both you and Anton correctly deduced the nature of the block
y. Both of your function methods produce the correct block. I wrote one of
my own:

>> p: y: [0]
== [0]
repeat j 10 [either j < 10 [
append/only p reduce [j] p: p/2][append/only p y]]
== [9 [0 [1 [2 [3 [4 [5 [6 [7 [8 [...]]]]]]]]]]]
>> y
== [0 [1 [2 [3 [4 [5 [6 [7 [8 [9 [...]]]]]]]]]]]

which is pretty similar to Antons function.

It is interesting to note that this structure of nested blocks of length 2
(without the ring link) is exactly the structure of Scheme and Lisp linked
list-structures which are built from Lisp pairs, represented here as REBOL
blocks of length 2. Based on this observation, one can easily build a
complete interface to Scheme list-structure with pairs implemented as blocks
of length 2 and the empty list as an empty block which terminates all proper
lists.

I have made such an interface, and using those functions y can be created
like this:

;create a linked list from a REBOL block
>> y: lst [0 1 2 3 4 5 6 7 8 9]
== [0 [1 [2 [3 [4 [5 [6 [7 [8 [9 []]]]]]]]]]]
;note the [] terminator after the 9

;create a cycle or ring improper list
>> set-cdr! last-pair y y
== [9 [0 [1 [2 [3 [4 [5 [6 [7 [8 [...]]]]]]]]]]]
>> y
== [0 [1 [2 [3 [4 [5 [6 [7 [8 [9 [...]]]]]]]]]]]

The nested look is due to the fact that MOLD is intrinsically recursive when
showing references to blocks (with self reference marked as ...).

Cheers
-Larry
```