[REBOL] Re: On mutability and sameness
From: joel:neely:fedex at: 7-Jun-2001 8:08
Hi, Ladislav,
Clearly Doug Henning and David Copperfield could use a man of
your talents!
Ladislav Mecir wrote:
> The problem with the proof of DATE! mutability is, that it
> doesn't work. The same "proof" could e.g. prove, that
> integers are mutable:
>
Of course it doesn't, but it was an interesting exercise in
how not to be deceived by the traditional parlor magic devices
of smoke and mirrors. ;-)
You've constructed a complex variation on the following theme:
incr: func ['wd [word!]] [
set :wd 1 + get :wd
]
>> a: 9 == 9
>> incr a == 10
>> incr a == 11
>> incr a == 12
>> a == 12
Of course we all know that INCR is not mutating the integer,
but constructing another integer from the content of A and
then replacing the old content of A with that new one.
We can, of course make this a little more obscure by hiding
the use of SET as follows:
sneaky-incr: func ['wd [word!] /local trick] [
trick: to-set-word :wd
trick (1 + get :wd)
]
>> sneaky-incr a == 13
>> sneaky-incr a == 14
>> sneaky-incr a == 15
>> a == 15
In which TRICK hides the setting of A (or whatever).
Your BM function adds to the above trick the ability to unwind
paths for access into blocks, as well as the *very* nice twist
of binding the constructed set back to the original argument's
context. (Since SNEAKY-INCR doesn't do the BIND, it will fail
when used in the following way
>> a
== 15
>> foo: func [arg /local a] [
[ a: arg
[ sneaky-incr a
[ print [a arg]
[ ]
>> foo 3
3 3
>> a
== 4
as the created set-word tampers with the global context rather
than the context of FOO.)
Nice touch!
...
> bm: function [
> block [block!]
...
> ] [
> (part-path: load mold to set-path! copy/part block-path len)
> (part-path: first bind reduce [:part-path] first :path)
> new-bit: second block
> if old-bit <> new-bit [
> to-add: to integer! 2 ** bitno
> if new-bit < old-bit [to-add: - to-add]
; All of which sets up PART-PATH as a setter, and
> part-path integer + to-add
; has the effect of REPLACING an integer, not MUTATING one.
> ]
> new-bit
> ]
> ]
>
That was a stimulating puzzle!
-jn-
------------------------------------------------------------
Programming languages: compact, powerful, simple ...
Pick any two!
joel'dot'neely'at'fedex'dot'com
