[REBOL] Re: Rebol Crash Was Mutability and sameness - a puzzle
From: lmecir:mbox:vol:cz at: 18-Jul-2001 12:54
Hi,
> 1) What is the answer to the puzzle?
In http://www.sweb.cz/LMecir/evaluation.html I wrote a function like:
mut-equal?: function [
{
a mutation discernibility test,
should discern SERIES! values
}
a [series!]
b [series!]
] [result] [
if not equal? length? :a length? :b [
return false
]
; perform a temporary mutation
insert tail :a #"1"
result: equal? length? :a length? :b
; perform the reverse mutation
remove back tail :a
result
]
to use it instead of SAME? for SERIES! values, because SAME? crashes Rebol.
BTW, I reported the crash to feedback on 23 July 2000 and received the tick
#3509. If I try it for the given example, I get:
a: [1] ; == [1]
insert a reduce [a] ;== [1]
b: [1] ;== [1]
insert b reduce [b] ;== [1]
mut-equal? a a/1 ;== true
mut-equal? b b/1 ;== true
mut-equal? a b ; == false
This looks like the way how the SAME? function should have worked.
The problem is, that there is still one case, where it doesn't produce a
result we would want to get. See this:
a: [1]
b: tail a
remove a
length? b
; ** Script Error: Out of range or past end
; ** Where: halt-view
; ** Near: length? b
same? b b
; ** Script Error: Out of range or past end
; ** Where: halt-view
; ** Near: same? b b
mut-equal? b b
; ** Script Error: Out of range or past end
; ** Where: mut-equal?
; ** Near: if not equal? length? :a
For the MUT-EQUAL? function to work more satisfactorily we need a better
LENGTH? function. Here is my trial:
indexz?: function [
series [series!]
] [orig-tail result] [
orig-tail: tail :series
while [
error? try [result: (index? :series) - 1]
] [
if empty? head :series [
append :series #"1"
]
append :series head :series
]
clear :orig-tail
result
]
lengthz?: func [
series [series!]
] [
(indexz? tail :series) - (indexz? :series)
]
now we can redefinne MUT-EQUAL? to use LENGTHZ? instead of LENGTH? and the
results are:
mut-equal?: function [
{
a mutation discernibility test,
should discern SERIES! values
}
a [series!]
b [series!]
] [result] [
if not equal? lengthz? :a lengthz? :b [
return false
]
; perform a temporary mutation
insert tail :a 1
result: equal? lengthz? :a lengthz? :b
; perform the reverse mutation
remove back tail :a
result
]
a: [1] ; == [1]
insert a reduce [a] ;== [1]
b: [1] ;== [1]
insert b reduce [b] ;== [1]
mut-equal? a a/1 ;== true
mut-equal? b b/1 ;== true
mut-equal? a b ; == false
a: [1]
b: tail a
remove a
lengthz? b ; == -1
mut-equal? b b ; == true
Regards
Ladislav
