[REBOL] Re: change/at
From: joel:neely:fedex at: 19-Sep-2002 13:32
Hi, Pat,
Think of the brackets around a literal block as being similar
to quotation marks. See below.
pat665 wrote:
> The original problem was to change false to true for the ith
> position in a block ( [ false false false ] becoming
> [ false true false ]
>
> Romano and Gabriele suggested poke, which I had obviously missed.
> Joel Neely's answer was interesting and pointed to another gotcha
>
> >> test: [[0] [0] [0]] == [[0] [0] [0]]
> >> i: 2 == 2
> >> test/:i/1: 1 == [1]
>
> >> neither of the expressions
> >> test: [false false false]
> >> nor
> >> test: [[false][false][false]]
> >> contain LOGIC! values ;-)
>
> Isn't odd ? Because I needed logic! values. I found out that the
> correct way to do it was
>
> test: reduce [false false false]
>
> How come it is so ? I frankly do not understand !
>
In the case of
foo: "1 23 456 -7"
a human being might say that there are four numbers in the
value of FOO, but to REBOL FOO is just set to a STRING! value.
If I want the numeric values I think I'm seeing, then I have
to ask REBOL to convert from a string to a block of numbers,
which I can do via
baz: load foo
which sets BAZ to a block of four numbers by "interpreting" the
string.
Similarly, if I say
foo: [false true none]
I have just created a block containing four "uninterpreted" words,
as illustrated by the following snippet:
>> foreach item foo [print type? item]
word
word
word
>>
If I want to ask for those words to be interpreted as the (non-
literal) REBOL values that my eyes keep trying to tell me are
present, I need to REDUCE the block.
>> baz: reduce foo
== [false true none]
>> foreach item baz [print type? item]
logic
logic
none
>>
Of course, there are drawbacks to this need to REDUCE what might
be arbitrary blocks of words/expressions. That's why RT has
added a couple of new features, CONSTRUCT and source syntax for
several previously un-expressible values.
For details, see:
3.1. CONSTRUCT Object Creator
and
4.1. Source Code Form for Values of NONE, TRUE, etc.
in the document at
http://www.reboltech.com/downloads/changes.html
-jn-
