[REBOL] Re: parse question
From: joel:neely:fedex at: 26-Jun-2001 18:42
Hi, Brett,
I must agree with Graham on this one. A zero-length string
is a completely legitimate value, but most definitely is *not*
the same thing as NONE!
Brett Handley wrote:
> Hi Graham,
>
> > If parse always returns a string
>
I think he meant "sets variables to string! values using
COPY" instead of "returns"...
> > shouldn't
> >
> > parse {a=""} [ thru {a="} copy test to {"} ]
> >
> > 'test now be an empty string rather than type none! ?
> >
>
> I guess is depends on how you see it, either as "nothing to
> copy" (none!) or "copied a string of zero length" (empty).
> Also, I don't see that it makes a lot of difference - except
> for perhaps some more if statements in some circumstances.
>
Any time you wish to build a new string from the parsed
substrings, or print their content, you have a problem with
NONE! instead of an empty string. For example, given:
addr1: "John Doe/123 Lonely St/Suite 16/Los Angeles/CA/90210"
addr2: "Joe Doaks/321 Hilltop Ln//Green Mtn/MA/02187"
print-address-label: function [
s [string!]
][
addr-name addr-line1 addr-line2
addr-city addr-state addr-ZIP
][
parse/all s [
copy addr-name to "/" skip
copy addr-line1 to "/" skip
copy addr-line2 to "/" skip
copy addr-city to "/" skip
copy addr-state to "/" skip
copy addr-ZIP to end
]
print [
addr-name newline
addr-line1 newline
addr-line2 newline
addr-city " " addr-state " " addr-ZIP newline
]
]
... we get ...
>> print-address-label addr1
John Doe
123 Lonely St
Suite 16
Los Angeles CA 90210
>> print-address-label addr2
Joe Doaks
321 Hilltop Ln
none
Green Mtn MA 02187
Clearly the address label should contain a blank line between
321 Hilltop Ln
and "Green Mtn Ma 02187".
This is only a trivial example; I'm not suggesting this is the
best way to represent addresses or print labels! If the
example is too trite, just pretend that the syntax of the
address were much more complex, with various punctuation, etc.
I've done a tremendous amount of text-flogging over the years,
converting address lists, reformatting data files among a
variety of representations, etc. In such applications, it's
very common to parse out the pieces of one format and
immediately construct a new string or write/print using some
combination of the just-parsed text fields.
Yes, I know it's possible to follow the PARSE statement with
a list of "fix-the-empty-strings" statements, as in
parse/all s [
;...
]
addr-name: any [addr-name ""]
addr-line1: any [addr-line1 ""]
addr-line2: any [addr-line2 ""]
;...
print [
;...
]
(or even to make a fix-up function and call it on all of
the fields), but all that bother hardly seems in keeping
with "make easy things easy and hard things possible" IMHO.
-jn-
------------------------------------------------------------
Programming languages: compact, powerful, simple ...
Pick any two!
joel'dot'neely'at'fedex'dot'com
