[REBOL] Re: Help me, Obi Reb Kenobi, you're my only hope!
From: lmecir:mbox:vol:cz at: 28-Aug-2002 17:37
Hi Jason,
even Rebol expressions such as:
try [print "hello"]
or
do [print "hello"]
or
()
yield a Rebol value, which, is of type UNSET!.
Check:
type? try [print "hello"] ; == unset!
or
probe head insert [] try [print "hello"] ; == [unset]
etc. The only trouble with a value of that type is, that you cannot use it
in an expression like:
a: ()
, which fires an error. OTOH, you can use UNSET! type value like this:
set/any 'a ()
probe head insert [] () ; == [unset]
type? () ; == unset!
error? () ; == false
unset? () ; == true
etc. Generally spoken, if your expression can yield an UNSET! type value,
you have to use (set/any 'a ...) instead of (a: ...). The trouble with this
approach is, that in the former case you lose the typo protection you
normally have. If no Rebol expression was allowed to yield an UNSET! type
value, this "problem" wouldn't exist.
----- Original Message -----
From: "Jason Cunliffe" <[jason--cunliffe--verizon--net]>
To: <[rebol-list--rebol--com]>
Sent: Wednesday, August 28, 2002 8:12 AM
Subject: [REBOL] Re: Help me, Obi Reb Kenobi, you're my only hope!
hmmm.. I also be glad if someone can explain this issue well
>> Fred: Try ["hello"]
== "hello"
>> fred
== "hello"
>> Fred: Try [print "hello"]
hello
** Script Error: Fred needs a value
** Near: Fred: Try [print "hello"]
>>
>> ? try
USAGE:
TRY block
DESCRIPTION:
Tries to DO a block and returns its value or an error.
TRY is a native value.
ARGUMENTS:
block -- (Type: block)
>> do [print "hello"]
hello
>> fred: do [print "hello"]
hello
** Script Error: fred needs a value
** Near: fred: do [print "hello"]
>> fred: do ["hello"]
== "hello"
>>
Looks like REBOL is being very consistent.
So RebGurus, how do you interpret "returns its value" in this context?
Which Rebol statements have value?
thanks
./Jason
