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In last example, probe value? 'a returns true as it detects 'a has value in global context. But I'd like to test 'a exclusively in local context, is there a syntax to do so ?

    context [
        probe value? 'a; false
    ]

    context [
        a: 1
        probe value? 'a; true
    ]

    ; would like false instead of true
    a: 1
    context [
        probe value? 'a; true
    ]
user310291
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3 Answers3

3

I think the simplest check would be

context [
    probe in self 'a ; none
]

It returns none instead of false, but that is the only other falsy value anyway, so it's totally appropriate to check against. If you need it to be false, you can put to-logic in front.

Dave Andersen
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  • really better. I was just before to add it. – sqlab Nov 13 '17 at 20:42
  • I did very similar one without seeing yours: `>> a: 1 context [probe same? self context? 'a] ; >> false` – endo64 Nov 13 '17 at 20:55
  • @daveanderson I don't understand even context [(a: 1) probe in self 'a] returns none whereas it shouldn't since there is a: 1 ? – user310291 Nov 15 '17 at 07:50
  • @user310291 I'm not sure about that one, but I notice that when you evaluate it, the returned object doesn't have an `a` value, so the parens must be preventing that word from being defined in the object. You could `compose` the block before it is given to `context` and the parens will be evaluated beforehand. – Dave Andersen Nov 15 '17 at 08:25
2

Is it possible in Red to test value? for a variable inside a context not in global context?

By definition, no. Because there is no such thing as "the global context".

Remember that "Rebol (Red) actually does not have scope at all" ... "Rebol (Red) fakes it."

(See: Is there a overall explanation about definitional scoping in Rebol and Red)

Rebol has BIND? for asking an arbitrary ANY-WORD! where it's bound to (if anywhere). Red calls this CONTEXT?. So what you can do is ask if the binding of a word is to a context you care about.

ctx1: context [a: 10]
ctx2: context [a: 20]

word: bind 'a ctx1

print ctx1 = context? word ; true
print ctx2 = context? word ; false

So if what you mean by "global context" is actually the so-called SYSTEM/WORDS object, then that can be your test:

context [
    probe value? 'a ; false
]

context [
    a: 1
    probe system/words <> context? 'a ; true
]

a: 1
context [
    probe system/words <> context? 'a ; false
]

As to whether this is appropriate for your purposes, I don't know. Just remember there's no scope in Rebol/Red, unless you rig up some fake approximation that works well enough for what you're doing.

HostileFork says dont trust SE
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1

Here a solution without searching in the 'global' system/words, but in the 'local' self

>> a: 1
== 1
>> context [
[        probe all [
[            find words-of self 'a 
[            value? pick find words-of self 'a 1
[        ]
[        probe value? 'a
[    ]
none
true
== make object! []
>> 
>> context [
[        probe all [
[            find words-of self 'a 
[            value? pick find words-of self 'a 1
[        ]
[        probe value? 'a
[        a: 2
[    ]
none
false
== make object! [
   a: 2
]
>> 
>> context [
[        a: 2
[        probe all [
[            find words-of self 'a 
[            value? pick find words-of self 'a 1
[        ]
[        probe value? 'a
[    ]
true
true
== make object! [
    a: 2
]
sqlab
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  • that's ok if I know the value of 'a in advance only ? – user310291 Nov 15 '17 at 07:57
  • I don't understand your question? – sqlab Nov 15 '17 at 09:26
  • I want to test if variable a has been declared within local context whatever the value of a: 1, 2, "test" ... in your example you test for value a=1 ? Or I misunderstood ? – user310291 Nov 15 '17 at 10:26
  • No, I test for existence of the word **a** in words-of object at first, then if the existent word has a value. I see there are some relics in my answer. I will correct that – sqlab Nov 15 '17 at 10:38