How does the VALUE? function work?

Question

I have reduced down to a small example some code that I have, which tests for whether a variable called class-name has a value assigned to it:

ask-params: function [
    config-file [file!]
    default-class-name
    default-fields
] [
    probe value? 'class-name
    input
    either (value? 'class-name) [
        probe class-name
    ] [
        ;-- omit code in this branch for now
    ]
]

ret-block: ask-params %simple-class.params.txt "Person" "First Name, Last Name"

The expression value? 'class-name returns false here. On the other hand, if I fill in the missing branch with an assignment:

ask-params: function [
    config-file [file!]
    default-class-name
    default-fields
] [
    probe value? 'class-name
    input
    either (value? 'class-name) [
        probe class-name
    ] [
        class-name: default-class-name
    ]
]

ret-block: ask-params %simple-class.params.txt "Person" "First Name, Last Name"

This will return true for value? 'class-name. But in this second case, class-name: default-class-name isn't even executed yet.

I would think that class-name shouldn't exist in memory, so value? 'class-name should be returning false. Why is value? returning true instead?

Answer 1

You are using function. This scans the body of the function and pre-creates the local variables for you, initialized to NONE. That's why value? 'class-name becomes true (because NONE is a legal value for a variable, distinct from the situation of being "unset").

If you used func instead, then both would return false.