Question

It is possible to overide rebol system words like print, make etc., so is it possible to do the same with the path operator ? Then what’s the syntax ?

Answer 1

Another possible approach is to use REBOL meta-programming capabilities and preprocess your own code to catch path accesses and add your handler code. Here’s an example :

apply-my-rule: func [spec [block!] /local value][
    print [
        "-- path access --" newline
        "object:" mold spec/1 newline
        "member:" mold spec/2 newline
        "value:" mold set/any 'value get in get spec/1 spec/2 newline
        "--"
    ]
    :value
]

my-do: func [code [block!] /local rule pos][
    parse code rule: [
        any [
            pos: path! (
                pos: either object? get pos/1/1 [
                    change/part pos reduce ['apply-my-rule to-block pos/1] 1
                ][
                    next pos
                ]
            ) :pos
            | into rule    ;-- dive into nested blocks
            | skip         ;-- skip every other values
        ]
    ]
    do code
]

;-- example usage -- 

obj: make object! [
    a: 5
]

my-do [
    print mold obj/a
]

This will give you :

-- path access --
object: obj
member: a
value: 5
--
5

Another (slower but more flexible) approach could also be to pass your code in string mode to the preprocessor allowing freeing yourself from any REBOL specific syntax rule like in :

my-alternative-do {
    print mold obj..a
}

The preprocessor code would then spot all .. places and change the code to properly insert calls to ‘apply-my-rule, and would in the end, run the code with :

do load code

There’s no real limits on how far you can process and change your whole code at runtime (the so-called “block mode” of the first example being the most efficient way).