[REBOL] Re: Passing word in a block as an argument

From: joel:neely:fedex at: 9-Feb-2003 23:04

Hi, Tim,

In a nutshell, the difference is that FOREACH creates a new context
for its word (or block of words) argument.  Consider this:

    >> i: "nobody home"
    == "nobody home"
    >> foreach i [0 2 4 6 8] [print i]
    0
    2
    4
    6
    8
    >> print i
    nobody home

The I that is used inside the FOREACH isn't the same I as was set
and printed outside that expression.  Now see below:

Tim Johnson wrote:
> ... I can use forskip with the desired result, as in:
>
> forskip blk length? cols[
>     set cols blk
>     ?? a ?? b ?? c ?? d
>     test cols
>     ]
> ; so why is there a difference?
>

Here the body of the loop uses SET to set the words, therefore
making them global, and therefore accessible to TEST, which is
trying to get a global word via 'A .  To see this in more detail,
we can change your original code to contain one extra line, as
follows:

    >> a: b: c: d: "Global!"
    == "Global!"
    >> test: func[e[block!]][
    [        ?? e
    [        print get 'a
    [        ]
    >> blk: [1 2 3 4 5 6 7 8 9 10 11 12]
    == [1 2 3 4 5 6 7 8 9 10 11 12]
    >> cols: [a b c d]
    == [a b c d]
    >> foreach :cols blk[
    [        ?? a ?? b ?? c ?? d
    [        test cols
    [        ]
    a: 1
    b: 2
    c: 3
    d: 4
    e: [a b c d]
    Global!
    a: 5
    b: 6
    c: 7
    d: 8
    e: [a b c d]
    Global!
    a: 9
    b: 10
    c: 11
    d: 12
    e: [a b c d]
    Global!

This shows (clearly, I hope!) that the A B C and D *inside* the
FOREACH loop aren't the same as any global variables that exist
already (and TEST is trying to see a global A as mentioned above).

-jn-

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