Limiting PRINT to BLOCK!, TEXT! (perhaps BLANK! to opt-out?)

[hostilefork]

@hostilefork

A kind-of-weird idea would be to also allow CHAR!, and have it print the character without a newline. Reason would be to be able to say print newline and have that mean just print literally one newline (not a newline and then a newline). It comes up kind of often, and seems a shame to have to say write-stdout newline or something more drawn out. But also worth pointing out you could get the same effect idiomatically with print [] or print {}

I decided print newline was clearer. It's the only character PRINT accepts (would be confusing otherwise as to whether it printed a single character or not).

• print newline prints a single newline (returns void)
• print {} does the same (returns void)
• print _ prints nothing (returns null)
• print [] will print nothing (returns null)

If you look at the implementation of PRINT, it shows how the rules are lining up:

print: func [
    {Textually output spaced line (evaluating elements if a block)}

    return: "NULL if blank input or effectively empty block, otherwise VOID!"
        [<opt> void!]
    line "Line of text or block, blank or [] has NO output, newline allowed"
        [<blank> char! text! block!]
][
    if char? line [
        if not equal? line newline [
            fail "PRINT only allows CHAR! of newline (see WRITE-STDOUT)"
        ]
        return write-stdout line
    ]

    (write-stdout try spaced line) then [write-stdout newline]
]

Notice how SPACED of returns NULL, which TRY converts into blank, which WRITE-STDOUT takes to mean it should return NULL. The THEN notices whether write-stdout returns null or not...if it returns non-null (including void) it will follow up with writing a newline.

Because of the way that <blank> works (name still being thought about), this has a nice behavior w.r.t. performance. A function receiving blank that has that parameter annotation won't even try to run.