Re: [Tile-dev] Scrollbar '-command' callbacks

[Joe E. <jen...@fl...>]

Jeff Hobbs wrote:
> [Joe English]:
> > The new formulation may have a small performance boost in
> > the common case where the -command is a pure list, but
> > it's not fully backwards-compatible, since there are
> > valid script prefixes that aren't lists.
>
> No there aren't.

They're tricky to construct and unlikely to appear in the real world,
but they do exist.  Example:

    text .left
    text .right
    set left 1
    set cmd {[expr {$left ? ".left" : ".right"}] yview}

    uplevel #0 [concat $cmd moveto 0.0]
	# ==> works
    llength $cmd
	# ==> error "list element in braces followed by "]" instead of space"


> The real trick is "script prefix" vs "command prefix".  I guess
> for most of Tk it's a script prefix, and that remains that way
> as long as we use uplevel.  It really matters at the C level,
> as there we can force it to be one command, vs a script.  I'm
> inclined to say "script" just because that's Tk-ish, but I'm
> sure I could be convinced otherwise.

I'm leaning that way also; consistency with "what Tk does"
instead of "what (I think) Tk ought to do".


--Joe English

  jen...@fl...