Re: [TCLCORE] TIP#103, argument expansion

[Peter S. <pet...@sp...>]

At 18:51 2003-02-18, Brent Welch wrote:

>I think there is a 4th alternative, which is to make eval (and possibly
>concat) more list aware.

>Have eval use a list-aware version of concat if it gets multiple arguments.
>In the past I have suggested the command name "leval" for this.
>The above would be
>
>leval {exec $prog} $opts [getMoreopts] {$file $file2}

Just to make sure we mean the same thing, here is my definition of leval:
Assume we have 'lconcat' that makes true list concatenation.
As 'eval ....' is equivalent to 'eval [concat ....]',
'leval ....' would be equivalent to 'eval [lconcat ....]'.

Now lets assume $opts contains a two element list, with the elements
{exec} and {$prog}, but with a non-pure string rep:
set opts {exec $prog}

How can leval know the difference between its first and second argument, so
it can substitute the first and expand the second?

leval {exec $prog} $opts [getMoreopts] {$file $file2}

Well, it can't. This means that a list based leval can't make an extra
round of substitution as eval does. So, leval would need to be used as:

leval [list exec $prog] $opts [getMoreopts] [list $file $file2]

And suddenly the improvement is not that great.
It would be secure, object safe and efficient, and thus better than
using eval, but it is still ugly.

I agree that a real lconcat would be great, but leval just
does not feel very good compared to expand.

/Peter

PS. A little lconcat/leval example:

proc lconcat {args} {
    set result {}
    foreach arg $args {
        eval [linsert $arg 0 lappend result]
    }
    return $result
}

proc leval {args} {
    eval [eval [linsert $args 0 lconcat]]
}

# This should be equivalent to eval and is
# just to emphasize the symmetry with leval.
proc myeval {args} {
    eval [eval [linsert $args 0 concat]]
}

catch {console show}

foreach p {leval myeval eval} {
    puts "Trying $p"

    set testlist [list a [expr 1.0/3.0] b]
    set apa [$p list $testlist]
    set bepa [lindex $apa 1]
    set cepa [expr {$bepa * 3.0}]
    puts "The double is now: $cepa"

    set testlist [list a [expr 1.0/3.0] b]
    set apa [$p [list list] $testlist]
    set bepa [lindex $apa 1]
    set cepa [expr {$bepa * 3.0}]
    puts "The double is now: $cepa"

    set testlist {a b ; puts hej}
    set apa [$p list $testlist]
    puts "The list is now: $apa"
}