From Newsgroup: comp.lang.tcl

I often find myself writing things like this:

set x [mycommand ...]
if {$x ne ""} { ... }

I'd like to be able to do this instead:

iff {set x [mycommand ...]} { ... }

To this end I've created a couple of commands shown below.

Are these reasonable? I've never used uplevel before so
I don't know if there are any problems that I've missed.

#!/usr/bin/env tclsh9

proc main {} {
iff {set a [expr {5 + -5}]} { puts FAIL }
iff! {set b [expr {5 + -5}]} { puts "$b OK" }
iff {set c {}} { puts FAIL }
iff {set d [list a b c]} { puts "$d OK" }
iff {set e false} { puts FAIL }
iff {set f true} { puts "$f OK" }
iff {set g ""} { puts FAIL }
iff {set h "H"} { puts "$h OK" }
}

proc iff {setcmd tcode {ncode ""}} {
set x [uplevel 1 $setcmd]
switch $x {
0 - false - no - off - "" - {} {
if {$ncode ne ""} { uplevel 1 $ncode }
}
default { uplevel 1 $tcode }
}
}

proc iff! {setcmd tcode {ncode ""}} {
set x [uplevel 1 $setcmd]
switch $x {
0 - false - no - off - "" - {} { uplevel 1 $tcode }
default { if {$ncode ne ""} { uplevel 1 $ncode } }
}
}

main
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From Newsgroup: comp.lang.tcl

On 22/10/2025 10:50, Mark Summerfield wrote:

I often find myself writing things like this:

set x [mycommand ...]
if {$x ne ""} { ... }

I'd like to be able to do this instead:

iff {set x [mycommand ...]} { ... }

You are aware that `set` returns the new value of the variable? So you
can just do:

if {[set x [mycommand ...]] ne ""} { ... }


Schelte.

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From Newsgroup: comp.lang.tcl

* Mark Summerfield <m.n.summerfield@gmail.com>
| To this end I've created a couple of commands shown below.

| Are these reasonable? I've never used uplevel before so
| I don't know if there are any problems that I've missed.
--<snip-snip>--
| proc iff {setcmd tcode {ncode ""}} {
| set x [uplevel 1 $setcmd]
| switch $x {

Better use
switch -- $x
here, in case the [set x] returns something with a leading "-".

Other than that, looks fine to me.

An exercise would be to check whether using
llength $ncode
brings any advantage with respect to shimmering over using
$ncode ne ""

R'
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From Newsgroup: comp.lang.tcl

On Wed, 22 Oct 2025 11:29:14 +0200, Schelte wrote:

On 22/10/2025 10:50, Mark Summerfield wrote:

I often find myself writing things like this:

set x [mycommand ...]
if {$x ne ""} { ... }

I'd like to be able to do this instead:

iff {set x [mycommand ...]} { ... }

You are aware that `set` returns the new value of the variable? So you
can just do:

if {[set x [mycommand ...]] ne ""} { ... }

Schelte.

Thanks for reminding me of that; I'd forgotten.
(Thanks also to Ralf for his reply.)

I've realised that my iff command was really going against Tcl's grain;
in effect trying to make empty lists and empty strings false as they
are in Python (which I'm more used to).

I'll try to use your approach if {[set ...] ne ""} { ... }; or in
fact I'll try to return 0 for empty lists or empty strings so I can
simplify to if {![set ...]} { ... }

Thanks again!
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