Thread from comp.lang.tcl (10 replies)

Consider this (Unix/Linux/bash) command line:

$ expect -- /dev/fd/3 3<<< 'eval spawn -noecho printf {{\t%s\n}} $argv;interact' $(seq 1 10)

This correctly generates the output (each line starts with a tab):

	1
	2
	3
	4
	5
	6
	7
	8
	9
	10

But notice how we have to use "eval" in order to split up the args in $argv.
And, since we are using "eval", we have to "double quote" (with {}) the
"format" arg to "printf".  It'd be nice if neither of these things were
necessary.

I've always believed that "eval" was "evil" and to be avoided if at all
possible - both in shell and in Tcl.  It has strange side effects, such as
we see here (the need to "double quote").  Is there any way to get the
above effect w/o using "eval" ?

-- 
"Every time Mitt opens his mouth, a swing state gets its wings."

(Should be on a bumper sticker)

Am 12.09.2024 um 17:07 schrieb Kenny McCormack:
> Consider this (Unix/Linux/bash) command line:
> 
> $ expect -- /dev/fd/3 3<<< 'eval spawn -noecho printf {{\t%s\n}} $argv;interact' $(seq 1 10)
> 
> This correctly generates the output (each line starts with a tab):
> 
> 	1
> 	2
> 	3
> 	4
> 	5
> 	6
> 	7
> 	8
> 	9
> 	10
> 
> But notice how we have to use "eval" in order to split up the args in $argv.
> And, since we are using "eval", we have to "double quote" (with {}) the
> "format" arg to "printf".  It'd be nice if neither of these things were
> necessary.
> 
> I've always believed that "eval" was "evil" and to be avoided if at all
> possible - both in shell and in Tcl.  It has strange side effects, such as
> we see here (the need to "double quote").  Is there any way to get the
> above effect w/o using "eval" ?
> 

Kenny,
thanks for the question. Here is the answer by a TCL'er without expect 
knowledge.

"eval" does an additional round of substitutions. This happens for all 
elements and is intended, if a whole command is included in a variable.

To only expand some arguments, the list expansion operator may be used:

In your case:
eval spawn -noecho printf {{\t%s\n}} $argv
equal to:
spawn -noecho printf {\t%s\n} {*}$argv
eventually, this works to:
spawn -noecho printf \t%s\n {*}$argv

Harald

In article <vbv8kd$blsb$1@dont-email.me>,
Harald Oehlmann  <wortkarg3@yahoo.com> wrote:
>Am 12.09.2024 um 17:07 schrieb Kenny McCormack:
>> Consider this (Unix/Linux/bash) command line:
>> 
>> $ expect -- /dev/fd/3 3<<< 'eval spawn -noecho printf {{\t%s\n}}
>$argv;interact' $(seq 1 10)
>> 
>> This correctly generates the output (each line starts with a tab):
>> 
>> 	1
>> 	2
>> 	3
>> 	4
>> 	5
>> 	6
>> 	7
>> 	8
>> 	9
>> 	10
>> 
>> But notice how we have to use "eval" in order to split up the args in $argv.
>> And, since we are using "eval", we have to "double quote" (with {}) the
>> "format" arg to "printf".  It'd be nice if neither of these things were
>> necessary.
>> 
>> I've always believed that "eval" was "evil" and to be avoided if at all
>> possible - both in shell and in Tcl.  It has strange side effects, such as
>> we see here (the need to "double quote").  Is there any way to get the
>> above effect w/o using "eval" ?
>> 
....
>To only expand some arguments, the list expansion operator may be used:
>
>In your case:
>eval spawn -noecho printf {{\t%s\n}} $argv
>equal to:
>spawn -noecho printf {\t%s\n} {*}$argv
>eventually, this works to:
>spawn -noecho printf \t%s\n {*}$argv

Indeed it does.  Thanks!  This solves my issue.

ISTM that this {*} thing is a relatively recent addition to the langauge,
since Expect has been around for a long time, and most of the scripts that
do this sort of thing were written long ago.

-- 
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Kenny McCormack <gazelle@shell.xmission.com> wrote:
> ISTM that this {*} thing is a relatively recent addition to the langauge,

Not really, it was added in 8.5, which was released Feb 12, 2016 [1].  
Although 2016 *is* relatively recent in regards to many expect scripts.

> since Expect has been around for a long time, and most of the scripts that
> do this sort of thing were written long ago.

Before 8.5, 'eval' was the way to perform the "splicing" that {*} 
provides.


[1] https://www.tcl.tk/software/tcltk/8.5.tml

Harald Oehlmann <wortkarg3@yahoo.com> wrote:
> spawn -noecho printf {\t%s\n} {*}$argv
> eventually, this works to:
> spawn -noecho printf \t%s\n {*}$argv

Do note that the second one causes \t and \n to be interpreted by Tcl's 
parser, so printf gets passed a literal tab and literal newline in its 
parameter list.  The first passes the \ and t characters on to printf, 
and then printf does the interpretation.

Both likely work fine for printf, but may not work fine for all 
possible external commands.

Am 12.09.2024 um 20:45 schrieb Kenny McCormack:
> ISTM that this {*} thing is a relatively recent addition to the langauge,
> since Expect has been around for a long time, and most of the scripts that
> do this sort of thing were written long ago.

"Relatively recent" is quite relative. Here is the relevant changes file 
entry:

2006-11-02 (feature change)[TIP 293] Replace {expand} with {*} (hobbs)
	*** POTENTIAL INCOMPATIBILITY with previous 8.5 alphas only ***

;-)

Am 13.09.2024 um 06:32 schrieb Rich:
> Kenny McCormack <gazelle@shell.xmission.com> wrote:
>> ISTM that this {*} thing is a relatively recent addition to the langauge,
> 
> Not really, it was added in 8.5, which was released Feb 12, 2016 [1].
> Although 2016 *is* relatively recent in regards to many expect scripts.
> 
>> since Expect has been around for a long time, and most of the scripts that
>> do this sort of thing were written long ago.
> 
> Before 8.5, 'eval' was the way to perform the "splicing" that {*}
> provides.
> 
> 
> [1] https://www.tcl.tk/software/tcltk/8.5.tml

Sorry, this date is TCL 8.5.19. The {*} was TCL 8.5.0...

In article <vc0f9n$ml8n$1@dont-email.me>, Rich  <rich@example.invalid> wrote:
>Kenny McCormack <gazelle@shell.xmission.com> wrote:
>> ISTM that this {*} thing is a relatively recent addition to the langauge,
>
>Not really, it was added in 8.5, which was released Feb 12, 2016 [1].  
>Although 2016 *is* relatively recent in regards to many expect scripts.

Well, yes, really, from the perspective of Expect.

That was my point - that Expect hasn't really changed much since the late
90s, and most of the active development was done then.  The scripts that I
have in mind - such as "unbuffer" - date back to that era.

>> since Expect has been around for a long time, and most of the scripts that
>> do this sort of thing were written long ago.
>
>Before 8.5, 'eval' was the way to perform the "splicing" that {*} 
>provides.

Right.  OK.

I'm glad I learned about {*}.

-- 
This is the GOP's problem.  When you're at the beginning of the year
and you've got nine Democrats running for the nomination, maybe one or
two of them are Dennis Kucinich.  When you have nine Republicans, seven
or eight of them are Michelle Bachmann.

In article <vc0ffk$ml8n$2@dont-email.me>, Rich  <rich@example.invalid> wrote:
>Harald Oehlmann <wortkarg3@yahoo.com> wrote:
>> spawn -noecho printf {\t%s\n} {*}$argv
>> eventually, this works to:
>> spawn -noecho printf \t%s\n {*}$argv
>
>Do note that the second one causes \t and \n to be interpreted by Tcl's 
>parser, so printf gets passed a literal tab and literal newline in its 
>parameter list.  The first passes the \ and t characters on to printf, 
>and then printf does the interpretation.
>
>Both likely work fine for printf, but may not work fine for all 
>possible external commands.
>

I think you misunderstand Harald's post (i.e., his notation).

The point is that if you pass {{\t%s\n}} to "eval", both sets of braces get
removed and what "printf" sees is (literally)

    backslash tee percent ess backslash en

If you only pass a single level of braces, and eval it, then yes, as you
said, printf will see

    tab percent ess newline

-- 
"Insisting on perfect safety is for people who don't have the balls to live
in the real world."

    - Mary Shafer, NASA Ames Dryden -

In article <vc0lm1$npv9$1@dont-email.me>,
Harald Oehlmann  <wortkarg3@yahoo.com> wrote:
>Am 12.09.2024 um 20:45 schrieb Kenny McCormack:
>> ISTM that this {*} thing is a relatively recent addition to the langauge,
>> since Expect has been around for a long time, and most of the scripts that
>> do this sort of thing were written long ago.
>
>"Relatively recent" is quite relative. Here is the relevant changes file 
>entry:
>
>2006-11-02 (feature change)[TIP 293] Replace {expand} with {*} (hobbs)
>	*** POTENTIAL INCOMPATIBILITY with previous 8.5 alphas only ***
>
>;-)

The other poster seemed to think 2016 (not 2006).  That's still 8 years
ago...

-- 
Republican Congressman Matt Gaetz claims that only ugly women want
abortions, which they will never need since no one will impregnate them.

Kenny McCormack <gazelle@shell.xmission.com> wrote:
> In article <vc0lm1$npv9$1@dont-email.me>,
> Harald Oehlmann  <wortkarg3@yahoo.com> wrote:
>>Am 12.09.2024 um 20:45 schrieb Kenny McCormack:
>>> ISTM that this {*} thing is a relatively recent addition to the langauge,
>>> since Expect has been around for a long time, and most of the scripts that
>>> do this sort of thing were written long ago.
>>
>>"Relatively recent" is quite relative. Here is the relevant changes file 
>>entry:
>>
>>2006-11-02 (feature change)[TIP 293] Replace {expand} with {*} (hobbs)
>>       *** POTENTIAL INCOMPATIBILITY with previous 8.5 alphas only ***
>>
>>;-)
> 
> The other poster seemed to think 2016 (not 2006).  That's still 8 years
> ago...

I pulled a date for a "point version" of 8.5 instead of 8.5.0.