Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From: Harald Oehlmann <wortkarg3@yahoo.com>
Newsgroups: comp.lang.tcl
Subject: Re: Why do we need "eval"? (Expect question)
Date: Thu, 12 Sep 2024 19:33:01 +0200
Organization: A noiseless patient Spider
Lines: 48
Message-ID: <vbv8kd$blsb$1@dont-email.me>
References: <vbv03t$1t4qm$1@news.xmission.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Injection-Date: Thu, 12 Sep 2024 19:33:04 +0200 (CEST)
Injection-Info: dont-email.me; posting-host="ae3a192b59388df87300ca25b56dbdf0";
	logging-data="382859"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX196iqgj8Gr6VNEFCQ05jRjb"
User-Agent: Mozilla Thunderbird
Cancel-Lock: sha1:JRBOslHVW6oXRGjZMk6gAM6Jjwk=
In-Reply-To: <vbv03t$1t4qm$1@news.xmission.com>
Content-Language: en-GB
Bytes: 2357

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