Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!eternal-september.org!.POSTED!not-for-mail
From: et99 <et99@rocketship1.me>
Newsgroups: comp.lang.tcl
Subject: Re: can this work?
Date: Tue, 25 Mar 2025 21:40:11 -0700
Organization: A noiseless patient Spider
Lines: 54
Message-ID: <vs00fd$10bgm$1@dont-email.me>
References: <vrjsse$1oane$1@dont-email.me>
 <20250321163420.39ecbc0b40151daab77dcc27@domain.invalid>
 <vrkkeq$2c2m0$1@dont-email.me> <vrkqq1$2hm9c$1@dont-email.me>
 <vrmbb6$3vamk$1@dont-email.me> <vrmjv9$6fer$1@dont-email.me>
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Injection-Date: Wed, 26 Mar 2025 05:40:13 +0100 (CET)
Injection-Info: dont-email.me; posting-host="59ae5c87b4ec8e8f95ef3c15933bc1c0";
	logging-data="1060374"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX1/QzNOplH1yTaRmAzGWbT4r"
User-Agent: Mozilla Thunderbird
Cancel-Lock: sha1:OfZbBHXg2rJ3KWIM8ec7uHDKmSA=
In-Reply-To: <vrmjv9$6fer$1@dont-email.me>
Content-Language: en-US
Bytes: 2986

On 3/22/2025 8:11 AM, Rich wrote:

--snip--

> The for command is defined as always running expr on the middle
> argument.  Whether you get a loop that looks up variable contents by
> that expr call to make the check dynamic, or a loop that runs expr on
> the exact same static values for each iteration, depends upon what you
> pass to the command.  That depends upon what you write that is parsed
> by the Tcl parser.
> 
> 
> 


Actually, the manual for the [for] command does not say it runs [expr], rather, it only says:

      Then it repeatedly evaluates test as an expression;

And the command [expr] is not mentioned at all. Also, in the page with the 12 rules, it never defines the word expression.

The [if] command, however, does mention the use of [expr]:

     The if command evaluates expr1 as an expression (in the same way that expr evaluates its argument).

I suppose one has to get deep into the weeds and fully understand the algorithm of [expr] to be able to parse it all. There, [expr] does define an expression.

One item that took me forever to understand is why in most commands, words such as in these 3,

    set foo bar
    set foo {bar}
    set foo "bar"

the 2 types of quotes don't change the result here from the unquoted version. But in [expr] and therefore also in the first argument to [if] and the second to [for] a string has to be quoted in one of the 2 ways. So that,

     if {$foo eq "bar"} ..
     if {$foo eq {bar}} ..

is ok, but

     if {$foo eq bar} ..

is not ok. And the reason is that,

     expr {$foo eq bar}

also produces an error since operands in [expr] are not the same as tcl words. Here, [expr] complains about a bare word - something I've also not seen defined.

Anyway, there's always something to learn here :)