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From: Alan Grunwald <nospam.nurdglaw@gmail.com>
Newsgroups: comp.lang.tcl
Subject: Re: can this work?
Date: Wed, 26 Mar 2025 12:35:07 +0000
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On 26/03/2025 04:40, et99 wrote:
> 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 :)
> 
> 
> 
> 
> 
FWIW, I remember when I first started to use [expr {$a eq "b"}] if took 
me *AGES* to get the syntax right. To be fair to [expr], its error 
message (if you omit the double-quotes) is pretty specific, but I still 
found it a struggle to get the syntax right.

The lesson I took from this was to stop writing

set i foo

and to start writing

set i "foo"

but the temptation to save a couple of keystrokes is still strong :-)