** Summary **

A '''gotcha''' is an ''unexpected side effect, behavior, consequence, requirement, etc.''

** Description **

As with any language, the syntax and semantics of [Tcl] can catch the
programmer off guard.  This page is an attempt to enumerate constructs and
behaviour's that can be construed as '''gotcha'''s.
----
**Gotchas**
*** [Tcl and octal numbers%|%The Octabug%|%] ***

See [Tcl and octal numbers]

*** Working [list%|%Lists%|%] ***

Another Tcl '''gotcha''' is to hand arbitrary strings, read from the user or a
file/command, directly into a list operation without first ensuring that the
contents is, in actuality, a [list].

[DKF]: The best way to deal with such user input, where users aren't expecting to write a Tcl list in the first place, is to use a sanitizing command to convert the input into a proper list. Examples of sanitization commands can include [split], [splitx] and (my favorite) this:
======
set elements [regexp -all -inline {\S+} $line]
======

*** Switch its Arguments ***

[RS] One possible gotcha is [switch] -- always use "--" before the switch
variable, since if the value of the switch variable starts with "-", you'll get
a syntax error.

[AMG]: This isn't required by Tcl 8.5 onward.


*** Delimiting Options from Argumennts ***

[KPV] Also, [comment]s w/i [switch], while possible, are tricky.

[RS] 2010-05-10 A similar gotcha is in the '''[text] search''' subcommand - although the misunderstanding could be avoided by counting non-optional arguments from the end,

======
set whatever -this
$t search $whatever 1.0
======

raises an error that "-this" is an undefined switch. For robustness, use

======
$t search -- $whatever 1.0
======

if the slightest possibility exists that $whatever might start with a dash.

*** Intepretations by [expr] ***

[RS] 2010-02-24: Yet another gotcha we ran into last night: Consider a function

======
proc f x {
    if {$x == 00000000} {
        puts "$x is NULL"
    }
}
======

which reported:

======none
0E123456 is NULL
======

How so? Bug? No -- feature. With the [==] comparison operator, the operands are
tried to match as integers, floats, or then as strings. And the $x in this
case, though meant as a pointer in hex, could be parsed as float - with the
numeric value of 0, which is numerically equivalent to 00000000. The solution
of course was to use the [eq] operator instead.

[AMG]: Another issue with expr interpreting stuff happens when said stuff was already interpreted by Tcl.  This creates performance, security, and correctness problems.  Always [brace your expr-essions]!

*** [[[set]]] and namespaces ***

[Twylite] 2012-12-12: Setting a variable in a '''namespace eval''' can clobber
a global variable.  See [Dangers of creative writing].

======
set foo 10
namespace eval bar { set foo 20 ; set bar 30 }
puts $foo ;# puts: 20
puts $bar ;# error: can't read "bar": no such variable
======

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