'''This page is intended as to summarize some coding pitfalls I encountered in my early days of Tcl programming. As far as I know, this has not been issued anywhere else.''' Further contributions welcome -- [RJM].


While coding, it is important to realize which type of argument is EXPECTED by the various Tcl commands. This saves unnecessary quotings or even erroneous quotings. Also superfluous commands could be entered as in this example:
 if [expr {$a < 0}] {set a 0}
Such silly sort of code lines may well occur when a beginner is getting confused about using the proper quoting. A more appropriate example is this:
 if {[command...] == xx} {
 for {[command...]} {..} {
(The curly braces may be omitted in the second example because the enclosed text is one word).

For a confused beginner, both lines seem to conform to a consistent syntax. Of course, the second case yields an error when the nested script does not return a valid command name. The beginner who is reading this should know that:
   * each command can have a "local" syntax of how to handle arguments. The [expr] command is one such command.
   * each argument may also be interpreted simply as tcl command/script, expr or the name of a variable.

The latter will be looked at briefly.

   * [if] expects an expression as its first argument, therefore no [expr] necessary.
   * [for] expects a command/script as its first argument, therefore no [[]] necessary.
The beginner must realize that the term "necessary" in the second example is incorrect. It suggests that where a command is expected, a [[]]-quoting would be harmless doubling. It must be emphasized that [[]] does '''substitution''' just as $ also substitution. To be precise: [[]] is not "quoting". See also [An Introduction to Tcl Scripting].

Further examples:
   * [time] expects a command as its first argument. time [[command...]] would substitute the script between [[]] upon Tcl parsing, and then the command [time] uses its result and treats this as a script.
   * [switch] is (at a first glance) confusing to C-programmers because they no see 'case'. The well defined order and interpretation of the switch arguments as 'pattern-body' pairs enables proper operation of switch.
   * [incr] expects a variable name. Perhaps a bit silly to notice that no $ is "necessary". But it may help a beginner to understand why sometimes not to use $ and when not. BTW: $ would effectively result in double dereferencing. See also [deferencing].

A bit confusing for beginners is the use of [list] in -command options of Tk widgets or as the ''script'' argument of [bind]. The usage of [[list .....]] in [[]] results in ''immediate'' command subsitution. Its result is treated as the effective command/script for the -command option or binding. It's all in the "Division of Responsibility" Parser <-> Command: Any substitution is carried out during Parsing.



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