Need some explanation about package in TCL - tcl

Question

I am having a little problem understanding the following command:
package ifneeded HelloWorld 1.0 [list source [file join $dir helloworld.tcl]]
in the pkgIndex.tcl,
I understand that when the pkgIndex.tcl is sourced and for example, we package require HelloWorld 1.0 , the helloworld.tcl will be sourced. I dont understand the list command...

Answer 1

The package ifneeded command is used to register (or query) how to make a package actually become present in a Tcl interpreter. This is done by evaluating a script, which is the argument generated with list in your example. Let's deconstruct it.
package ifneeded HelloWorld 1.0 [list source [file join $dir helloworld.tcl]]
---------------- ========== --- =============================================
command name package ver how to make it present,
name result of [list ...]
So far, so good. Now, a little aside: the list command is not just used for making lists, but it also makes guaranteed-substitution-free commands. That is, its result is a scrip that consists of an invocation of the command with its arguments, exactly as they were when they went into the list command.
This means that we're producing a script that is source somefilename, where somefilename is the result of the file join. In other words, you're getting almost the same thing as:
package ifneeded HelloWorld 1.0 "source $dir/helloworld.tcl"
Except that there is no assumption that the filename separator is / (that's formally a feature of the OS, not of Tcl, and file join knows about the difference) and it is safe if $dir happens to contain a space or other metacharacters (rather more common than you might hope).
What is $dir? Well, it's a special feature of pkgIndex.tcl scripts that they are (normally) evaluated in a context that sets the dir variable to the absolute name of the directory that contains the pkgIndex.tcl script. (You mustn't make assumptions about the current directory at this point; that belongs to the user of the main Tcl program, not to the package author.) This makes it enormously easier to relocate a package, as you can place all its component files relative to the one script and just move the whole lot in one chunk.

Answer 2

The package ifneeded command expects the following inputs:
package ifneeded package version ?script?
You can see that in your case, the package is HelloWorld, and the version is 1.0. Finally, the script is [list source [file join $dir helloworld.tcl]]. The reason list is used is that the script parameter expects a list.

Answer 3

The package ifneeded command expects a script as its last argument. A script is expected (in a common sense) to be well-formed, that is, to be parsable by the Tcl parser.
In this case of a rather standard pkgIndex.tcl, the thing to ensure is: no matter what the "dir" variable contains at the time the code from that pkgIndex.tcl is processed, the script should be constructed in such a way, that later the Tcl parser sees in it the source command with exactly one argument — no matter if $dir expanded to contain whitespace or funky characters like { etc.
Enter the list command. Here, it's used to construct a list of two elements: the string "source" and a string containing a file name (to serve as the sole argument to that source command). Now, when that list is interpreted as a script (a string), Tcl ensures that string representation contains all the needed quoting to remove any ambiguity about whitespace etc.
This ensures when the parser later interprets our constructed script, the source command in it will receive exactly one argument.
You can read much more of better written information on using list to prevent quoting issues here.