Dev:TclTk Tutorial
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Tcl/Tk Theories
List & Substitution Theory
In Tcl everything is eventually a list. Everything.
A common error with lists is this:
set file "/home/tom/My Pictures/Some Picture.jpg" button .b -command "open $file"
Then clicking on the button will throw an error. This is because when the command argument is evaluated (before "button .b ..." is called), it is returned like this:
button .b -command {open {/home/tom/My} {Pictures/Some} {Picture.jpg}}
So each space in our path is interpreted as a new argument to open. Yay!
Trying to run:
button .b -command {open $file}
Will also fail, because variables between {} will not be substituted (ie. the open command will try to open a file on disk literally called $file):
The correct way to do this is therefore:
button .b -command [list open $file]
Which will be evaluated to:
button .b -command {open {/home/tom/My Pictures/Some Picture.jpg}}
Magic Keywords
args
args is a "magic keyword" for procs in Tcl. You can use it to concatenate all trailing arguments to a proc into a single list. Consider this example where you would expect the same result:
proc foo { args } {
puts ${args}
}
proc bar { data } {
puts ${data}
}
foo hello world
bar hello world
Evaluates to:
hello world
wrong # args: should be "bar data"
while executing
"bar hello world"
(file "test.tcl" line 9)
A more "real-world" example using args:
# A drop-in replacement for exec that catches errors.
rename exec exec_tcl
proc exec { args } {
set exec_str [list exec_tcl]
foreach arg ${args} {
lappend exec_str ${arg}
}
if {[ catch ${exec_str} result ]} {
return "EXEC_ERROR: ${result}"
} else {
return ${result}
}
}
puts [exec uname -n] ;# Executes correctly.
puts [exec uname -X] ;# Throws Error
Command Theory
In the end everything is interpreted as a command. Even built-in statements, like if or for are Tcl commands.
Therefore running this will work:
set cmd [list if [list 5 > 4] [list puts [list Yes 5 is bigger than 4]]] eval $cmd
$cmd evaluates to:
if {5 > 4} {puts {Yes 5 is bigger than 4}}
Some Useful Links
TclTk Command Manuals
TkDocs (An Introduction to Building Tk GUI's)
Links Regarding These Theories
Kakaroto's Mail (From Which This Is Derived)
Forum Pages With Useful Info
http://www.amsn-project.net/forums/viewtopic.php?p=17467
http://www.amsn-project.net/forums/viewtopic.php?p=5801#5801
http://www.amsn-project.net/forums/viewtopic.php?p=4461#4461
