Boolean logic in Tcl

Many people who write Tcl script get confused when it comes to boolean logic, especially within if, while construct. To clear up some of these confusions, let’s take a look at the following interactive session:

	(tcl) 66 % # What is true?
	(tcl) 66 % set var -1
	-1
	(tcl) 67 % string is true $var
	0
	(tcl) 68 % # So, -1 is not considered a token for true. Let see now:
	(tcl) 68 % if {$var} {puts "$var is true"}
	-1 is true
	(tcl) 69 % # It seems the expression in the if statement only care if var is zero or not
	(tcl) 69 % # Try a couple of other tokens:
	(tcl) 69 % string is true yes
	1
	(tcl) 70 % string is true no
	0
	(tcl) 71 % string is false no
	1
	(tcl) 72 % # So yes=true, no=false, so far so good
	(tcl) 72 % set var yes
	yes
	(tcl) 73 % if {$var} {puts "$var is true"}
	yes is true
	(tcl) 74 % set var neither
	neither
	(tcl) 75 % if {$var} {puts "$var is true"}
	expected boolean value but got "neither"
	(tcl) 76 % # Now that is the behavior I expected
	(tcl) 76 % set var no
	no
	(tcl) 77 % if {!$var} {puts "$var is false"}
	no is false
	(tcl) 78 % # Worked as expected
	(tcl) 78 %

Normally, Tcl defines true as 1 and false as 0, but as line 68 shows, Tcl considers any non-zero numerical value as true, not just 1.

Lines 69-77: Tcl’s boolean vocabulary also extends to other tokens such as true/false, y/n, yes/no, and on/off. These tokens are case insensitive, which means Yes, yes, and YES are the same.

Line 74-75: If Tcl does not understand a token, it will flag as an error.

Here are the summary points:

1. For numeric values 0=false and non-zero=true
2. For non numeric values, Tcl understands true/false, yes/no, y/n, and on/off. The language might understand more tokens than that, but those are the ones I tested.
3. Anything other than 1 and 2 will raise an error