Question

I was expecting

if {[some_func $some_args]} {
   ....
}

to eval true and simply work as soon as some_func returns some string. However there are errors.

Learning it the hard way, Tcl accepts only the

• empty string -> False
• 0 -> False
• false -> False
• true -> True
• 1 -> True

To be precise:

% expr 1
1
% expr 0
0
% expr
wrong # args: should be "expr arg ?arg ...?"
% expr {{}}
% expr true
true
% expr false
false
% expr True
syntax error in expression "True": variable references require preceding $
% expr False
syntax error in expression "False": variable references require preceding $

It seems expr does not normalize it’s return value. Note especially the empty string result if an empty string is given.

So how do I easily convert strings into booleans?

Answer 1

After thinking some more, and looking some more, the answer probably should be: “It depends”

With my perl background I was expecting something similar to perl. My fault.

In Tcl string is true seems to be the official cast function. Valid strings for casting are defined here:

If string is any of 0, false, no, or off, then Tcl_GetBoolean stores a
zero value at *boolPtr. If string is any of 1, true, yes, or on, then
1 is stored at *boolPtr. Any of these values may be abbreviated, and
upper-case spellings are also acceptable

In my case, I don’t care about the string’s content so I can just

if {[string length [some_func $some_args]] > 0} {
  ...
}

Another way to write this also is much simpler:

if {[some_func $some_args] != {}} {
  ...
}