True is a [boolean] value. It is recognised by [expr]. 

   1. True values are '''yes''', '''on''', '''true''', their ''unique'' abbrevations, and non-zero integers, 
   2. False values are '''no''', '''off''', '''false''', their ''unique'' abbrevations, and the integer '''0'''.

The function '''[string is] true''' returns true if its argument obeys the condition 1. above (and '''[string is] false''' does the opposite).

Unless the '''-strict''' flag is passed both '''[[[string is] true ""]]''' and '''[[[string is] false ""]]''' (i.e., testing the empty string) return 1.  This is the case to facilitate entry validation in Tk.

----
[LV] I guess I must be really too old for programming languages, because making a command return a wrong answer to facilitate some other action, in AN EXTENSION, seems just wrong to me.

[AM] (18 january 2007) If it is any consolation, in mathematics one needs to deal with similar special
cases:

   * The sum over zero elements
   * The product over zero elements
   * Quantifiers over the empty set
   * In geometry one usually deals with objects "in general position" to avoid thinking about a bunch of collinear points for instance

It may seem to be a bizarre hack to make some long forgotten program work, and it probably is.
But taking care of these nauseating edge cases is a part of life.

----

The canonical form for true, like in C, is integer 1. To wit:
 % expr 42==42
 1

[1S] See also: [Boolean], [magic names]


----
[Category Glossary] [Category Internals]