Scalar values in awk are either numbers or strings.
gawk also supports values of type regexp
(see Strongly Typed Regexp Constants).
As described in True and False in awk, Boolean values in awk
don’t have a separate type: a value counts as “true” if it is nonzero
or non-null, and as “false” otherwise.
When interchanging data with languages that do have a real Boolean type,
using a standard format such as JSON or XML, the lack of a true Boolean
type in awk is problematic.
(See, for example, the json extension provided by
the gawkextlib project.)
It’s easy to import Boolean data into awk, but then the fact
that it was originally Boolean is lost. Exporting data is even harder;
there’s no way to indicate that a value is really Boolean.
To solve this problem, gawk provides a function named mkbool().
It takes one argument, which is any awk expression, and it
returns a value of Boolean type.
The returned values are normal awk numeric values, with
values of either one or zero,
depending upon the truth
value of the original expression passed in the call to mkbool().
The typeof() function (see Getting Type Information) returns
"number|bool" for these values.
Thus Boolean-typed values are numbers as far as gawk
is concerned, except that extension code can treat them as Booleans
if desired.
While it would have been possible to add two new built-in variables
of Boolean type named TRUE and FALSE, doing so would
undoubtedly have broken many existing awk programs. Instead,
having a “generator” function that creates Boolean values gives
flexibility, without breaking as much existing code.