Re: Primitive Boolean type?

[Michael Lazzaro]

Larry Wall wrote:
> Please don't think of C<bit> as a boolean type. There is no boolean
> type in Perl, only a boolean context. Or looking at it from the
> other direction, *every* type is a boolean type.

Er... dang. While I certainly agree with the concept in principle, I'm having a hard time sticking to it when I'm writing beginner-level docs. Suppose I'm declaring something like:

class Appliance {

has bit $.is_plugged_in;
has bit $.is_turned_on;
has bit $.has_ul_approval;
}

I'm declaring three attributes that are specifically always either true or false. They can *only* be true/false, or yes/no, or 1/0 -- they have no other possible values, and I want to declare them in such a way as to clearly enforce that behavior. C<$.is_plugged_in> is never "5" or "timmy" -- only true or false, period! By definition, my _intention_ is therefore to declare these three attributes as containing "boolean" values.

The problem I'm having is that there are few good ways to describe the above behavior without using the word "boolean". The word is conspicuous in its absence; it's like someone trying to describe Daffy Duck to you without using the word "duck". Perhaps boolean is spelled bit (or type bool ::= bit), but the associated value is unambiguously "boolean", which makes the attribute logically "boolean" in type.

Similarly I would assume that we want the capability to refer to boolean context by name (just as we can refer to scalar, list (array?), numeric, string, and other contexts) if for no other reason than to be able to make declarations like:

sub foo (int $i; str $s; bool $y) { ... }

which, from appearances, seems to strongly _imply_ to the user that there is indeed a 'bool' type, just as there is an 'int' type and a 'str' type. (They may be wrong, but that's intuitively what the above signature _appears_ to say.)

So while I understand the philosophical/semantic reasons for the absence of a true boolean type, I wonder how easy it will be to describe the principle to newcomers, and if it's worth it. When someone asks "what's the boolean type in Perl?" I'd rather answer "bit" than "Perl doesn't have one", if for no other reason than the latter answer will completely freak them out. :-)

Feedback? What am I missing?

MikeL