Feature #17265


Add `Bool` module

Added by marcandre (Marc-Andre Lafortune) over 2 years ago. Updated over 2 years ago.

Target version:


1-line Summary: rbs would benefit from the existence of common ancestor Bool for TrueClass and FalseClass.

Matz: I am aware you rejected a similar request, but could we revisit this in light of RBS?

One use case was for an easy way to check for true or false values, instead of simply for truthiness (e.g. for data transfer, strict argument checking, testing, etc.)

I believe there's a new use case: RBS

In RBS, the most used types like String and Integer have types for "string-like" and "integer-like" objects: string and integer (all lowercase).

For example the signature for Integer#>> is:

def >>: (int) -> Integer

It accepts an Integer or an object responding to to_int (summarized by int) and returns an Integer (and never another class of object responding to to_int or not).

There is a similar idea with boolean values, where a method may accept any object and will use it's truthiness, while returning true | false. For example one of the interface for Enumerable#all? should look like:

def all?: () { (Elem) -> bool } -> true | false

The user supplied block can return any value, and its truthiness (anything else than nil or false) will be used to determine the result of all?. That result will be true | false, and no other value.

If RBS is to be popular, there will be many signatures for such predicates (in builtin Ruby, stdlib, any gems, applications, etc.). I feel the best option would be Bool, if this would be reflected in Ruby itself.

Proposal: a new global module called Bool, without any method of constant, included in TrueClass and FalseClass.

Following reasons for rejection were given at the time:

many gems and libraries had already introduced Boolean class. I don't want to break them.

I looked and found the bool gem that defines a Bool module. My proposal is compatible. In any case, this gem looks abandoned, the author Aslak Helles√ły doesn't have the code on github, the gem has had 7000 downloads in the past 6 years and has no public reverse dependency. It also fails to install on my machine.

I am not aware of incompatibilities.

true and false are the only representative of true-false values. In Ruby. nil and false are falsy values, and everything else is a true value. There's no meaning for having a superclass of TrueClass and FalseClass as Boolean.

The proposal is exactly to be able to easily write about this duality of Bool as having only true and false as members, and every Ruby object as being implicitly convertible as being truthy or falsy (bool in RBS).

Discussion in RBS:

Previous feature requests for Boolean:


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