Why can't I do "if zero" in ruby?

Question

>> current_user.first_visit
=> 0
>> if current_user.first_visit
>> puts "test"
>> end
test
=> nil

Why does it print test?

Answer 1

In Ruby only nil can be considered as analogue to false. There is no assumption, that 0, "" or [] means false, so you must use == 0, .blank?, .empty?, .zero?, etc.

But nil doesn't always behave as false. For example, in string interpolation, the .to_s method is applied to #{} contents, that works differently for FalseClass and NilClass:

irb(main)> "qwe #{false} rty"
=> "qwe false rty"
irb(main)> "qwe #{nil} rty"
=> "qwe  rty"

Answer 2

You can think of Ruby's if as testing either a boolean, or for the availability of data (vs nil). The implicit conversion from 0 to false supported by C and (hence other languages like C++) was more an historical artefact from days before C had a distinct boolean type. There, it relies on 0 being a convenient sentinel value or having an intuitive meaning. For many things (e.g. some country where clothing sizes range from 0 to 8, POSIX libC function call results) zero does not convert nicely to a logically equivalent boolean, so it's not a bad thing that Ruby goes its own way.

From this perspective, the issue with your code is that current_user.first_visit - the name of which implies a boolean type - actually holds 0 and not false. Alternatively, if you had the clearly numeric current_user.visit_counter, it would be natural and correct to use one of:

current_user.visit_counter > 0
current_user.visit_counter >= 1
current_user.visit_counter != 0