Keyword Precedence

The Ruby keyword not and the unary ! aren’t the same. They have a different precedence, and therefore are not interchangeable, despite seeming very similar.

irb > not 3 == 4
=> true
irb > !3 == 4
=> false

The first example is evaluated as not (3 == 4), or not false, which is true. The second example is evaluated as (not 3) == 4, or false == 4, which is false.

Just use !. It’s terser, it’s less ambiguous, and it can be overridden.

Ruby’s and and or have similar gotchas, which I plan to write about here in the future.