Ruby uses `nil` to represent the absence of a meaningful value. It is still an object—the sole instance of `NilClass`—and it responds to methods. For example, `nil.to_s` returns an empty string and `nil.to_i` returns `0`; neither conversion makes `nil` an empty string or a number. section 7.5.3
An expression’s result and its truth value are different questions. `nil` and `false` are the only objects that fail an `if` test. Zero, an empty string, and empty collections pass it. Thus `nil` can signal a negative test outcome without being the same object as `false`. section 7.5.2 section 7.5.3
Several ordinary operations produce `nil`:
- `puts "Hello"` prints the string but returns `nil`. In irb, the printed string and the displayed `=> nil` are two different outputs. section 1.4.2
- An `if` expression with a failed condition and no `else` evaluates to `nil`. section 2.2.4
- An uninitialized instance variable or an out-of-range array element yields `nil`. Reading an undefined local variable instead raises an error. section 7.5.3
Absence matters when calling methods. If `roster.players` is `nil`, `roster.players.first` raises `NoMethodError`. Safe navigation calls the next method only when its receiver is not `nil`: `roster.players&.first&.position` yields `nil` instead of raising when a link is missing. section 6.4.4
Seeing an unassigned value during debugging
At a breakpoint before a temperature conversion, `rdbg` displays `celsius = nil`. After stepping past its assignment, inspecting `celsius` displays `100`. The two inspections distinguish a value not yet assigned at that point from the value later used in the calculation. section 1.4.5
