Programming Crystal

Nilable Types

How do you declare a variable that’s of a certain type but can also be nil, sometimes called nilable? You saw earlier that in certain situations, Crystal will assign a union type to an object at compile time. You could declare a nilable type as a union type, but you can also declare its type with a ? appended, as shown here:

types_and_control_flow/type_nil.cr

n : Int32 | Nil
n = 42
p typeof(n) # => Int32

a : Int32?
a = 42
p typeof(a) # => Int32

b : Int32?
b = nil
p typeof(b) # => Nil

a.try { p a + 1 } # => 43
b.try { p b + 1 } # => no error!

When using nilable types, the try method can come in handy: the compiler won’t signal any errors when the value might be nil. But be aware that if you lock ...