struct Nil

• Nil
• Value
• Object

Overview

The Nil type has only one possible value: nil.

nil is commonly used to represent the absence of a value. For example, String#index returns the position of the character or nil if it's not in the string:

str = "Hello world"
str.index 'e' # => 1
str.index 'a' # => nil

In the above example, trying to invoke a method on the returned value will give a compile time error unless both Int32 and Nil define that method:

str = "Hello world"
idx = str.index 'e'
idx + 1 # Error: undefined method '+' for Nil

The language and the standard library provide short, readable, easy ways to deal with nil, such as Object#try and Object#not_nil!:

str = "Hello world"

# The index of 'e' in str or 0 if not found
idx1 = str.index('e') || 0

idx2 = str.index('a')
if idx2
  # Compiles: idx2 can't be nil here
  idx2 + 1
end

# Tell the compiler that we are sure the returned
# value is not nil: raises a runtime exception
# if our assumption doesn't hold.
idx3 = str.index('o').not_nil!

See Nil literal in the language reference.

Defined in:

Constructors

• .new(ctx : YAML::ParseContext, node : YAML::Nodes::Node)
• .new(pull : JSON::PullParser) : self

Instance Method Summary

• #==(other : Nil)

  Returns true: Nil has only one singleton value: nil.

• #clone
• #hash(hasher)

  See Object#hash(hasher)

• #inspect(io : IO) : Nil

  Writes "nil" to the given IO.

• #inspect : String

  Returns "nil".

• #not_nil!(message = nil) : NoReturn

  Raises NilAssertionError.

• #object_id : UInt64

  Returns 0_u64.

• #presence : Nil

  Returns self.

• #same?(other : Nil)

  Returns true: Nil has only one singleton value: nil.

• #same?(other : Reference) : Bool

  Returns false.

• #to_json(json : JSON::Builder) : Nil
• #to_json_object_key : String
• #to_s(io : IO) : Nil

  Doesn't write anything to the given IO.

• #to_s : String

  Returns an empty string.

• #to_yaml(yaml : YAML::Nodes::Builder) : Nil
• #try(&)

  Doesn't yield to the block.

Constructor Detail

Instance Method Detail

config = {"empty" => ""}
config["empty"]?.presence   # => nil
config["missing"]?.presence # => nil