struct Nil

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!

Superclass hierarchy

Defined in:

Class Method Summary

• .new(pull : JSON::PullParser)

Instance Method Summary

• #!

  Returns true.

• #==(other : Nil)

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

• #hash

  Returns zero.

• #inspect

  Returns "nil".

• #inspect(io)

  Writes "nil" to the given IO.

• #nil?

  Returns true.

• #not_nil!

  Raises an exception.

• #object_id

  Returns 0_u64.

• #same?(other : Nil)

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

• #same?(other : Reference)

  Returns false.

• #to_i

  Returns zero.

• #to_json(io)
• #to_s

  Returns an empty string.

• #to_s(io : IO)

  Doesn't write anything to the given IO.

• #try(&block)

  Doesn't yields to the block.

Class Method Detail

Instance Method Detail