# 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:

json/to_json.cr
nil.cr
uri/params/to_www_form.cr
yaml/to_yaml.cr

##
Constructors

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

##
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.

### Instance methods inherited from struct `Value`

==(other : JSON::Any)
==(other : YAML::Any)
==(other) ==, dup dup

### Instance methods inherited from class `Object`

! : Bool !, !=(other) !=, !~(other) !~, ==(other) ==, ===(other : JSON::Any)
===(other : YAML::Any)
===(other) ===, =~(other) =~, as(type : Class) as, as?(type : Class) as?, class class, dup dup, hash(hasher)
hash hash, in?(collection : Object) : Bool
in?(*values : Object) : Bool in?, inspect(io : IO) : Nil
inspect : String inspect, is_a?(type : Class) : Bool is_a?, itself itself, nil? : Bool nil?, not_nil!(message)
not_nil! not_nil!, pretty_inspect(width = 79, newline = "\n", indent = 0) : String pretty_inspect, pretty_print(pp : PrettyPrint) : Nil pretty_print, responds_to?(name : Symbol) : Bool responds_to?, tap(&) tap, to_json(io : IO) : Nil
to_json : String to_json, to_pretty_json(indent : String = " ") : String
to_pretty_json(io : IO, indent : String = " ") : Nil to_pretty_json, to_s(io : IO) : Nil
to_s : String to_s, to_yaml(io : IO) : Nil
to_yaml : String to_yaml, try(&) try, unsafe_as(type : T.class) forall T unsafe_as

### Class methods inherited from class `Object`

from_json(string_or_io, root : String)
from_json(string_or_io) from_json, from_yaml(string_or_io : String | IO) from_yaml

### Macros inherited from class `Object`

class_getter(*names, &block) class_getter, class_getter!(*names) class_getter!, class_getter?(*names, &block) class_getter?, class_property(*names, &block) class_property, class_property!(*names) class_property!, class_property?(*names, &block) class_property?, class_setter(*names) class_setter, def_clone def_clone, def_equals(*fields) def_equals, def_equals_and_hash(*fields) def_equals_and_hash, def_hash(*fields) def_hash, delegate(*methods, to object) delegate, forward_missing_to(delegate) forward_missing_to, getter(*names, &block) getter, getter!(*names) getter!, getter?(*names, &block) getter?, property(*names, &block) property, property!(*names) property!, property?(*names, &block) property?, setter(*names) setter

##
Constructor Detail

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

##
Instance Method Detail

def ==(other : Nil) #

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

def clone #
def hash(hasher) #

See `Object#hash(hasher)`

def inspect(io : IO) : Nil #

Writes `"nil"` to the given `IO`.

def inspect : String #

Returns `"nil"`.

def not_nil!(message = nil) : NoReturn #

Raises `NilAssertionError`.

If message is given, it is forwarded as error message of `NilAssertionError`.

See also: `Object#not_nil!`.

def object_id : UInt64 #

Returns `0_u64`. Even though `Nil` is not a `Reference` type, it is usually mixed with them to form nilable types so it's useful to have an object id for `nil`.

def presence : Nil #

Returns `self`. This method enables to call the `#presence` method (see `String#presence`) on a union with `Nil`. The idea is to return `nil` when the value is `nil` or empty.

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

def same?(other : Nil) #

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

def same?(other : Reference) : Bool #

Returns `false`.

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

Doesn't write anything to the given `IO`.

def to_s : String #

Returns an empty string.

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

Doesn't yield to the block.

See also: `Object#try`.