struct Nil

Overview

TheNil 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 ornil 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 bothInt32 andNil 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 withnil, such asObject#try andObject#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!

SeeNil literal in the language reference.

Defined in:

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

Constructors

Instance Method Summary

Instance methods inherited from struct Value

==(other : Log::Metadata::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 : String | IO, root : String)
from_json(string_or_io : String | 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) : self #

Instance Method Detail

def ==(other : Nil) #

Returnstrue:Nil has only one singleton value:nil.


def clone #

def hash(hasher) #

SeeObject#hash(hasher)


def inspect(io : IO) : Nil #

Writes"nil" to the givenIO.


def inspect : String #

Returns"nil".


def not_nil!(message = nil) : NoReturn #

RaisesNilAssertionError.

Ifmessage is given, it is forwarded as error message ofNilAssertionError.

See also:Object#not_nil!.


def object_id : UInt64 #

Returns0_u64. Even thoughNil is not aReference type, it is usually mixed with them to form nilable types so it's useful to have an object id fornil.


def presence : Nil #

Returnsself. This method enables to call the#presence method (seeString#presence) on a union withNil. The idea is to returnnil when the value isnil or empty.

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

def same?(other : Nil) #

Returnstrue:Nil has only one singleton value:nil.


def same?(other : Reference) : Bool #

Returnsfalse.


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 givenIO.


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.