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!
See Nil
literal in the language reference.
Defined in:
json/to_json.crnil.cr
yaml/to_yaml.cr
Constructors
Instance Method Summary
-
#==(other : Nil)
Returns
true
:Nil
has only one singleton value:nil
. - #clone
- #hash(hasher)
-
#inspect(io : IO) : Nil
Writes
"nil"
to the givenIO
. -
#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
Instance Method Detail
Raises NilAssertionError
.
If message is given, it is forwarded as error message of NilAssertionError
.
See also: Object#not_nil!
.
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
.
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