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! : 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! 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
Constructor Detail
def self.new(ctx : YAML::ParseContext, node : YAML::Nodes::Node)Source
def self.new(pull : JSON::PullParser)Source
Instance Method Detail
def cloneSource
def hash(hasher)Source
def not_nil! : NoReturnSource
Raises NilAssertionError
.
See also: Object#not_nil!
.
def presence : NilSource
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 to_json(json : JSON::Builder) : NilSource
def to_yaml(yaml : YAML::Nodes::Builder) : NilSource
def try(&)Source
Doesn't yield to the block.
See also: Object#try
.
© 2012–2021 Manas Technology Solutions.
Licensed under the Apache License, Version 2.0.
https://crystal-lang.org/api/1.2.1/Nil.html