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!

Defined in:

nil.cr
json/to_json.cr
yaml/to_yaml.cr

Constructors

Instance Method Summary

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
hash(hasher)
hash
, 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)
to_json
to_json
, to_pretty_json(indent : String = " ")
to_pretty_json(io : IO, indent : String = " ")
to_pretty_json
, to_s : String
to_s(io : IO) : Nil
to_s
, to_yaml(io : IO)
to_yaml
to_yaml
, try(&) try, unsafe_as(type : T.class) forall T unsafe_as

Constructor methods inherited from class Object

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

Constructor Detail

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

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def self.new(pull : JSON::PullParser) #

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Instance Method Detail

def ==(other : Nil) #

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


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def clone #

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def hash(hasher) #

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def inspect(io : IO) : Nil #

Writes "nil" to the given IO.


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def inspect : String #

Returns "nil".


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def not_nil! #

Raises NilAssertionError.

See also: Object#not_nil!.


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def object_id #

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.


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def presence #

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

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def same?(other : Nil) #

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


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def same?(other : Reference) #

Returns false.


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def to_json(json : JSON::Builder) #

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def to_json_object_key #

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def to_s(io : IO) : Nil #

Doesn't write anything to the given IO.


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def to_s : String #

Returns an empty string.


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def to_yaml(yaml : YAML::Nodes::Builder) #

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def try(&) #

Doesn't yield to the block.

See also: Object#try.


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