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

Class Method Summary

Instance Method Summary

Instance methods inherited from struct Value

! !, ==(other) ==, nil? nil?

Instance methods inherited from class Object

! !, !=(other) !=, !~(other) !~, ==(other) ==, ===(other) ===, =~(other) =~, class class, clone clone, crystal_type_id crystal_type_id, dup dup, hash hash, inspect
inspect(io : IO)
inspect
, itself itself, not_nil! not_nil!, tap(&block) tap, to_json to_json, to_pretty_json(io : IO)
to_pretty_json
to_pretty_json
, to_s
to_s(io : IO)
to_s
, to_yaml(io : IO)
to_yaml
to_yaml
, try(&block) try

Class methods inherited from class Object

==(other : Class) ==, ===(other) ===, cast(other) : self cast, from_json(string_or_io) from_json, from_yaml(string : String) from_yaml, hash hash, inspect(io) inspect, name : String name, to_s(io) to_s

Class Method Detail

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

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

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Instance Method Detail

def ! #

Returns true.


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def ==(other : Nil) #

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


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def hash #

Returns zero.


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def inspect #

Returns "nil".


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def inspect(io) #

Writes "nil" to the given IO.


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def nil? #

Returns true.


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def not_nil! #

Raises an exception. See 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 same?(other : Reference) #

Returns false.


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def same?(other : Nil) #

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


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def to_json(io) #

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def to_s(io : IO) #

Doesn't write anything to the given IO.


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def to_s #

Returns an empty string.


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def to_yaml(yaml : YAML::Generator) #

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def try(&block) #

Doesn't yields to the block. See Object#try.


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