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/Crystal 0.31

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

Constructor Detail

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

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

Instance Method Detail

def ==(other : Nil)Source

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

def cloneSource

def hash(hasher)Source

def inspect(io : IO) : NilSource

Writes "nil" to the given IO.

def inspect : StringSource

Returns "nil".

def not_nil!Source

Raises NilAssertionError.

See also: Object#not_nil!.

def object_idSource

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.

def same?(other : Nil)Source

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

def same?(other : Reference)Source

Returns false.

def to_json(json : JSON::Builder)Source

def to_json_object_keySource

def to_s(io : IO) : NilSource

Doesn't write anything to the given IO.

def to_s : StringSource

Returns an empty string.

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

def try(&block)Source

Doesn't yield to the block.

See also: Object#try.

© 2012–2019 Manas Technology Solutions.
Licensed under the Apache License, Version 2.0.
https://crystal-lang.org/api/0.31.1/Nil.html