Method

Class

Method objects are created by Object#method, and are associated with a particular object (not just with a class). They may be used to invoke the method within the object, and as a block associated with an iterator. They may also be unbound from one object (creating an UnboundMethod) and bound to another.

class Thing
  def square(n)
    n*n
  end
end
thing = Thing.new
meth  = thing.method(:square)

meth.call(9)                 #=> 81
[ 1, 2, 3 ].collect(&meth)   #=> [1, 4, 9]

[ 1, 2, 3 ].each(&method(:puts)) #=> prints 1, 2, 3

require 'date'
%w[2017-03-01 2017-03-02].collect(&Date.method(:parse))
#=> [#<Date: 2017-03-01 ((2457814j,0s,0n),+0s,2299161j)>, #<Date: 2017-03-02 ((2457815j,0s,0n),+0s,2299161j)>]
Instance Methods

meth << g → a_proc
#

Returns a proc that is the composition of this method and the given g. The returned proc takes a variable number of arguments, calls g with them then calls this method with the result.

def f(x)
  x * x
end

f = self.method(:f)
g = proc {|x| x + x }
p (f << g).call(2) #=> 16

meth == other_meth → true or false
#

Two method objects are equal if they are bound to the same object and refer to the same method definition and the classes defining the methods are the same class or module.

===(*args)
#
An alias for call

meth >> g → a_proc
#

Returns a proc that is the composition of this method and the given g. The returned proc takes a variable number of arguments, calls this method with them then calls g with the result.

def f(x)
  x * x
end

f = self.method(:f)
g = proc {|x| x + x }
p (f >> g).call(2) #=> 8

[](*args)
#
An alias for call

meth.arity → integer
#

Returns an indication of the number of arguments accepted by a method. Returns a nonnegative integer for methods that take a fixed number of arguments. For Ruby methods that take a variable number of arguments, returns -n-1, where n is the number of required arguments. Keyword arguments will be considered as a single additional argument, that argument being mandatory if any keyword argument is mandatory. For methods written in C, returns -1 if the call takes a variable number of arguments.

class C
  def one;    end
  def two(a); end
  def three(*a);  end
  def four(a, b); end
  def five(a, b, *c);    end
  def six(a, b, *c, &d); end
  def seven(a, b, x:0); end
  def eight(x:, y:); end
  def nine(x:, y:, **z); end
  def ten(*a, x:, y:); end
end
c = C.new
c.method(:one).arity     #=> 0
c.method(:two).arity     #=> 1
c.method(:three).arity   #=> -1
c.method(:four).arity    #=> 2
c.method(:five).arity    #=> -3
c.method(:six).arity     #=> -3
c.method(:seven).arity   #=> -3
c.method(:eight).arity   #=> 1
c.method(:nine).arity    #=> 1
c.method(:ten).arity     #=> -2

"cat".method(:size).arity      #=> 0
"cat".method(:replace).arity   #=> 1
"cat".method(:squeeze).arity   #=> -1
"cat".method(:count).arity     #=> -1

meth.call(args, ...) → obj
#

Invokes the meth with the specified arguments, returning the method’s return value.

m = 12.method("+")
m.call(3)    #=> 15
m.call(20)   #=> 32

method.clone → new_method
#

Returns a clone of this method.

class A
  def foo
    return "bar"
  end
end

m = A.new.method(:foo)
m.call # => "bar"
n = m.clone.call # => "bar"

meth.curry → proc

meth.curry(arity) → proc
#

Returns a curried proc based on the method. When the proc is called with a number of arguments that is lower than the method’s arity, then another curried proc is returned. Only when enough arguments have been supplied to satisfy the method signature, will the method actually be called.

The optional arity argument should be supplied when currying methods with variable arguments to determine how many arguments are needed before the method is called.

def foo(a,b,c)
  [a, b, c]
end

proc  = self.method(:foo).curry
proc2 = proc.call(1, 2)          #=> #<Proc>
proc2.call(3)                    #=> [1,2,3]

def vararg(*args)
  args
end

proc = self.method(:vararg).curry(4)
proc2 = proc.call(:x)      #=> #<Proc>
proc3 = proc2.call(:y, :z) #=> #<Proc>
proc3.call(:a)             #=> [:x, :y, :z, :a]

meth.eql?(other_meth) → true or false
#
An alias for ==

meth.hash → integer
#

Returns a hash value corresponding to the method object.

See also Object#hash.

meth.inspect → string
#

Returns a human-readable description of the underlying method.

"cat".method(:count).inspect   #=> "#<Method: String#count(*)>"
(1..3).method(:map).inspect    #=> "#<Method: Range(Enumerable)#map()>"

In the latter case, the method description includes the “owner” of the original method (Enumerable module, which is included into Range).

inspect also provides, when possible, method argument names (call sequence) and source location.

require 'net/http'
Net::HTTP.method(:get).inspect
#=> "#<Method: Net::HTTP.get(uri_or_host, path=..., port=...) <skip>/lib/ruby/2.7.0/net/http.rb:457>"

... in argument definition means argument is optional (has some default value).

For methods defined in C (language core and extensions), location and argument names can’t be extracted, and only generic information is provided in form of * (any number of arguments) or _ (some positional argument).

"cat".method(:count).inspect   #=> "#<Method: String#count(*)>"
"cat".method(:+).inspect       #=> "#<Method: String#+(_)>""

meth.name → symbol
#

Returns the name of the method.

meth.original_name → symbol
#

Returns the original name of the method.

class C
  def foo; end
  alias bar foo
end
C.instance_method(:bar).original_name # => :foo

meth.owner → class_or_module
#

Returns the class or module on which this method is defined. In other words,

meth.owner.instance_methods(false).include?(meth.name) # => true

holds as long as the method is not removed/undefined/replaced, (with private_instance_methods instead of instance_methods if the method is private).

See also Method#receiver.

(1..3).method(:map).owner #=> Enumerable

meth.parameters → array
#

Returns the parameter information of this method.

def foo(bar); end
method(:foo).parameters #=> [[:req, :bar]]

def foo(bar, baz, bat, &blk); end
method(:foo).parameters #=> [[:req, :bar], [:req, :baz], [:req, :bat], [:block, :blk]]

def foo(bar, *args); end
method(:foo).parameters #=> [[:req, :bar], [:rest, :args]]

def foo(bar, baz, *args, &blk); end
method(:foo).parameters #=> [[:req, :bar], [:req, :baz], [:rest, :args], [:block, :blk]]

meth.private? → true or false
#

Returns whether the method is private.

meth.protected? → true or false
#

Returns whether the method is protected.

meth.public? → true or false
#

Returns whether the method is public.

meth.receiver → object
#

Returns the bound receiver of the method object.

(1..3).method(:map).receiver # => 1..3

meth.source_location → [String, Integer]
#

Returns the Ruby source filename and line number containing this method or nil if this method was not defined in Ruby (i.e. native).

meth.super_method → method
#

Returns a Method of superclass which would be called when super is used or nil if there is no method on superclass.

meth.to_proc → proc
#

Returns a Proc object corresponding to this method.

meth.to_s → string
#
An alias for inspect

meth.unbind → unbound_method
#

Dissociates meth from its current receiver. The resulting UnboundMethod can subsequently be bound to a new object of the same class (see UnboundMethod).