How do I reference a function in Ruby?

Question

In python, it's fairly straightforward to reference a function:

>>> def foo():
...     print "foo called"
...     return 1
... 
>>> x = foo
>>> foo()
foo called
1
>>> x()
foo called
1
>>> x
<function foo at 0x1004ba5f0>
>>> foo
<function foo at 0x1004ba5f0>

However, it seems to be different in Ruby since a naked foo actually calls foo:

ruby-1.9.2-p0 > def foo
ruby-1.9.2-p0 ?>  print "foo called"
ruby-1.9.2-p0 ?>  1
ruby-1.9.2-p0 ?>  end
 => nil 
ruby-1.9.2-p0 > x = foo
foo called => 1 
ruby-1.9.2-p0 > foo
foo called => 1 
ruby-1.9.2-p0 > x
 => 1 

How do I actually assign the function foo to x and then call it? Or is there a more idiomatic way to do this?

Answer 1

Ruby doesn't have functions. It only has methods (which aren't first-class) and Procs which are first-class, but are not associated with any object.

So, this is a method:

def foo(bar) puts bar end

foo('Hello')
# Hello

Oh, and, yes, this is a real method, not a top-level function or procedure or something. Methods defined at the top-level end up as private(!) instance methods in the Object class:

Object.private_instance_methods(false) # => [:foo]

This is a Proc:

foo = -> bar { puts bar }

foo.('Hello')
# Hello

Notice that Procs are called differently from methods:

foo('Hello')  # method
foo.('Hello') # Proc

The foo.(bar) syntax is just syntactic sugar for foo.call(bar) (which for Procs and Methods is also aliased to foo[bar]). Implementing a call method on your object and then calling it with .() is the closest thing you will get to Python's __call__ables.

Note that an important distinction between Ruby Procs and Python lambdas is that there are no restrictions: in Python, a lambda can only contain a single statement, but Ruby doesn't have the distinction between statements and expressions (everything is an expression), and so this limitation simply doesn't exist, therefore in a lot of cases where you need to pass a named function as an argument in Python because you cannot express the logic in a single statement, you would in Ruby simply pass a Proc or a block instead, so that the problem of the ugly syntax for referencing methods doesn't even arise.

You can wrap a method in a Method object (which essentially duck-types Proc) by calling the Object#method method on an object (which will give you a Method whose self is bound to that particular object):

foo_bound = method(:foo)

foo_bound.('Hello')
# Hello

You can also use one of the methods in the Module#instance_method family to get an UnboundMethod from a module (or class, obviously, since a class is-a module), which you can then UnboundMethod#bind to a particular object and call. (I think Python has the same concepts, albeit with a different implementation: an unbound method simply takes the self argument explicitly, just like the way it is declared.)

foo_unbound = Object.instance_method(:foo) # this is an UnboundMethod

foo_unbound.('Hello')
# NoMethodError: undefined method `call' for #<UnboundMethod: Object#foo>

foo_rebound = foo_unbound.bind(self)       # this is a Method

foo_rebound.('Hello')
# Hello

Note that you can only bind an UnboundMethod to an object which is an instance of the module you took the method from. You cannot use UnboundMethods to "transplant" behavior between unrelated modules:

bar = module Foo; def bar; puts 'Bye' end; self end.instance_method(:bar)
module Foo; def bar; puts 'Hello' end end

obj = Object.new
bar.bind(obj)
# TypeError: bind argument must be an instance of Foo

obj.extend(Foo)
bar.bind(obj).()
# Bye
obj.bar
# Hello

Note, however, that both the Method and the UnboundMethod are wrappers around the method, not the method itself. Methods are not objects in Ruby. (Contrary to what I have written in other answers, BTW. I really need to go back and fix those.) You can wrap them in objects, but they aren't objects, and you can see that because you essentially get all the same problems you always get with wrappers: identity and state. If you call method multiple times for the same method, you will get a different Method object every time. If you try to store some state on that Method object (such as Python-style __doc__strings, for example), that state will be private to that particular instance, and if you try to retrieve your docstring again via method, you will find that it is gone.

There is also syntactic sugar in the form of the method reference operator .::

bound_method = obj.:foo

Which is identical to

bound_method = obj.method(:foo)