ruby--ruby/doc/syntax/calling_methods.rdoc

= Calling Methods

Calling a method sends a message to an object so it can perform some work.

In ruby you send a message to an object like this:

  my_method()

Note that the parenthesis are optional:

  my_method

Except when there is difference between using and omitting parentheses, this
document uses parenthesis when arguments are present to avoid confusion.

This section only covers calling methods.  See also the {syntax documentation
on defining methods}[rdoc-ref:syntax/methods.rdoc].

== Receiver

+self+ is the default receiver.  If you don't specify any receiver +self+ will
be used.  To specify a receiver use <code>.</code>:

  my_object.my_method

This sends the +my_method+ message to +my_object+.  Any object can be a
receiver but depending on the method's visibility sending a message may raise a
NoMethodError.

You may use <code>.?</code> to designate a receiver, then +my_method+ is not
invoked and the result is +nil+ when the receiver is +nil+.  In that case, the
arguments of +my_method+ are not evaluated.

You may also use <code>::</code> to designate a receiver, but this is rarely
used due to the potential for confusion with <code>::</code> for namespaces.

== Arguments

There are three types of arguments when sending a message, the positional
arguments, keyword (or named) arguments and the block argument.  Each message
sent may use one, two or all types of arguments, but the arguments must be
supplied in this order.

All arguments in ruby are passed by reference and are not lazily evaluated.

Each argument is separated by a <code>,</code>:

  my_method(1, '2', :three)

Arguments may be an expression, a hash argument:

  'key' => value

or a keyword argument:

  key: value

Hash and keyword arguments must be contiguous and must appear after all
positional arguments, but may be mixed:

  my_method('a' => 1, b: 2, 'c' => 3)

=== Positional Arguments

The positional arguments for the message follow the method name:

  my_method(argument1, argument2)

In many cases parenthesis are not necessary when sending a message:

  my_method argument1, argument2

However, parenthesis are necessary to avoid ambiguity.  This will raise a
SyntaxError because ruby does not know which method argument3 should be sent
to:

  method_one argument1, method_two argument2, argument3

If the method definition has a <code>*argument</code> extra positional
arguments will be assigned to +argument+ in the method as an Array.

If the method definition doesn't include keyword arguments the keyword or
hash-type arguments are assigned as a single hash to the last argument:

  def my_method(options)
    p options
  end

  my_method('a' => 1, b: 2) # prints: {'a'=>1, :b=>2}

If too many positional arguments are given an ArgumentError is raised.

=== Default Positional Arguments

When the method defines default arguments you do not need to supply all the
arguments to the method.  Ruby will fill in the missing arguments in-order.

First we'll cover the simple case where the default arguments appear on the
right.  Consider this method:

  def my_method(a, b, c = 3, d = 4)
    p [a, b, c, d]
  end

Here +c+ and +d+ have default values which ruby will apply for you.  If you
send only two arguments to this method:

  my_method(1, 2)

You will see ruby print <code>[1, 2, 3, 4]</code>.

If you send three arguments:

  my_method(1, 2, 5)

You will see ruby print <code>[1, 2, 5, 4]</code>

Ruby fills in the missing arguments from left to right.

Ruby allows default values to appear in the middle of positional arguments.
Consider this more complicated method:

  def my_method(a, b = 2, c = 3, d)
    p [a, b, c, d]
  end

Here +b+ and +c+ have default values.  If you send only two arguments to this
method:

  my_method(1, 4)

You will see ruby print <code>[1, 2, 3, 4]</code>.

If you send three arguments:

  my_method(1, 5, 6)

You will see ruby print <code>[1, 5, 3, 6]</code>.

Describing this in words gets complicated and confusing. I'll describe it
in variables and values instead.

First <code>1</code> is assigned to +a+, then <code>6</code> is assigned to
+d+.  This leaves only the arguments with default values.  Since
<code>5</code> has not been assigned to a value yet, it is given to +b+ and
+c+ uses its default value of <code>3</code>.

=== Keyword Arguments

Keyword arguments follow any positional arguments and are separated by commas
like positional arguments:

  my_method(positional1, keyword1: value1, keyword2: value2)

Any keyword arguments not given will use the default value from the method
definition.  If a keyword argument is given that the method did not list an
ArgumentError will be raised.

=== Block Argument

The block argument sends a closure from the calling scope to the method.

The block argument is always last when sending a message to a method.  A block
is sent to a method using <code>do ... end</code> or <code>{ ... }</code>:

  my_method do
    # ...
  end

or:

  my_method {
    # ...
  }

<code>do end</code> has lower precedence than <code>{ }</code> so:

  method_1 method_2 {
    # ...
  }

Sends the block to +method_2+ while:

  method_1 method_2 do
    # ...
  end

Sends the block to +method_1+.  Note that in the first case if parentheses are
used the block is sent to +method_1+.

A block will accept arguments from the method it was sent to.  Arguments are
defined similar to the way a method defines arguments.  The block's arguments
go in <code>| ... |</code> following the opening <code>do</code> or
<code>{</code>:

  my_method do |argument1, argument2|
    # ...
  end

==== Block Local Arguments

You may also declare block-local arguments to a block using <code>;</code> in
the block arguments list.  Assigning to a block-local argument will not
override local arguments outside the block in the caller's scope:

  def my_method
    yield self
  end

  place = "world"

  my_method do |obj; place|
    place = "block"
    puts "hello #{obj} this is #{place}"
  end

  puts "place is: #{place}"

This prints:

  hello main this is block
  place is world

So the +place+ variable in the block is not the same +place+ variable as
outside the block.  Removing <code>; place</code> from the block arguments
gives this result:

  hello main this is block
  place is block

=== Array to Arguments Conversion

Given the following method:

  def my_method(argument1, argument2, argument3)
  end

You can turn an Array into an argument list with <code>*</code> (or splat)
operator:

  arguments = [1, 2, 3]
  my_method(*arguments)

or:

  arguments = [2, 3]
  my_method(1, *arguments)

Both are equivalent to:

  my_method(1, 2, 3)

If the method accepts keyword arguments the splat operator will convert a hash
at the end of the array into keyword arguments:

  def my_method(a, b, c: 3)
  end

  arguments = [1, 2, { c: 4 }]
  my_method(*arguments)

You may also use the <code>**</code> (described next) to convert a Hash into
keyword arguments.

If the number of objects in the Array do not match the number of arguments for
the method an ArgumentError will be raised.

If the splat operator comes first in the call, parentheses must be used to
avoid a warning.

=== Hash to Keyword Arguments Conversion

Given the following method:

  def my_method(first: 1, second: 2, third: 3)
  end

You can turn a Hash into keyword arguments with the <code>**</code> operator:

  arguments = { first: 3, second: 4, third: 5 }
  my_method(**arguments)

or:

  arguments = { first: 3, second: 4 }
  my_method(third: 5, **arguments)

Both are equivalent to:

  my_method(first: 3, second: 4, third: 5)

If the method definition uses <code>**</code> to gather arbitrary keyword
arguments they will not be gathered by <code>*</code>:

  def my_method(*a, **kw)
    p arguments: a, keywords: kw
  end

  my_method(1, 2, '3' => 4, five: 6)

Prints:

  {:arguments=>[1, 2, {"3"=>4}], :keywords=>{:five=>6}}

Unlike the splat operator described above the <code>**</code> operator has no
commonly recognized name.

=== Proc to Block Conversion

Given a method that use a block:

  def my_method
    yield self
  end

You can convert a proc or lambda to a block argument with the <code>&</code>
operator:

  argument = proc { |a| puts "#{a.inspect} was yielded" }

  my_method(&argument)

If the splat operator comes first in the call, parenthesis must be used to
avoid a warning.

Unlike the splat operator described above the <code>&</code> operator has no
commonly recognized name.

== Method Lookup

When you send a message Ruby looks up the method that matches the name of the
message for the receiver.  Methods are stored in classes and modules so method
lookup walks these, not the objects themselves.

Here is the order of method lookup for the receiver's class or module +R+:

* The prepended modules of +R+ in reverse order
* For a matching method in +R+
* The included modules of +R+ in reverse order

If +R+ is a class with a superclass, this is repeated with +R+'s superclass
until a method is found.

Once a match is found method lookup stops.

If no match is found this repeats from the beginning, but looking for
+method_missing+.  The default +method_missing+ is BasicObject#method_missing
which raises a NameError when invoked.

If refinements (an experimental feature) are active the method lookup changes.
See the {refinements documentation}[rdoc-ref:syntax/refinements.rdoc] for
details.