Methods

To avoid duplication of the same message, instead of using a variable we can define a method and call it multiple times.

A method definition is indicated by the keyword def followed by the method name. Every expression until the keyword end is part of the method body.

def say_hello
  puts "Hello Penny!"
end

say_hello
say_hello
say_hello() # syntactically equivalent method call with parentheses

Tip

Method calls are unambiguously indicated by parentheses after the name, but they can be omitted. It would only be necessary for disambiguation, for example, if say_hello was also a local variable.

Arguments

What if we want to greet different people, but all in the same manner? Instead of writing individual messages, we can define a method that allows customization through a parameter. A parameter is like a local variable inside the method body. Parameters are declared after the method name in parentheses. When calling a method, you can pass in arguments that are mapped as values for the method's parameters.

def say_hello(recipient)
  puts "Hello #{recipient}!"
end

say_hello "World"
say_hello "Crystal"

Tip

Arguments at method calls are typically placed in parentheses, but it can often be omitted. say_hello "World" and say_hello("World") are syntactically equivalent.

It's generally recommended to use parentheses because it avoids ambiguity. But they're often omitted if the expression reads like natural language.

Default arguments

Arguments can be assigned a default value. It is used in case the argument is missing in the method call. Usually, arguments are mandatory but when there's a default value, it can be omitted.

def say_hello(recipient = "World")
  puts "Hello #{recipient}!"
end

say_hello
say_hello "Crystal"

Type Restrictions

Our example method expects recipient to be a String. But any other type would work as well. Try say_hello 6 for example.

This isn't necessarily a problem for this method. Using any other type would be valid code. But semantically we want to greet people with a name as a String.

Type restrictions limit the allowed type of an argument. They come after the argument name, separated by a colon:

def say_hello(recipient : String)
  puts "Hello #{recipient}!"
end

say_hello "World"
say_hello "Crystal"

# Now this expression doesn't compile:
# say_hello 6

Now names cannot be numbers or other data types anymore. This doesn't mean you can't greet people with a number as a name. The number just needs to be expressed as a string. Try say_hello "6" for example.

Overloading

Restricting the type of an argument can be used for positional overloading. When a method has an unrestricted argument like say_hello(recipient), all calls to a method say_hello go to that method. But with overloading several methods of the same name can exist with different argument type restrictions. Each call is routed to the most fitting overload.

# This methods greets *recipient*.
def say_hello(recipient : String)
  puts "Hello #{recipient}!"
end

# This method greets *times* times.
def say_hello(times : Int32)
  puts "Hello " * times
end

say_hello "World"
say_hello 3

Overloading isn't defined just by type restrictions. The number of arguments as well as named arguments are also relevant characteristics.

Returning a value

Methods return a value which becomes the value of the method call. By default, it's the value of the last expression in the method:

def adds_2(n : Int32)
  n + 2
end

puts adds_2 40

A method can return at any place in its body using the return statement. The argument passed to return becomes the method's return value. If there is no argument, it's nil.

The following example illustrates the use of an explicit and an implicit return:

# This method returns:
# - the same number if it's even,
# - the number multiplied by 2 if it's odd.
def build_even_number(n : Int32)
  return n if n.even?

  n * 2
end

puts build_even_number 7
puts build_even_number 28

Return type

Let's begin defining a method that we expect it will return an Int32 value, but mistakenly returns a String:

def life_universe_and_everything
  "Fortytwo"
end

puts life_universe_and_everything + 1 # Error: no overload matches 'String#+' with type Int32

Because we never told the compiler we were expecting the method to return an Int32, the best the compiler can do is to tell us that there is no String#+ method that takes an Int32 value as an argument (i.e. the compiler is pointing at the moment when we use the value but not at the root of the bug: the type of the method's return value).

The error message can be more accurate if using type information, so let's try again the example but now specifying the type:

def life_universe_and_everything : Int32
  "Fortytwo"
end

puts life_universe_and_everything + 1 # Error: method top-level life_universe_and_everything must return Int32 but it is returning String

Now the compiler can show us exactly where the problem is originated. As we can see, providing type information is really useful for finding errors at compile time.