Methods

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

    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.

    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.

    1. def say_hello(recipient)
    2. puts "Hello #{recipient}!"
    3. end
    4. say_hello "World"
    5. 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.

    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.

    1. def say_hello(recipient = "World")
    2. puts "Hello #{recipient}!"
    3. say_hello
    4. say_hello "Crystal"

    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 .

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

    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.

    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.

    1. # This methods greets *recipient*.
    2. def say_hello(recipient : String)
    3. puts "Hello #{recipient}!"
    4. end
    5. # This method greets *times* times.
    6. def say_hello(times : Int32)
    7. puts "Hello " * times
    8. end
    9. say_hello "World"

    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:

    1. def adds_2(n : Int32)
    2. n + 2
    3. end

    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:

    Return type

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

    1. def life_universe_and_everything
    2. "Fortytwo"
    3. end
    4. 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:

    1. def life_universe_and_everything : Int32
    2. "Fortytwo"
    3. end