CODEWITHSUNDEEP
crystal-lang
shadowtalker
shadowtalker

Reputation: 13893

In Crystal, what's the difference between inheritance and inclusion?

In most of the Crystal docs, class inheritance is used, with < syntax (e.g. https://stackoverflow.com/a/61053311/2954547).

However, HTTP::Handler says that custom handlers must include the HTTP::Handler module and not inherit from some class.

I can't find a description in the Crystal docs of what include-ing a module is supposed to do, or how it differs from <-inheritance of classes.

What does it mean when a class includes a module?

Upvotes: 1

Views: 125

Answers (2)

Johannes M&#252;ller
Johannes M&#252;ller

Reputation: 5661

Inclusion is also a form of inheritance.

The main difference is really that extending a type is limited to exactly one parent. The extension inheritance graph of the entire program is a tree. In contrast, a type can include multiple modules. And there can be multiple include inheritance paths between two types.

Upvotes: 2

shadowtalker
shadowtalker

Reputation: 13893

The description of inclusion is buried in the "Module" specification: https://crystal-lang.org/reference/syntax_and_semantics/modules.html

The example in the docs page is:

An include makes a type include methods defined in that module as instance methods:

module ItemsSize
  def size
    items.size
  end
end

class Items
  include ItemsSize

  def items
    [1, 2, 3]
  end
end

items = Items.new
items.size # => 3

There is also the related extend, which includes module members as class-level members and not instance-level members.

This is a useful way to define mixins or other namespaces, that are not meant to be instantiated as objects.

Upvotes: 0

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