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Generics allow you to parameterize a type based on other type. Consider a Box type:

class MyBox(T)
  def initialize(@value : T)

  def value

int_box = MyBox(Int32).new(1)
int_box.value # => 1 (Int32)

string_box = MyBox(String).new("hello")
string_box.value # => "hello" (String)

another_box = MyBox(String).new(1) # Error, Int32 doesn't match String

Generics are especially useful for implementing collection types. Array, Hash, Set are generic types, as is Pointer.

More than one type parameter is allowed:

class MyDictionary(K, V)

Any name can be used for type parameters:

class MyDictionary(KeyType, ValueType)

Type variables inference

Type restrictions in a generic type's constructor are free variables when type arguments were not specified, and then are used to infer them. For example:       # : MyBox(Int32)"hello") # : MyBox(String)

In the above code we didn't have to specify the type arguments of MyBox, the compiler inferred them following this process:

  • delegates to initialize(@value : T)
  • T isn't bound to a type yet, so the compiler binds it to the type of the given argument

In this way generic types are less tedious to work with.

Generic structs and modules

Structs and modules can be generic too. When a module is generic you include it like this:

module Moo(T)
  def t

class Foo(U)
  include Moo(U)

  def initialize(@value : U)

foo =
foo.t # Int32

Note that in the above example T becomes Int32 because makes U become Int32, which in turn makes T become Int32 via the inclusion of the generic module.

Generic types inheritance

Generic classes and structs can be inherited. When inheriting you can specify an instance of the generic type, or delegate type variables:

class Parent(T)

class Int32Child < Parent(Int32)

class GenericChild(T) < Parent(T)

Generics with variable number of arguments

We may define a Generic class with a variable number of arguments using the splat operator.

Let's see an example where we define a Generic class called Foo and then we will use it with different number of type variables:

class Foo(*T)
  getter content

  def initialize(*@content : *T)

# 2 type variables:
# (explicitly specifying type variables)
foo = Foo(Int32, String).new(42, "Life, the Universe, and Everything")

p typeof(foo) # => Foo(Int32, String)
p foo.content # => {42, "Life, the Universe, and Everything"}

# 3 type variables:
# (type variables inferred by the compiler)
bar ="Hello", ["Crystal", "!"], 140)
p typeof(bar) # => Foo(String, Array(String), Int32)

In the following example we define classes by inheritance, specifying instances for the generic types:

class Parent(*T)

# We define `StringChild` inheriting from `Parent` class
# using `String` for generic type argument:
class StringChild < Parent(String)

# We define `Int32StringChild` inheriting from `Parent` class
# using `Int32` and `String` for generic type arguments:
class Int32StringChild < Parent(Int32, String)

And if we need to instantiate a class with 0 arguments? In that case we may do:

class Parent(*T)

foo = Parent().new
p typeof(foo) # => Parent()

But we should not mistake 0 arguments with not specifying the generic type variables. The following examples will raise an error:

class Parent(*T)

foo = # Error: can't infer the type parameter T for the generic class Parent(*T). Please provide it explicitly

class Foo < Parent # Error: generic type arguments must be specified when inheriting Parent(*T)