Type grammar

When:

• specifying type restrictions
• specifying type arguments
• declaring variables
• declaring aliases
• declaring typedefs
• the argument of an is_a? pseudo-call
• the argument of an as expression
• the argument of a sizeof expression
• the argument of an instance_sizeof expression
• a method’s return type

a convenient syntax is provided for some common types. These are especially useful when writing C bindings, but can be used in any of the above locations.

Paths and generics

Regular types and generics can be used:

Int32
My::Nested::Type
Array(String)

Union

alias Int32OrString = Int32 | String

The pipe (|) in types creates a union type. Int32 | String is read “Int32 or String”. In regular code, Int32 | String means invoking the method | on Int32 with String as an argument.

Nilable

alias Int32OrNil = Int32?

is the same as:

alias Int32OrNil = Int32 | ::Nil

In regular code, Int32? is an Int32 | ::Nil union type itself.

Pointer

alias Int32Ptr = Int32*

is the same as:

alias Int32Ptr = Pointer(Int32)

In regular code, Int32* means invoking the * method on Int32.

StaticArray

alias Int32_8 = Int32[8]

is the same as:

alias Int32_8 = StaticArray(Int32, 8)

In regular code, Int32[8] means invoking the [] method on Int32 with 8 as an argument.

Tuple

alias Int32StringTuple = {Int32, String}

is the same as:

alias Int32StringTuple = Tuple(Int32, String)

In regular code, {Int32, String} is a tuple instance containing Int32 and String as its elements. This is different than the above tuple type.

NamedTuple

alias Int32StringNamedTuple = {x: Int32, y: String}

is the same as:

alias Int32StringNamedTuple = NamedTuple(x: Int32, y: String)

In regular code, {x: Int32, y: String} is a named tuple instance containing Int32 and String for x and y. This is different than the above named tuple type.

Proc

alias Int32ToString = Int32 -> String

is the same as:

alias Int32ToString = Proc(Int32, String)

To specify a Proc without parameters:

alias ProcThatReturnsInt32 = -> Int32

To specify multiple parameters:

alias Int32AndCharToString = Int32, Char -> String

For nested procs (and any type, in general), you can use parentheses:

alias ComplexProc = (Int32 -> Int32) -> String

In regular code Int32 -> String is a syntax error.

self

self can be used in the type grammar to denote a self type. Refer to the type restrictions section.

class

class is used to refer to a class type, instead of an instance type.

For example:

def foo(x : Int32)
  "instance"
end
def foo(x : Int32.class)
  "class"
end
foo 1     # "instance"
foo Int32 # "class"

class is also useful for creating arrays and collections of class type:

class Parent
end
class Child1 < Parent
end
class Child2 < Parent
end
ary = [] of Parent.class
ary << Child1
ary << Child2

Underscore

An underscore is allowed in type restrictions. It matches anything:

# Same as not specifying a restriction, not very useful
def foo(x : _)
end
# A bit more useful: any two-parameter Proc that returns an Int32:
def foo(x : _, _ -> Int32)
end

In regular code _ means the underscore variable.

typeof

typeof is allowed in the type grammar. It returns a union type of the type of the passed expressions:

typeof(1 + 2)  # => Int32
typeof(1, "a") # => (Int32 | String)