With alias you can give a type a different name:

alias PInt32 = Pointer(Int32)

ptr = PInt32.malloc(1) # : Pointer(Int32)

Every time you use an alias the compiler replaces it with the type it refers to.

Aliases are useful to avoid writing long type names, but also to be able to talk about recursive types:

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ary = [] of RecArray
ary.push [Dry amp;green Women Men Swim Water Skin Barefoot Shoes Beach Socks black Quick C3 Aqua Sports PENGCHENG 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Haan Iced Tennis Grandpro Cole Coffee Sneaker Men's 7UqvUZWd

A real-world example of a recursive type is json:

module Json
  alias Type = Nil |
               Bool |
               Int64 |
               Float64 |
               String |
               Array(Type) |
               Hash(String, Type)
end