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:

Support EQT Gray Gray Gray 93 Three Running Two 17 adidas Originals Shoe Men's Two alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Three Two EQT Shoe adidas 93 Gray Two Running Men's Originals Gray Support 17 Gray 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Boot Pampa Palladium Linen Hi Women's Canvas x8xARO

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