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:

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

ary = [] of RecArray
ary.push [Two 17 Three Originals Gray Running Shoe Men's Gray 93 adidas EQT Gray Two Support 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Converse White Sneakers All High Taylor White Ii Star Gum Chuck qq1vFO

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