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:

White Balance Fresh Trail Foam Running Grey Trail Kaymin New Men's v1 Shoe alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Trail Fresh v1 Running Foam Trail Balance White New Men's Kaymin Grey Shoe 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Knit Dearfoams Space Women's Clog dye Cable Black Slipper FZHTwIqZ

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