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:

bin Running Casual wei Fashion Lightweight Wu Sport Red Shoes Mens Breathable Flyknit Sneakers alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Wu bin Mens Lightweight Running Breathable Sneakers Fashion wei Shoes Red Flyknit Casual Sport 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Aldric On Leather Black CLARKS Mens Walk Un Slip w0qqRTXx

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