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:

Summer Flip Flops Heel Blue Wedge Fashion High Platform Hotmarzz Slippers Stylish Sandals Women's Beach alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Platform Women's Beach Slippers Sandals Hotmarzz Wedge Blue Flops Stylish Summer Heel Fashion Flip High 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Glory 362217 Gray 01 Grey PUMA PD Blaze Men of Xd4Xxaw

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

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