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:

Oxford Whitford H amp; Men's fBuaLpzCZX G Tan Bass alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Men's Tan amp; Bass H Oxford Whitford G fBuaLpzCZX 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Men Shoes Sneakers Breathable New 2018 Outdoor Neartime Running Sneakers Red Elastic Sport Casual Mesh Clearance❤️Men Athletic xACFqF

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