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:

ECCO Metallic Jab Grey Warm Toggle Sandal Women's Warm Grey alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Metallic Grey Warm Grey Warm ECCO Women's Jab Sandal Toggle 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Shoe Ivory Men's Burnt Grant Sebago vwUqEq

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