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:

Shoe RN Blue tough Hydrogen Red NIKE Running Free Men's Black alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Running Red Shoe tough Free Blue Men's NIKE RN Hydrogen Black 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Edmond Tuxedo Shoes Dress Oxfords Alligator with Black Men's Prints Up Lace Fashion r1rHwX

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