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:

Toe 6 Charcoal Waterproof 0 Boot Comp Charcoal Baltimore Work Magnum Men's alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [6 0 Magnum Boot Charcoal Men's Comp Toe Baltimore Work Charcoal Waterproof 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Tactical Men's 1 Boot Black Valsetz Under Black Military Armour 5 and Rts IFCq5n8x5w

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