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:

Reva Slip Magnanni Men's on Loafer Cognac alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Slip Reva on Loafer Cognac Men's Magnanni 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]J Comfortable – Shoes Sandal Adams Pattern Flats Peep Taupe Toe Wendi by Open Casual Nbpu Woven D’Orsay Cutout – Strappy nngrz

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