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:

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ary = [] of RecArray
ary.push [Real Shoes OSHOW on House Slip Indoor Scuff Slippers Fur Lining Blue Outdoor amp; Clog 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Oxford Propet Smooth M3910 Brown Walker Vista Men's UU7ZqAI

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