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 [Fury 9 NIKE blue Tour Men's KD Basketball Zoom Black Shoe Yellow 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Vans Trwt Indigo Tropical Authentic blu 1TP1f

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