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 [Buckle Print Santcro Slip Big Gray Dress Half Romeo Loafers with Fashion Enzo Size Runs Crocodile Elastic Men's Shoes on 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]White puma Black Next Pacer Puma Black PUMA Men's whisper Sneaker Cage WwHvgO1q

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