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 [OSHOW Indoor Fur Outdoor on Slip Scuff Slippers amp; Lining Blue Shoes House Clog Real 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Slippers rismart Mens' Loafers Shoes Driving Minimalism Navy Moccasin Suede Blue Soft rSApxAFqw8

A real-world example of a recursive type is json:

module Json
  alias Type = Nil |
               Bool |
               Int64 |
               Float64 |
               String |
               Array(Type) |
               Hash(String, Type)