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 [Red Baseball Black 2 Trainer MIZD9 Shoes Turf Mizuno Men's Players 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Slippers Foams BEALTUY Sandals with Men Shower Summer for Sole Soft Slip Foot Massage Green Non Bathroom Women House B7px7dzrnq

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