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 [Yoga Quick Socks Pool Barefoot White Boating Slip Lightweight Dry for HOBIBEAR on Aqua Dot Water Men Beach Shoes 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Wine Sneaker Plus Nylite4 Men's Fashion Wine Tretorn IYq7x

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