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 [Romeo on Runs Men's Shoes Dress Crocodile Enzo Buckle Loafers Elastic with Fashion Slip Half Santcro Big Size Gray Print 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Red Summer Home Slippers Sandal Leather Women Wine Indoor Maylian for Flat Slide qCBqdP

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