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 [Two Men's Originals Two Three adidas Gray Running Gray Shoe 17 Gray 93 EQT Support 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Farjing Low Women Boots Fashion Cylinder HOT Boots Boots SALE Flat Black Tassel For Boots Women's Flock fUgwISqc0

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

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