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 [Orange Glacier Running Gemini Speedform Men's Shoe Phoenix Blaze 3 Gray Fire Under Armour 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Leather Sneaker Recreation Fashion Creative Carda White Men's Hi 08xBqBzf

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