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 [Toggle Warm Grey ECCO Sandal Metallic Warm Women's Grey Jab 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Castle Men's 5 ASICS Running Silver Venture Red Fiery Shoe Rock GEL fHqAUT

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