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 [Stylish Sandals Wedge Platform Flip Summer High Heel Slippers Flops Hotmarzz Blue Women's Fashion Beach 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Sandal Venetia Nina Women's Dress Champagne AawtBqw

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