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:

Fire Shoe Blaze Orange Phoenix Running 3 Men's Speedform Armour Glacier Under Gemini Gray alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Shoe Phoenix Glacier Orange Armour Gemini Blaze Under Running Fire Gray 3 Men's Speedform 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Toe Brown Loafers Moc ZRO Leather Shoes Stitching Casual Mens's q16RZt

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