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:

Charcoal Work Toe Men's Waterproof Boot Comp Charcoal Magnum 6 Baltimore 0 alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Baltimore 0 Toe Work Charcoal Men's Charcoal Boot Waterproof Magnum 6 Comp 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Floor Socks Slippers Warm GRE Slip Grey Boots Indoor Knit Snow Ankle Lined Non Fleece Greenery Cable Cotton High Women's Winter ATqKSU

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