Question

What are the various use cases for union types and intersection types? There has been lately a lot of buzz about these type system features, yet somehow I have never felt need for either of these!

Answer 1

If you want a more practice-oriented answer:

With union and recursive types you can encode regular tree types and therefore XML types.

With intersection types you can type BOTH overloaded functions and refinement types (what in a previous post is called coherent overloading)

So for instance you can write the function add (that overloads integer sum and string concatenation) as follows

let add ( (Int,Int)->Int ; (String,String)->String )
      | (x & Int, y & Int) -> x+y
      | (x & String, y & String) -> x@y ;;

Which has the intersection type

(Int,Int)->Int & (String,String)->String

But you can also refine the type above and type the function above as

(Pos,Pos) -> Pos & 
(Neg,Neg) -> Neg & 
(Int,Int)->Int & 
(String,String)->String.

where Pos and Neg are positive and negative integer types.

The code above is executable in the language CDuce ( http://www.cduce.org ) whose type system includes union, intersections, and negation types (it is mainly targeted at XML transformations).

If you want to try it and you are on Linux, then it is probably included in your distribution (apt-get install cduce or yum install cduce should do the work) and you can use its toplevel (a la OCaml) to play with union and intersection types. On the CDuce site you will find a lot of practical examples of use of union and intersection types. And since there is a complete integration with OCaml libraries (you can import OCaml libraries in CDuce and export CDuce modules to OCaml) you can also check the correspondence with ML sum types (see here).

Here you are a complex example that mix union and intersection types (explained in the page “http://www.cduce.org/tutorial_overloading.html#val”), but to understand it you need to understand regular expression pattern matching, which requires some effort.

type Person   = FPerson | MPerson 
type FPerson  = <person gender = "F">[ Name Children ] 
type MPerson  = <person gender = "M">[ Name Children ] 
type Children = <children>[ Person* ] 
type Name     = <name>[ PCDATA ]

type Man = <man name=String>[ Sons Daughters ]
type Woman = <woman name=String>[ Sons Daughters ]
type Sons = <sons>[ Man* ]
type Daughters = <daughters>[ Woman* ]

let fun split (MPerson -> Man ; FPerson -> Woman)
  <person gender=g>[ <name>n <children>[(mc::MPerson | fc::FPerson)*] ] ->
  (* the above pattern collects all the MPerson in mc, and all the FPerson in fc *)
     let tag = match g with "F" -> `woman | "M" -> `man in
     let s = map mc with x -> split x in
     let d = map fc with x -> split x in    
     <(tag) name=n>[ <sons>s  <daughters>d ] ;; 

In a nutshell it transforms values of type Person into values of type (Man | Women) (where the vertical bar denotes a union type) but keeping the correspondence between genres: split is a function with intersection type

MPerson -> Man & FPerson -> Woman