OCaml: List that could contain two types?

Question

Instead of specifying a int list or string list, could I specify a list whose members must be either strings or ints, but nothing else?

Answer 1

You could do:

type element = IntElement of int | StringElement of string;;

and then use a list of elements.

Answer 2

One option is polymorphic variants. You can define the type of the list using:

# type mylist = [`I of int | `S of string] list ;;
type mylist = [ `I of int | `S of string ] list

Then define values such as:

# let r : mylist = [`I 10; `S "hello"; `I 0; `S "world"] ;;
val r : mylist = [`I 10; `S "hello"; `I 0; `S "world"]

You have to be careful to add type annotations, though, because polymorphic variants are "open" types. E.g., the following is legal:

# let s = [`I 0; `S "foo"; `B true]
val s : [> `B of bool | `I of int | `S of string ] list =
  [`I 0; `S "foo"; `B true]

To prevent the type of a list from allowing non-integer-or-string values, use an annotation:

# let s : mylist = [`I 0; `S "foo"; `B true];;
This expression has type [> `B of bool ] but is here used with type
  [ `I of int | `S of string ]
The second variant type does not allow tag(s) `B