Can discriminated unions refer to each other?

Question

I'm building an expression tree using discriminated unions. The below code:

type IntExpression =
    | TrueIsOne of BoolExpression

type BoolExpression =
    | LessThan of IntExpression * IntExpression
    | And of BoolExpression * BoolExpression
    | Or of BoolExpression * BoolExpression
    | Bool of bool

throws an error because BoolExpression is not defined. Swapping the definitions just results in the reverse (IntExpression is not defined) as you would expect.

Is there a way around this?

Answer 1

Yes, use and to group type definitions with inter-dependencies:

type IntExpression =
    | TrueIsOne of BoolExpression

and BoolExpression =
    | LessThan of IntExpression * IntExpression
    | And of BoolExpression * BoolExpression
    | Or of BoolExpression * BoolExpression
    | Bool of bool