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?

Burushaski

possible duplicate of F# forward type declarations – Mountain

@Mountain It is the same question, but the terminology is different enough that I failed to find it with either Google or the site search. That alone might be a reason to leave both open. – Burushaski

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

Oogenesis

Answer 2

"and" works generally for types with mutual dependencies. That is, it works for all types, such as discriminated unions, as shown by Mau, classes, records and mutually recursive functions.

Non terminating example:

let rec foo x = bar x
and bar x = foo x

Gilbart

Answer 3

Perhaps this will work:

type IntExpression =
  ...
and BoolExpression = 
  ...

(Information taken from this page on MSDN.)

Glabrescent