F# forward type declarations

Question

I stumbled across this problem in F#. Suppose, I want to declare two types that reference each other:


type firstType = 
     | T1 of secondType
     //................

type secondType =
     | T1 of firstType  
     //................

How do I do that, so the compiler does not generate an error?

See also http://stackoverflow.com/questions/680606/f-how-to-have-two-methods-calling-each-other — Brian, Sep 04 '09 at 12:23

score 55 · Accepted Answer · answered Sep 04 '09 at 11:23

55

You use 'and':

type firstType = 
     | T1 of secondType

and secondType =
     | T1 of firstType

answered Sep 04 '09 at 11:23

Johan Kullbom

4

Every time I feel something isn't elegant in F# I'm pleasantly surprised there's an elegant solution. – CodeMonkey Aug 18 '16 at 10:36

score 4 · Answer 2 · answered Sep 04 '09 at 11:28

4

I figured it. It's:


type firstType = 
     | T1 of secondType
     //................

and secondType =
     | T1 of firstType  
     //................

answered Sep 04 '09 at 11:28

Max

6

you use the same notation for mutually recursive functions as well - in case you didn't already know that. – Massif Sep 04 '09 at 13:17

score 2 · Answer 3 · answered Sep 04 '09 at 20:55

2

The limitation is that the types have to be declared in the same file.

answered Sep 04 '09 at 20:55

t0yv0

3 Answers3