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
     //.....         


        

Answer 1

I figured it. It's:

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

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

Answer 2

You use 'and':

type firstType = 
     | T1 of secondType

and secondType =
     | T1 of firstType

Answer 3

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