F# forward type declarations

问题

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?

回答1:

You use 'and':

type firstType = 
     | T1 of secondType

and secondType =
     | T1 of firstType

回答2:

I figured it. It's:

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

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

回答3:

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

来源：https://stackoverflow.com/questions/1378575/f-forward-type-declarations