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?

Persas

See also #681106 – Stanfill

Answer 1

You use 'and':

type firstType = 
     | T1 of secondType

and secondType =
     | T1 of firstType

Numerary

Every time I feel something isn't elegant in F# I'm pleasantly surprised there's an elegant solution. – Nochur

Answer 2

I figured it. It's:

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

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

Persas

you use the same notation for mutually recursive functions as well - in case you didn't already know that. – Promethium

Answer 3

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

Flightless