f# 3.0 - Choice type implementation in F#

Question

I am fairly new in F# and having some problems with choice type implementation. I have three types.

type A = p of string
type B = {q:string;r:int}
type C = s of string

Then I've a choice type like this.

type D = A | B | C

Now I am having problems in creating D type of variable. I am trying to do something like this,

let v = {q="abc";r=12}
let vofD = D.B v

But it gives this error,

This value is not a function and cannot be applied

I have tried to search for documentation on this thing and it seems that I'm unable to find any good document or solution on this. Any help will be greatly appreciated.

question from:https://stackoverflow.com/questions/65861245/choice-type-implementation-in-f

Answer 1

The problem is that the F# compiler doesn't understand that D.B refers to type B. If you define your discriminated union like this:

type D =
   | FromA of A
   | FromB of B
   | FromC of C

Then you can write let vofD = D.FromB v.